Electronic and Optical Properties of Amorphous Semiconductors: Their Principles and Applications

ELECTRONIC AND OPTICAL PROPERTIES OF AMORPHOUS SEMICONDUCTORS: THEIR PRINCIPLES AND APPLICATIONS

Alan E Owen PhD, FRSE

PART 2: APPLICATIONS

Thesis submitted for the degree of Doctor of Science University of Edinburgh 1994

- xlii - CONTENTS - PART 2 APPLICATIONS

Index to Cited Publications: Applications...... (xv)

Cited Publications: Applications...... 341-576

APPENDIX:

Complete List of Publications...... (Apl-Ap12)

- xlv -

APPLICATIONS

(V/here there are co-authors, the number in parenthesis indicates my position in the list of authors.) Page Al. (2) Electronically Assisted Thermal Breakdown, with J M Robertson. J 341 Non-cryst Sol, Vols. 8-10, 439-444, (1972). (1) Electronic Conduction and Switching in Chalcogenide Glasses, with J 347 M Robertson. Proc IEEE Transactions on Electron Devices. Vol. ED-20, 105-123, (1973). Invited Contribution. (2) The Characterisation of Metal-Thin-Insulator—n—p Silicon 365 Switching Devices, with J Buxo, G Sarrabayrouse and J-P Sebaa. Rev de Physique Applique, Vol. 13, 767-770 (1978). (1) The Threshold Characteristics of Chalcogenide Glass Memory 369 Switches, with J M Robertson and C Main, J Non-cryst So!, Vol. 32, 29-52 (1979). (1) New Amorphous Silicon Electrically Programmable Non-Volatile 393 Switching Device, with P G Le Comber, G Sarrabayrouse and W E Spear. TEE Proc, Part I, Solid-State and Electron Dev, Vol. 129, 51-54, (1982). (1) Memory Switching in Amorphous Silicon Devices, with P G Le 397 Comber, W E Spear and J Hajto. J Non-cryst Sol, Vols. 59 and 60, 1273-1280, (1983). An invited paper presented at the 10th mt Coni on Amorphous and Liquid Semiconductors, Tokyo, August 1983. AT (2) Electronic Switching in Amorphous Silicon Junction Devices, with P 405 G Le Comber, W E Spear, J Hajto and W K Choi. Chapter 15, pp 275-289, of "Hydrogenated Amorphous Silicon, Part D - Device Applications", Ed J J Pankove, Vol 21, Part D of the series "Semiconductors and Semimetals", Ser Eds R K Willardson and A C Beer, Academic Press, (1984). (3) Inorganic Resists Based on Photo-Doped As-S Films, with A P Firth, 420 P J S Ewen and C M Huntley. Proc SPIE, Vol. 539, (Advances in Resist Technology II), 160-165, (1985). (2) The Switching Mechanism of Amorphous Silicon Junctions, with P G 426 Le Comber, W E Spear, J Hajto, A J Snell, W K Choi, M J Rose and S Reynolds. J Non-cryst Sal, Vols. 77 and 78, 1373-1382, (1985). AlO. (5) Chalcogenide Gratings Produced by the Metal Photodissolution 436 Effect, with A Zakery, C Slinger, P J S Ewen and A P Firth. J Phys D, (Applied Physics), Vol.21, S78-S81 (1988). All. (4) Dry Etched High Resolution Positive and Negative High Resolution 440 Photoresist System Ag-As-S, with E Hajto, R E Belford and P J S Ewen. J Non-cryst Sal, Vol.115, 129-131 (1989).

-xv - Al2. (4) Interference Grating Fabrication in Spin-Coated As 2 S3 Films, with E 443 Hajto, P J S Ewen and R E Belford. Thin Solid Films, Vol 200, 229-237 (1991). (2) Analogue Memory and Ballistic Electron Effects in Metal- 452 Amorphous Silicon Structures, with J Hajto, A J Snell, P G Le Comber and M J Rose. Phil Mag B, Vol 63 (Special Festchrift issue for Professor W E Spear FRS), 349-369 (1991).

(3) PASS - A Chalcogenide-Based Lithography Scheme for I.C. 473 Fabrication, with M N Kozicki, S W Hsia and P J S Ewen. J Non- cryst Sol Vols 137 and 138, 1341-1344 (1991). (4) Photodoped Chalcogenides as Potential Infrared Holographic Media, 477 with C W Slinger, A Zakery and P J S Ewen. App! Optics, Vol 31, 2490-2498 (1992). (2) Photo-Induced Changes in Chalcogenide Glasses and their 486 Applications, with P J S Ewen, Chapter 14, pp 287-309 in "High Performance Glasses". Eds: M Cable and J M Parker, pub!: Blackie (1992). (2) Electronic Switching in Amorphous Semiconductor Thin Films, with 515 J Hajto, A J Snell, P G Le Comber and M J Rose. Chap 14, pp 641-701 in "Amorphous and Microcrystalline Semiconductor Devices: Volume II, Materials and Device Physics". Ed: J Kanicki, pubi: Artech House, Boston, USA (1992).

-xvi - 341

JOURNAL OF NON-CRYSTALLINE SOLIDS 8-10 (1972) 439-444 North-Holland Publishing Co.

ELECTRONICALLY-ASSISTED THERMAL BREAKDOWN IN CHALCOGENIDE GLASSES

J. M. ROBERTSON and A. E. OWEN Department of Electrical Engineering. University of Edinburgh, Edinburgh, Scotland

The threshold characteristics of switches prepared from compositions in the As2Te3 + Si system have been examined. With low resistivity compositions (i.e. low Si content) the characteristics are adequately described by a simple thermal model, irrespective of the geometry of the device. As the resistivity is increased (i.e. increased Si content) the field- dependence of the conductivity becomes significant and has an important effect on the threshold characteristics of thin sandwich devices.

1. Introduction

Chalcogenide glasses have low thermal conductivities and their electrical conductivity increases rapidly with temperature so thermal runaway must always be considered as a possible high-field breakdown mechanism. Alter- natively, if switching cannot be completely attributed to thermal runaway, the thermal effects of the current and voltage conditions which prevail at threshold may be significant enough to influence the device characteristics. The simplest form of thermal breakdown occurs when o=o(T) 1). In glasses of higher resistivity the field dependence of conductivity is more important, i.e. a=o(T, E). When this is taken into account, with a suitable expression for o, good quantitative prediction can be obtained of switching characteristics2' 3). This may be considered as a form of "electronically- assisted" thermal breakdown. In this paper, the effects of a steady increase in glass resistivity are presented along with some consequences of this type of breakdown. 2. Sample preparation

In the system As 2Te3 +Si, glass resistivity increases approximately loga- rithmically as the proportion of silicon is increased. The range is from 50

ohm m for As2Te3 to 108 ohm m for (As2Te3 ) 60 Si40. On evaporation, dissociation occurs and the films are deficient in silicon with respect to the bulk glass. However, X-ray fluorescence analysis has shown that the As: Te ratio in the thin film is still approximately 0.67 so the general form of the composition does not change.

439 342

440 J. M. ROBERTSON AND A. E.O\.rS

All samples were deposited on Corning 7059 glass. In most cases there was a simple cross-over geometry with a device area of 2 x 10 m 2 and glass film thickness about 1 gm. Gold electrodes were used.

Thermal breakdown

Conductivity may be written in the form:

a(E, T) = a exp(-?i) f(E), (1) where 0a is the conductivity when the temperature deviation from ambient, T, is zero and T1 is a constant, typically 20 K for usual chalcogenide switching glasses; f (E) expresses the field modification of conductivity and must be determined experimentally. This equation is quite accurate for small values of AT. The steady state heat dissipation may be expressed by the usual Newtonian expression:

VI=FAT, (2) where F is the thermal conductance of the film. If eq. (1) is substituted into eq. (2) and differentiated with respect to temperature, the breakdown condition may be established:

V2 1 ,h f(E) RaFTi . (3)

With a constant device geometry. F is approximately constant, so R. the low field sample resistance is the most controllable parameter.

Pulse measurements

When a rectangular voltage pulse is applied to a sample the current rises from an initial value I to a final value which is AI greater. This is shown in fig. 1. The capacitive spike is of short duration and the increase in current is characterised by a time constant T. For three types of sample: (a) thick (100 jim) with sandwich electrodes, (b) thin film with coplanar electrodes 100 gm apart and (c) thin (1 gm thick) with candwich electrodes and very low resistivity glass (<60 ohm m), the

variation of I with voltage was ohmic. Just below Vlh, the magnitudes of M and r were appropriate to values expected of the device were being internally heated so it is reasonable to take Al as the Joule heating component of current. The variation of (i +AI) with voltage followed the usual ohmic 343

ELECTRONICALLY-ASSISTED THERMAL BREAKDOWN 441 then exponential relation expected for simple thermal breakdown where cr=a(T) only. As glasses of higher and higher resistivity were used, the breakdown field, Eth, increased slowly but the current pulse remained of the same general form as that shown in fig. 1. Characteristics for a 300 ohm film are shown in fig. 2. The 10—V relation can be approximately described by:

cy(E)=cra exp (VI V 1 ). (4)

The variation of V1 with thickness is shown in fig. 3. The slope, E1 , is 3.3 x 106 V m 1; E1 increases slightly with film resistivity and decreases slowly with increasing ambient temperature. When eq. (4) is substituted into the thermal breakdown eq. (3) the threshold voltage is given by:

= V1 log(RFTj/eV). (5)

This equation gives a good quantitative prediction of V, over a wide range of glass film resistivities from the simple ohmic case with T1th < V1 and p <50 ohm in to the electronically-assisted case with p< 1000 ohm m and where Vh is limited to only 3-5 times V1 . The usual temperature, thickness, and delay time expressions also follow the analysis used to derive eq. (5).

VOLTAGE

TIME

CURRENT

TINE

Fig. 1. Schematic forms of the voltage and current pulses just below Vu,; Al represents the increase in current due to Joule heating.

344

442 J. M. ROBERTSON AND A. E. OWEN

JOULE HEATING TRAILING EDGE.

10 LEADING EDGE

CURRENT (MA)

3 / / EXTENSION OF LOW-FIELD I ' OHMIC CHARACTERISTIC I ,

/ 1

0.3 0 5 10 15 20 VOLTAGE (VOLTS)

Fig. 2. Current—voltage characteristics (log-linear) for Jo and (Jo + Al) of fig. 1 in a 3000 ohm film.

V 1

3 (VOLTS)

0 0 0.5 1.0 ELECTRODE SEPARATION (pm)

Fig. 3. The constant V1 of eq. (4) as a function of electrode separation. 345

ELECTRONICALLY-ASSISTED THERMAL BREAKDOWN 443

5. Localised breakdown At fields greater than E1 thermal breakdown occurs preferentially at any small region of slightly increased field. Thus the position of a conducting filament may be determined by physical factors such as glass or substrate irregularities or stresses over lower electrode edges. As glass films of higher resistivity are used, one of the most noticeable effects is that the Joule heating component in the current waveform decreases. This we suggest is due to more localised breakdown as the a. (E) term becomes more dominant. The thermal time constant for a sandwich device, area A, is approximately given by: r = CAd/r where C is the heat capacity of the glass. For our devices with r = 10 mW K' and C=2x 106J M - 3 K - 1 , r=40 jis which is the value measured for samples made from low resistivity glass films. The thermal conductance is determined largely by the substrate and electrodes so any reduction in the effective heated area should result in a corresponding reduction in T. This occurs when either the contact area is reduced, i.e. TocA, or if glass conductivity is increased i.e. icca.. These results have been combined in fig. 4 but in this case t has been plotted against R/d.

100

3- 30

- 10

El z I-. z 0 C)

I-Il E- -:1

0.3

0.1 0.1 0.3 1 3 10 30

Rid (Kn.jm')

Fig. 4. The thermal time constant, measured by a pulse experiment of the kind illustrated in fig. 1, as a function of the ratio of device resistance (R) to electrode separation (d). 346

444 J. M. ROBERTSON AND A. E.OWEN

6. Conclusions

As glass film resistivity is increased, the field-dependence of conductivity becomes steadily more important. There is a steady change from the simple thermal breakdown case with an ohmic sample to the electronically-assisted case. At all times, however, the Joule heating contribution to current can be identified and eliminated from any conductivity expression used to calculate breakdown conditions. With high resistivity films local breakdown is significant. Although other effects or processes may be present in these devices, particularly when the voltage approaches Vth, the thermal breakdown analysis is sufficient to determine the major functional relationships and their numerical values in the devices we have studied.

References

I) F. M. Collins, J. Non-Crystalline Solids 2 (1970) 496. J. M. Robertson and A. E. Owen, Symposium on Electrotechnical Glasses, Imperial College, London, September 1970. A. C. Warren and J. C. Male, Electronics Letters 6 (1970) 567. 347

- Reprinted lynn: IEEE TRANSACTIONS ON ELECTRON DEVICES Voiume ED-20. Number 2, February, 1973 pp. 105-122 COPYRIGHT @ 1973—THE INSTITUTE OF ELECTRICAL AND ELECTRONICS ENGINEERS; INC. - PRINTED IN THE U.S.A.

Electronic Conduction and Switching in Chalcogenide GIases

ALAN E. OWEN AND JOHN M. ROBERTSON

invited Paper

Abstract—Evidence for electronic conduction in the preswitching explicitly or implicitly, with a region of current-con- region of chalcogenide glasses is outlined and discussed in terms of trolled negative resistance (CCNR). An essential condi- several simple models. Electronic processes likely to contribute to tion for CCNR is a positive feedback mechanism which, the switching process itself are reviewed and compared, where possible, with the predictions of relevant models from conventional in the region of instability, allows the system to carry semiconductor physics. the same, or larger currents, with smaller voltages. In the thermal mechanisms of switching discussed by I. INTRODUCTION Warren [t], the feedback loop is F an increasing electric field is applied to a dielectric. high field - increased current -. increased power an unstable situation will eventually arise where the dissipation rate of energy input exceeds the capacity of the I .1. system to dissipate it. The results can be catastrophic greater conductivity - internal temperature and after breakdown the dielectric may be left in a rise. conducting (short-circuit breakdown) or nonconducting The essential link is the sensitivity of the conductivity (open-circuit breakdown) state. In some circumstances to temperature. Other examples, relevant to electronic ON state, breakdown may lead to a conducting state, or processes, will be mentioned later (Section IV-B, Q. which requires a small holding current and voltage to Whether the result is breakdown or switching, the condi- OFF state, is sustain it. The original high resistance, or tions leading to the electrical instability can usually recovered if the sustaining voltage is removed. In other be formally described by an energy-balance equation cases it is possible to establish a permanent ON state [2], [3] that persists in the absence of any voltage, but the OFF state may be recoverable by the application of a suitable A(T,E,a) = B(T,a) (1) current pulse. These two cases of reversible breakdown where A is the rate at which energy is gained from the are described as threshold switching and memory field E at temperature T and B is the rate at which it is switching, respectively. In all cases reported so far, absorbed or dissipated. The parameter a is introduced to switching in chalcogenide glasses is associated, either denote any other relevant property of the current car - riers pertinent to a particular situation. Manuscript received April 29, 1972. The variation of the two sides of this energy-balance The authors are with the Department of Electrical Engineering, University of Edinburgh, Edinburgh, Scotland. equation will generally be as shown in Fig. 1 in which 348

106 IEEE TRANSACTIONS ON ELECTRON DEVICES. FEBRUARY 1973

,rC5E.SIWG where r, is the mean time between collisions and is a E, function of energy and N is the average number of phonons of frequency c. Thus in Fig. 1 the abscissa RATE OF represents energy e, and the field E2 again corresponds to CHANGE or ENERGY the onset of instability. If the electron density is high (e.g., at high temperatures) electron—electron collisions may become important and the single-electron theories are no longer valid. With a high collision rate and a rate of exchange of energy due to electron—phonon interactions, the electron energy-distribution function is Maxwellian with a mean temperature T1 above the ambient temperature Ti,, or the lattice temperature T. This means T • £ that the energy rate of change terms A and B can be Fig. 1. Schematic representation of solutions to averaged to A and W having a similar functional de- the energy-balance equations. pendence so that the general picture of Fig. 1 remains valid at least up to energies €,,, at the maximum of the B the abscissa represents some appropriate parameter curve in Fig. 1. This "collective" description has a strong such as temperature T, energy E, or injected charge analogy with thermal breakdown and the abscissa in qiu, (see below). Fig. I corresponds to the electron temperature T. I. In The rate of gain of energy from the field will normally this case, therefore, it is the steady-state electron tem- be given by perature that rises until, at a critical value, no equilibrium is possible and instability sets in. Near €.,,, how- A = (T, E)E 2 (2) ever, the electron—phonon energy transfer increases, or some straightforward variant of this; the conductivity implying an increase in the internal temperature of the r(T, E) is generally a function of temperature and field. material with consequential thermal effects. With a sufficiently large conductivity that is very tem- Of the many other breakdown and/or switching perature dependent, the dielectric will heat up through mechanisms, at this stage it is worth mentioning double Joule heating, and if thermal processes are dominant injection with recombination [4]. In this case the en- then ergy input is either stored in the dielectric as injected charge Q, or is lost through recombination. An energy- ÔT B=l---V(Pl-\ 2 T. (3) balance equation could be written, therefore, as K / at - (6) The thermal energy-balance condition A = B is then the cr(T, E)E2 = -- Jrec \&i n t same as in [i, eq. (1)] (the same notation is used). Thus in Fig. 1 the abscissa represents temperature and the where ,c is the dielectric constant, D is a suitable geo- field E2 would be the point at which instability sets in. metrical factor, and j is the recombination current It is worth noting that for a thermal instability, it is driven by some appropriate internal potential 'it. In not necessary that the conductivity be a function of this case the abscissa of Fig. 1 would represent Qj,,j . field [1]. The isothermal conductivity may be ohmic at In general, therefore, we can describe the condition of all times, but the dynamic conductance will not be a electrical instability, whatever its origin, in terms of a constant because of Joule heating. diagram such as Fig. 1. This will normally be preceded 'Many other conditions of instability leading to break- by a region in which the conductivity is field dependent down have, of course, been investigated. For intrinsic (nonohmic). What follows, i.e., breakdown or switching, electronic breakdown in the single electron approxima- depends upon the properties of the dielectric and on the tion, for example, the function A is of the form [3] presence, or absence, of a suitable positive feedback e2E2 r(e) process in the system. For threshold switching the ma- A = e(€)E2 = ( 4) terial must be capable of carrying a much increased cur- m rent, either uniformly or locally (i.e., in a filament), where ju the mobility and r the relaxation time are func- but reversing spontaneously to the original nonconduct- tions of energy. The electron loses energy to the lattice ing OFF state when the sustaining or holding voltage is via phonon interactions and removed. For memory switching the dielectric material must be capable of changing in some way (e.g., an over- kT\ all or localized change in the atomic or microscopic B = (5) r)(2X + 1) (i - structure) into a permanent conducting state. but one 349

.)IVEN AND ROBERTSON: ELECTRONIC CONDUCTION AND SWITCHING that can be reversed to the OFF state by a suitable current (energy) pulse.' Obviously the system must also be able to absorb the reversing pulse without destructive breakdown. \l CURRENT II. ELECTROTHERMAL PROCESSES (WA) Ioi- Switching of either the threshold or memory kind has been observed in a wide variety of semiconducting 1 glasses of the chalcogenide [5], [6] and transition metal oxide types [1]. The general form of switching characteristics for chalcogenide glass is shown by Warren [1]. 2 Q.C. (100 HT) He also gives typical values for threshold voltage Vth, 3 100,ps PULSES ,$ PUl.sz5 delay- time Id, and describes how these parameters vary 'I. with temperature, field strength, and geometry. More detailed treatments of various aspects of switching behavior can be found in the proceedings of recent con- 0-3 ferences [8]–[10]. The main point of issue has been to what extent switching can be attributed to thermal or VOLTAGE (7s) electronic. processes and both viewpoints were represented.at these three conferences [8]–[10]. There have, 30- in addition, been suggestions that some subtle struc- AILING EC tural rearrangement akin to a dielectric or ferroelectric CURRENT transition may be responsible for switching [7], [ii]. The purpose of this review is to consider what electronic JO EDGE mechanisms may be appropriate, and mechanisms that CURRENT CURRENT (A) rely on structural changes will therefore not be con- (io n sidered. It is not proposed to argue the pros and cons of thermal versus electronic theories of switching, but it should be recognized from the outset that there must be some convergence of the thermal and nonthermal viewpoints. The properties of semiconducting glasses are such that some thermal effects must be present; the question is— how much? It is now acknowledged, for example, that 03 in chalcogenide switches with electrode separations of 10 &m or more, the threshold conditions can be quantitatively predicted from a solution of the thermal energy- balance equations (2) and (3) in a one-dimensional form 0 .r tO Is 30 with conductivity expressed only as a function of tem- VOLTAGE (VOLTS) perature, i.e., a =cr(T) [i], [12], [13]. With smaller electrode separations, however, there Fig. 2. (a) Illustration of the effect of measurement technique on the current—voltage characteristics of a chalcogenide glass. Mea- are significant changes [1] (e.g., in the temperature and surements made at room temperature. Composition: .As 2Te7Ge1 ; thickness dependence of the threshold voltage) and thickness: 1.7 Mm; repetition rate in pulse measurements: 100 Hz. (b) Current—voltage characteristics taken at the beginning and this has been taken to indicate that electronic effects are end of a 70-MS pulse. Sample and conditions same as in Fig. 2(a). predominant [13]. The simple thermal models are certainly no longer adequate, but thermal effects may still be present. Fig. 2(a), for example, shows the current– end of the pulse. In the dc case and to a lesser extent in voltage dependence on a log–log scale for a thin film the 100-Hz ac measurements, a region of negative resis- (1.7 Am) of glass of nominal composition As20Te70Ge10 tance was observed before the device switched. Such re- [14]. The pulse measurements were made at a repetition suits are to be expected if self-heating occurs, but note rate of 100 Hz and the current is that recorded at the that at voltages below the dc threshold, the characteristics are practically identical. To establish the conduction mechanism at high fields, 'Some care is required to distinguish memory switching from "self-healing" breakdown in which small regions of the dielectric are it is obviously necessary to measure the I–V charac- progressively destroyed through a sequence of short-circuit, open- teristics without the complication of self-heating, and it circuit breakdown events. See, for example, N. Klein, Thin Solid Films. vol. 7. p. 149, 1971. should not necessarily he assumed that pulse measure- 350

IEEE TRANSACTIONS ON ELECTRON DEVICES, FEBRUARY 1973 108 ments will do this the pulse length has to be shorter The conduction mechanism may be identified and than the thermal time constant for the active glass film. possibly linked with an electronic switching mechanism. Fig. 2(b) shows I— V curves plotted on a log—linear scale This is reviewed in Section III. with current measured at the leading and trailing edges The thermal consequences of this conduction of a 70-As pulse (50 ns rise time, 100-Hz repetition mechanism may be calculated. This approach, with con- rate). The capacitance spike at the beginning of the ductivity a function of temperature and field, has been pulse was very short and it was possible to extrapolate used recently by several authors [13], [15], [18], [19]. back to zero time to obtain the initial current value. Their predictions show reasonable correspondence with The current rise during the pulse could not be conclu- experimental data, but while they show that field-as- sively attributed to Joule heating, but the power dissi- sisted thermal runaway is sufficient to explain switching pated during one pulse was sufficient to give a tempera- observations, they do not prove that switching is exclu- ture rise of several degrees, which in turn was sufficient sively thermal in origin. to account for the current increase during the pulse [14]. The role of the nonohmic conduction mechanism Furthermore, the time taken for the current to reach a therefore becomes very important. We take the view steady value was approximately the same as the esti- that in a purely thermal process the exact mechanism is mated thermal time constant for the materials and immaterial and an empirical relation is all that is re- geometry used. Fig. 2(b) emphasizes that characteristics quired to predict the results. The nonohmic conductivity with and without a Joule heating component of current provides a means whereby energy is delivered to the are of almost identicalforin. This can make it very diffi- switch at a faster rate than would be possible under cult to identify effects of Joule heating particularly in ohmic conditions. If, on the other hand, there are fea- higher resistivity films where the heating component of tures of switching that cannot be accounted for by a current is smaller and the total current distribution may complete solution of the thermal energy-balance equa- be nonuniform even in the prebreakdown condition [15]. tion with all the parameters given their correct de- The straight-line region of a log—linear plot is often very pendence on temperature, field, etc., then we must ex- restricted. If the transition from an apparently ohmic to amine the conduction mechanism for sufficient condi- exponential characteristic occurs at a voltage Vol then tions to allow some electronic or other nonthermal mech- intrinsic contribution to the switching typically VIh+3Vo at room temperature [141, [16]. anism to make an Thus the voltage range over which experimental ob- process. In the following sections the likely electronic servations can be compared with theoretical models is processes are discussed and, wherever possible, com- often very limited and the interpretation is correspond- pared with experimental information. ingly uncertain. The "leading-edge" current characteris- The memory type of device will not be discussed ex- ON state in a chal- tic of Fig. 2(b) shows noticeable nonohmic behavior plicitly. The formation of a permant typical of that seen in many chalcogenide films at fields cogenide switch involves the precipitation of a crystalline filament from the glassy matrix [20]. This is a prob- as low as 106_10 6 Vm'. A recent paper by de Wit and Crevecoeur [17] suggests indeed that in AsSe3 the con- lem involving the thermodynamic stability of the various ductivity is nonohmic essentially down to zero fields. We phases involved (glass, liquid, and possible crystalline shall return to this question of the onset of nonohmic compounds) and the kinetics of nucleation and crystal conduction in a later section, but for the moment it is growth. Threshold switching always precedes memory sufficient to note that at high fields an exponential de- switching, however, so any mechanism established for pendence of current (or conductance) on voltage is often the former is implicitly applicable to the latter. observed: III. THE HIGH-FIELD OFF STATE I = Io exp (V/V 0) ( 7a) A. Experimental Data I—V I There is surprisingly little published data on the G - = Go exp(V/Vo) (7b) characteristics of the kind of chalcogenide compositions most often used in switching devices, and hence for the where G0 is the zero field conductance. For V<< V0 it may purposes of illustration in the following discussion data be very difficult, experimentally, to distinguish any de- obtained by one of the authors [14] will be used. Typical viation from ohmic behavior. The constant V0 in (7a) results for a glass of approximate composition (As 2Teo)s.s are shown in Fig. 3(a) for three different thicknesses and (7b) will not of course be the same for a given set of Si1 . 5 data and if they are to be interpreted in terms of some and a limited temperature range. These data conform specific model, attention must be given to the appropri- approximately to (7). Fig. 3(b), (c) reproduce, for et at. [lib] on the ate form of plotting the data to arrive at V0 . comparison, the data of Walsh The establishment of a nonohmic characteristic that composition (weight percent): Te l 49 percent; As, 33 is unambiguously free from thermal effects allows two percent; Ge, 6 percent; Si, 3 percent; Ga l 9 percent steps to be taken. (thickness 1.2 m) and Fagen and Fritzsche [21] on the 351

OVEN AND ROBERTSON: ELECTRONIC CONDUCTION AND SWITCHING 109

3401c ,/•o ; L• 3201< ?' 10 •/ / /•_ •1 M. k CURRENT ) (.A 3 V

L' I.Ij 03

0.1

o3 0 VOLTAGE (voLra)

(a)

O•

CURREN (AMPS)

:::~ Icr-

t62I23 : K ;

'd-01 10 VOLTAGE (VOLTS) VOLTAGE (VOLTS) (b) (c) Fig. 3. (a) Typical I—V characteristics without Joule heating illustrating the influence of temperature and electrode separation. Composition: (AsITei)LiSiI.i. (b) Typical I—V characteristics from Walsh el al. [16]. (c) Typical (1/1') — V characteristics, from Fagen and Fritzsche [21].

composition AsTe3Ge2 (thickness 0.9 tim); both cover C = GOA exp (V/V0A) + G08 exp (V/Voa). (8b) a reasonably wide range of temperature. Fagen and There is information available on the high-field be- Fritzsche's results are plotted in the form (1/ V) versus havior of simpler but related amorphous semiconductors V (7b). Note, in Fig. 3(b) and (c), that a second well- such as vitreous Se and As 2Se3, and although completely defined exponential (log I V) region develops, at low analogous switching behavior has not been demon- temperatures, i.e., strated, the conduction mechanisms probably have a I = 10.1 exp (V/VOA) + lOB exp (V/V 05) (8a) good deal in common with the more complex chalco- 352

110 IEEE TRANSACTIONS ON ELECTRON DEVICES, FEBRUARY 1973 genide glasses. Amorphous or vitreous Se. in particular, has been widely studied and high-field conduction has generally been interpreted in terms of various single- carrier space-charge-limited current models [22}—[26]. L It should be noted, however, that except in the case of [25] there has been no systematic investigation of the \ thickness dependence of current—a crucial factor in RESISTANCE space-charge-limited current mechanisms. In contrast, -oLO "Sfr' Muller and Muller [27] have recently concluded that the high-field I—V characteristics of amorphous Se are determined by the Poole—Frenkel mechanism [28]. 03 L02p Kolomiets and Lebedev [29] interpreted their results on As2Se3 in terms of space-charge-limited current with the Fermi level in an exponential distribution of traps, but from measurements on samples covering a very wide 0.1 range of thickness (5 m to 2 mm) de Wit and Creve- a 1 2 I. 5 coeur [17] conclude that the current is not space-charge VOLTAGE (ct1s limited. Recent results from the authors' own labora- (a) tory (Renouf [30]) on thin evaporated As2Se3 films (-1 Mm) over a wide range of temperature (370-100 K) a&s show similarities with the I—V characteristics of Fig. 3(b), the main difference being that although the curves SLOPE become steeper as the temperature is lowered, a well- () defined second I a exp V region does not develop.

B. Space-Charge-Limited Current 0.10 Some of the models proposed to explain threshold switching (see Section Ill-C) have much in common with double injection and it might be expected therefore that in the preswitching region the current flow is space- charge limited, either single or double carrier [4]. Single- 005 carrier space-charge-limited current flow with the Fermi level in a uniform distribution of traps predicts an (1/ T)— V characteristic exactly of the form of (7b) with

1 KK, -- (9) Vo - eN gkTd 2 0 - 0 I 2 3 1. 0 where 'Co is the permittivity of free space, K, 15 the dielectric constant, Ng is the trap density, and d is the sample thickness. Hall [31] has suggested that both exponential regions of the data of Walsh et al. [lib] should he inter- OLS preted in this way; the increased slope at lower tempera- SLOPE 1L00 S tures corresponding to the Fermi level entering a second V uniform trap distribution of higher density (e.g., Hall obtains trap densities of 2 X 10 23 and 6X 10 2 m 3 eV' from the two V0 values). Unfortunately, the crucial thickness dependence was not investigated. According to (9), (1/ V0) should be proportional to (I/ T) and (1/d2).

Some of Robertson's [14] results for three As—Te—Si 0 glasses are plotted to illustrate these relationships in 0 ' 2 3 '. 5 I. ELEC1RO3C SEPP.QATION. L .2. Fig. 4(a)—(c), and although the data show the expected functional dependence on temperature and thickness, Fig. 4. (a) Current—voltage data without Joule heating represented none of the lines in Fig. 4(b) and (c) pass through the as log. resistance (R) versus V for films of different thickness. origin. This is confirmed by a comparison with some re- Measurements made at room temperature. Composition as in Fig. 3(a). (b) Influence of temperature on the slope of lines on a sults of Fagen and Fritzsche, also shown in Fig. 4(b). !og. R versus V graph, e.g., Fig. 4(a), for two different thicknesses; Robertson [14] obtained trap densities in the region X: 0.45 A m, 0: 1.1 jum. Data from Fagen and Fritzsche (Fig. 1024 3(c) and [211) are also plotted for comparison. (c) Influence of 2 x 10-2 x m 3 eV' for a range of compositions, electrode separation on the slope of lines on a log. R versus V but there was no systematic variation with composition. zraph, e.g.. Fig. 4(a I. 353

C,VEN AND ROBERTSON: ELECTRONIC CONDUCTION AND SWITCHING

These trap densities are very similar to those derived by 5- the same methods for amorphous Se [23] and Se-Ge glasses [32], but, as Hall [31 ] points out, are much less -4- than those obtained by other electrical techniques for vs the more complex chalcogenide glasses [33]. The latter '.VLTs) Or point is not, perhaps, too serious, as there is some doubt about the large trap densities calculated from some electrical measurements, but the nonzero intercepts of Fig. 4(b), (c) must cast considerable doubt on the applicability of the 'simple form of the space-charge-limited model. It was mentioned in the previous paragraph that Kolomiets and Lebedev [29] reported space-charge- 05 10 limited current in vitreous As 2Ses, but again the thick- ELECTOODE 5tPAR'10'.. L ness dependence does not support this. Fig. 5. Variation of V0 in (12) with electrode separation. According to the recent measurements by de Wit and Composition same as in Fig. 3(a). Crevecoeur [17], Vo is proportional to d, i.e.. the field parameter E0 is constant at, in this case, a value of about TABLE I 1.6-1.8X10 7 V/rn. Renouf's data are also consistant with this [30]. Unit I Activation energies (eV) C. Hopping Conduction for V I for 1 01I Differencel for

In an amorphous semiconductor with a high density 1 0.06 - 0.40 - 0.46 - 0.48 of localized states, a large fraction of the carriers is likely 2 0.12 - 0.40 - 0.51 - 0.49 to be trapped at any instant. Hopping conduction, either 3 0.07 - 0.33 - 0.40 - 0.44 directly between traps or via the conduction (or valence) - 0.30 - 0,44 - 0.48 band, is therefore a possible means of transport and, as 4 0.14 Bagley [34] has pointed out by anology with well- Note: All samples have nominal turn composition (As2Te3 ) 3 Si1 . known treatments of ionic conduction, the I- V relationship should be film thickness. Experimentally it is found that both V0 I = lo sinh (V/V 0 ) ( 10) and 10 vary exponentially with (1/T), but in opposite where now senses. Thus activation energies can be derived and 2k Td typical values are listed in Table I, along with the low- (11) ea field activation energy for the "ohmic" conductance G 0 . The lo activation energies correspond, of course, to where a is the 'jump" distance. in (12) and the difference in the values for V 0 and TO The preexponential constant is given by Jo does roughly agree with that for G 0 as expected from Jo = A neap exp (-/kT) (12) (13). The apparent "activation" of V0 is puzzling; the only suitable term in (11) that could be temperature with n the trapped carrier concentration, 0 the height of activated is the jump distance a, and the implication is. the potential barrier, v a phonon frequency, and A the area. Equation (10) leads to an exponential I-V charac- therefore, that teristic (7a) for V> Vo. From (10) the low-field ohmic a = Go exp (-'I'/kT) conductance (V-+0) is given by with 'I'=0.06-0.14 eV (Table I) and Go is in the range = 2(10/V 5). (13) G0 0.2-4 ,um. From the calculated values of 0 and a and a Superficially, at any rate, the data of Fig. 3(a) and for typical value for v (1011 s'), an estimate of the carrier other samples, conform approximately to (11) as shown density ii can be made, and hence also of the mobility j, in Fig. 5, in which the straight line corresponds to a con- using the ohmic conductivity. Some representative re- stant field E 0 =3.3X108 V'm'. At room temperature sults are given in Table II. this gives a value for the jump distance a of about 15 The spread in the possible values of 0 introduces an nm. This is a rather larger value than expected, but uncertainty of about one order of magnitude in n and s. seems to be typical (other values are summarized later). Thus although superficially the simple hopping-type The temperature and thickness dependence of the model for conduction fits the functional form of the I-V "constants" V0 and lo appear to be anomalous. In Fig. characteristics, a detailed analysis of the parameters in- 3(a), which is representative of Robertson's [14] data, volved leads to inconsistencies. Abnormally large jump an increase in ambient temperature results in an in- distances seem to be a typical consequence, the appar- crease in I and a decrease in 1 70 ; increased thickness ently "temperature-activated" behavior of V 0 (and a) is

has the opposite effect. According to (11) and (12). 110 difficult to reconcile with the simple model, and V o and should increase with T and Jo should be independent of Jo do not vary with temperature in the required way. 354

112 IEEE TRANSACTIONS ON ELECTRON DEVICES. FEBRUARY 1973

TABLE II

I Thickness Area V a Ti -32 -1.-1 Unit 7 2 (X 22 -3 - I 10 ) (xlO M ) IC is 10 (Vol?s) (,,,) n) (x ) (

8 3.7 0 . 1 15 1.4 1 1.1 3.6 1.8 0.18 17 2.4 9 3.4 3.3 2.3 2 0.8 1.9 0.5 12 6.7 3 0.45 4.8 3.6 6 1.5 0.52 15 12 0.45 2.2 12.0 e. 16 1.7 0.3 22 3.5 0.75 3.0 9.1 3 0.9 0.45 11 12 0.2 2.6 5.9

Nole: All samples have nominal film composition (As,Tes)s,.Si,., Units 1-4 are the same as those listed in Table!.

D. Field-Assisted Emission from Traps: The Poole- Frenkel Effect The Poole-Frenkel effect has been widely used to in- 10 5 terpret the current-voltage characteristics of many E amorphous oxide and polymeric dielectrics [281, [35]. It describes the effect of an applied field on the attrac- Ui tive Coulombic potential between a carrier and an op- 107 positively charged trap from which the carrier has been released. In this case, therefore, the traps are neutral when occupied and correspond to donors (for electrons) zzz or acceptors (for holes). For one-dimensional current I flow, the Poole-Frenkel effect leads to a field dependent o22 10 23 10, 1025 id conductivity of the form [28], [35] N m 3 Fig. 6. Critical field E as a function of trap density in the Poole- /$E"2 \ Frenkel effect calculated for ft = 3 X 1O eV/V" m'. Below E = r, exp (14) the potential maximum is "fixed" midway between traps; only kT above E,, is the position of the maximum affected by the field. See text for explanation (Section 1ll-D). where 2 1) and it varies with the charged trap 32_ . as 0 (i.e.. as ic (13) \T213 7 density as ( 5rj Typical experimental data for a chalcogenide glass The precise value of 0 depends upon the dimensionality are shown in Fig. 7(a) in the form of log G versus of the problem [36] and on the position of the Fermi and the general behavior agrees with that expected for level relative to the various charged and uncharged the Poole-Frenkel mechanism. At constant tempera- traps [37]. ture, lines representing films of different thickness extra- The overlapping of the Coulombic potential between polate to a constant value at V=0 (allowing for small adjacent charged sites must also be considered and this errors due to slightly different electrode areas), and the radically affects the form of the I-V dependence [28]. slope decreases as thickness and temperature increase. The maximum in the combined potential occurs at the A slightly more detailed comparison is less satisfactory, midpoint between traps and it stays essentially fixed, however, as demonstrated in Fig. 7(b) and (c), which independently of field, until some critical field E, is show the variation of slope with temperature and thick- reached. Only above Ecr is the position of the Coulombic ness, respectively. Neither line passes through the origin, barrier affected by the field, and this is a necessary con- and this cannot be accounted for by experimental error. dition for (14). Below E the position of the potential The data of Fig. 7(a) also fit a log G versus V plot barrier is not changed by the field. This is identical with equally well, however, [see Fig. 4(a)],2 and as noted the situation in "hopping conduction" (Section 111-C) above this is consistent with field-assisted emission in and hence, below E, equations of exactly the same form circumstances where the overlapping of the Coulonibic as (10) and (11) are obtained [28]. As before, the "sinh" form of (10) goes over to exponential (7a) for large This emphasizes a major experimental difficulty. There is only voltages. The value of the critical field can be estimated a small voltage range over which appreciable nonohmic behavior can be observed in chalcogenide switching glasses (threshold fields and it is shown in Fig. 6 as a function of the density of are 407 V/rn) and the discrimination between different functional charged sites for 0= 3X 10 eV 'V' m 112 ; Ecr increases I-V characteristics is correspondingly uncertain. 355

OWEN AND ROBERTSON: ELECTRONIC CONDUCTION AND SWITCHING 113

30 310. as_.____o°______

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0' a' 1.0 .5 1.9 1.9 25 2.4 2.1. 2.5 3.0 3.1 3.'. 0 05 (.ELEtTRODE 5ERATlOK,L)" 0)4. (i

barriers is significant. Equation (7a) applies with the chalcogenide glass switches of thickness less than 10 tLm. values of Section 111-B and V0 given by (11). The jump The overlapping Coulombic potential modification to distances derived in Section Ill-B will therefore be ap- the conventional Poole—Frenkel effect does, therefore, plicable and taking 15 nm as a typical value gives the offer an alternative interpretation that overcomes one of concentration of charged traps as 3X10 23 m 3. This is the objections to the simple hopping model (Section probably not an unreasonable figure for chalcogenide Ill-B), but the problem of the apparently temperature- glasses, although rather higher values have sometimes activated jump distance remains. It should also be rec- been quoted [21]. According to Fig. 6, the (exp E"2) ognized that at the charge densities involved, the Cou- Poole—Frenkel behavior would not be expected to de- lomb potentials will probably be considerably. -modified velop until fields approaching 107 \1 /m were reached. by screening (overall charge neutrality is required) and This is the magnitude of the threshold field for typical this will affect the shape of the barriers, and hence the 356

114 IEEE TRANSACTIONS ON ELECTRON DEVICES, FEBRUARY 1973

0.45 r--

- 0.40.._.

- 0.35

0 - 0.30 ~ . 0 0 02 ' a, + L0 - .20 3 ± 50 0 0 0:10

neofsiope04,,/",- 0.05 I I 0 0-2 04 0.6 0-8100 (E/Eo)

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cc.cuc,iviyy (OR cucms.ce) DAT A r 0â Go 1b I — (om'm') (ohm') I(sl0*'v/ml 0 A 25.3 0 2965 Bold " 116 0 / x 129 23zI 042 V 101 2xlO30 3 (AsTs,) 5 Si0 320 6oIl 103 [j

— UOIILJIVTA 1 °K p (o - (uS /vs) (xI0'ynl + 208 4.6 1101 I•95 S. 3gJ )( 169 1116 8 1.16)

0 1 2 3 4 5 6 7 8 9 (E/Eo)

Fig. 8. The dependence of reduced conductivity (a/oo) or mobility (;//o) on reduced field (E, E0) for several typical chalcogenide glasses (see Section III-E).

predicted 1—V characteristics. Vet another difficulty, ent at low temperatures (200 K), but in As 2Sea [17], and one which raises more general problems, is that the [30], for example, it is observed at room temperature or (I/V)—V, characteristics do not show the ex- and above. In all cases the break from the low to high pected change from exponential to sinh behavior when slope region occurs at a field value of about 1-5X10 7 (V/ V 0) <1. This is discussed more fulls' in the following V. m— I with only small variations with temperature. section. The major problem in explaining the lower field region is that it extends to smaller (V/ V 0) [or (E/E0)] ratios E. Comments and Discussion than expected for any of the simple variants of hopping Electronic processes certainly make an important con- conduction. This is illustrated in Fig. 8 where (loo) is tribution to the nonohmic conductivity of chalcogenide plotted versus (E/E0) for a variety of data. The ex- glasses in the preswitching region, but none of the com- ponential field dependence is followed down to (E/E0) mon models give an adequate description, at least in ratios at least 0.05 and probably 0.01. Beyond this it their elementary forms. becomes an extremely difficult matter to distinguish ex- The principal experimental features seem to be two perimentally between ohmic and nonohmic conduction exponential I—V or G—V regions [i.e., equations (8a) or and it is a moot point whether or not an ohmic charac- (8b)], the second region at higher fields having a higher teristic is ever reached. De Wit and Crevecoeur [ii] slope (lower V 0 value). In the more conducting complex imply that it is not. This behavior, although apparent compositions used for switching [Te, 49 percent; As, in some of the published data, does not appear to have 33 percent; Ge, 6 percent; Si, 3 percent; Ga, 9 per- been explicitly recognized until the recent work of de cent (all in weight percent)] [11b]; (As4. 5T33.Ge [211; Wit and Crevecoeur [17] and Marshall and Owen [ 38 ]. GeiaTes .s [38]). the high-field region only becomes appar- The former authors briefly speculate that their results 357

OWEN AND ROBERTSON: ELECTRONIC CONDUCTION AND SWITCHING

cally used for switching devices the latter effects only on As~.,Se3 may be explained by potential fluctuations with a wavelength greater than some characteristic come into play at temperatures below about 250 K; at "jump distance" related to E. Marshall and Owen also higher temperatures switching occurs before the appro- find that the same exponential field dependence is ob- priate fields are reached [111) J, [21]. served in the trap-limited hole drift mobility in vitreous At the present time, models and theories of high-field Se down to surprisingly low fields [38] and typical re- phenomena are not developed well enough to apply con- sults are included in Fig. 8. It would be premature to fidently to the even more complex switching process. All ascribe this seemingly general behavior to a common that can be done is to speculate on the likely connection mechanism. The Ge15Te and (As2Tea) s.6Sii .s glasses are between the behavior in the preswitching region and the probably phase separated, for example, and this may actual threshold conditions. affect their field-dependent properties. The Circumstan- tial evidence is strong, however, and Marshall and Owen IV. THRESHOLD CONDITIONS discuss the consequences in qualitative terms. The same A. Experimental Features field dependence of the conductivity and trap-limited As mentioned previously, the experimental data on mobility means that the field is not affecting the free switching are reviewed by Warren [1] and relevant carrier concentration n. The other relevant parameters references can also be found there. For the purposes of that may be affected are the trapping time r, (involving the present discussion, however, the main experimental the capture cross section), the thermal release time r,, facts concerning the threshold conditions are sum- and the free mobility A o. Marshall and Owen eliminate marized below. It must be remembered at all Limes that the the first two and conclude, therefore, that the field de- concern here is with devices less than about 10 &m thick. In pendence of both conductivity and trap-limited mobility thicker films, and particularly for thicknesses greater enters through g o, although as yet there is no direct ex- than 100 &m, the switching process is almost certainly perimental proof of this. They suggest, tentatively, that thermally induced and some of the features listed below the field increases zo by increasing the percolation prob- do not apply. The controversy is really about the ap- ability of carriers in the "diffusive" extended states at parently different behavior of "thin" (<10 Am) devices. the bandedge and also, possibly, by increasing the over- The threshold voltage Vh is a slowly decreasing lap of the wave functions of such states. function of ambient temperature. Very approximately The apparent similarity in behavior of conductivity V 1ha(1/T), V h a(—T),or V h VhO (1bT), where b and mobility extends to the breakaway to an enhanced is a constant. field dependence at fields in the region of 1-5 X 107 V/rn, The threshold voltage is proportional to thickness and this is also indicated in Fig. 8 [the breakaway d, i.e., the switching field is essentially independent of different (E/E 0) values be- occurs. in this diagram, at thickness for d10 Am. The magnitude of threshold Ec varies]. This higher field region can only be ob- cause field is typically 8X10 6-2X107 V/rn. served over a limited range as it is quickly followed by decreases exponentially with switching or breakdown. Marshall and Owen argue that The delay time td a exp — V), but tends to this probably represents the onset of hot carrier phe- pulse amplitude T'. i.e., 'd ( For overvoltages (V— ITth) of a few nomena which, they suggest, will start at fields of the infinity as V--Vb. volts, La is typically 0.1-1 As. If the applied voltage is order of removed during the delay time the switch returns to the /kT) h I2 (16) OFF state. \2e o The delay time is followed by the instability leading to the ON state. The switching time 1. is of the order where v is the frequency of the energy loss process. A similar equation has been proposed by Mott [13]. The of 100 Ps. free mobility io is typically 3 X 10 m 2 V's in amor- In the ON state the evidence is that the current phous chalcogenide semiconductors [38] and substitut- flows in a narrow channel or filament. For a threshold and current A ing this value with v-..v,j-10 13 s' gives switch a minimum holding voltage VA must be maintained, otherwise the device returns to the £ - 3 X 10 V/rn. OFF state. The ON state will be discussed in Section V. Clearly there is a need for a much more thorough in- A number of electronic processes have been suggested vestigation and understanding of high-field phenomena, as applicable to the threshold behavior, and the ON state, conductivity, and mobility in chalcogenide glasses. of chalcogenide glasses, but none has been developed to Any of the mechanisms discussed in the previous the stage where quantitative calculations and predic- paragraphs could be very significant in electronic switch- tions can be made [13], [39]—[41]. Generally speaking, ing processes and hot carrier phenomena especially, however, the suggested models make few, if any, con- leading probably to impact ionization and avalanching, cessions to the amorphous nature of the material, and could promote electrical instabilities. It must be they usually draw heavily on concepts familiar in con- remembered. however, that in the compositions tvpi- ventional semiconductor physics. The chalcogenide glass 358

116 IEEE TRANSACTIONS ON ELECTRON DEVICES. FEBRUARY 1973 is often regarded as a typical semiconductor modified only by the presence of a larger than normal density of Anode traps and/6r recombination centers—a view which, in the present circumstances and context, may be quite MobIity L. Anode tt. gap justifiable. The most fruitful appräch at the moment. Cathode 4, (a) therefore, is to review the suggested mechanisms and, - CcthoG- . where it is relevant, to appeal to related data and theories developed for crystalline semiconductors. d) B. Carrier Injection Mechanisms The electronic process so far having most appeal— FL. probably because it is basically simple to visualize is n7 gap double injection of both carriers. Excess electrons are PT awelwa-, injected from the cathode, holes from the anode, forming a negative and positive space charge at the cathode and anode, respectively. The space-charge clouds build (c) (e) up from the electrodes and when they overlap neutralize Fig. 9. Schematic illustration of the switching sequence in one of each other, causing the field in the interior to collapse the "injection" models (following [411). See text for explanation (Section IV-13 and Section V-B). and the same, or larger, current can flow with a lower applied voltage, i.e., theçe is a negative-resistance characteristic. A qualitative explanation along the above ally they meet and overlap—Fig. 9(c). The situation lines was first explicitly suggested by Henisch [39] and then rapidly becomes unstable. With recombination Henisch et at. [40]. A variation has recently been pro- centers filled, the carrier lifetimes, and hence the elec- posed, in slightly more quantitative teriiis, by Lucas tron and hole conductivities, increase. In the region of [41]. Mott [42] has also suggested a related process to overlapping space charge the material is neutral; the describe the situation in the ON state; this will be men- conductivity in the center increases and the field de- tioned in Section V. creases while the field near the electrodes increases A schematic representation of Lucas' [41] model, Fig. 9(d). Electrons and holes are accelerated rapidly freely interpreted, is shown in Fig. 9. The vertical axis across the neutral region and because of the increased in this diagram is not drawn to scale, nor is the scale in- field, injection of carriers at the electrodes increases. tended to be consistent in the various parts. The as- Both effects increase the rate at which space-charge sumption is that the 'tails" of localized states extending overlap occurs. i.e., there is positive feedback and hence from the conduction and valence bands overlap some- an unstable situation. The stable state, corresponding to where near the center of the energy gap (or mobility the ON state, is shown in Fig. 9(e). Here, the space gap). This provides a completely compensated set of charge has spread right across the diode and the bands positively and negatively charged states above and be- are practically flat. Shottky-type barriers are established low the Fermi level, respectively, with the Fermi level at the electrodes, but because of the high density of pinned at or near the center of the gap. These features traps (the CFO model [43]) they are thin (about 1 nm) are inherent in the Cohen–Fritzsche–Ovshinsky (CFO) and electrons can easily tunnel through from the Fermi [43] model of the electronic-band structure of a chalco- level of the metal into the conduction band of the chal- genide glass. cogenide glass and, similarly, holes from the anode tun- For simplicity in Fig. 9, the charged states on either nel into the valence band. Because the space charge has side of the Fermi level are denoted by a single row of been neutralized and the traps filled, the conductivity positive and negative signs. The injected electrons and in the device is now high (the mobility is no longer holes will recombine with and neutralize the positively limited by trapping). The minimum voltage required to and negatively charged states setting up a negative and keep the ON state stable (i.e., the holding voltage Vh) is positive space charge, Fig. 9(a), (b), adjacent to the an- approximately equal, to, or a little greater than, the ode and cathode, respectively. Charge will also be mobility gap of the chalcogenide glass. Further con- trapped in the neutral localized states, of course (not sideration will be given to this point in Section V. shown in Fig. 9), but Lucas ignores this. Certainly one The model of Henisch [39] and Henisch el at. [40] is would expect that recombination via the charged states similar except that the charge is trapped in the neutral would be the dominant process in establishing the space localized states of the "tail" adjacent to the electrodes charge, at least initially. The regions of space charge at which they are injected. Thus a negative space charge will limit the current flow in the vicinity of the elec- again develops at the cathode and a positive space trodes and the field will be redistributed—decreasing charge at the anode. near the electrodes and increasing in the center—Fig. Apart from the !nagnitude of the holding voltage Vh , 9(b). As the applied voltage is increased more charge is no quantitative predictions have been made on the basis injected and the space-charge regions grow until eventu- of the Henisch–Fagen–Ovshinskv model, which can be 359

.:OVEN AND ROBERTSON: ELECTRONIC CONDUCTION AND SWITCHING compared with the basic experimental features outlined NA, respectively [46]. This is exactly equivalent to in Section IV-A. Lucas [41] has made a semiquantita- Lucas' [41 ] model for the chalcogenide glasses, de- tive analysis of the model outlined in Fig. 9, which she scribed earlier. Wright and Ibrahirn [46] show by a very describes as a recombination instability, and finds that simple and approximate analysis, which nevertheless gives good agreement with experiment (on their Si the critical current j,, at which the high-resistance state collapses [i.e., corresponding to Fig. 9(c)] is given by p+i..n+ structure), that d2eiYAND 2LeN — V 2h = . (19) (li) icr = 2K(NA + -VD) T,I The d2 dependence of Vth still obtains, as one would ex- is the diffusion length (a function of carrier where L, pect for any model of this kind. The donor and acceptor N is the density of lifetime and hence of injection level), concentrations will be approximately the same; in fact, recombination centers, and r is the relaxation time of in the present context [41] they will be equal: NA = No excess charge. 3 A complete quantitative analysis of these = N. Using a value of N = 1026 m 3 [43], inserting d = 1 charge injection models of switching, relevant in all its Am as a typical device thickness, and taking the relative particulars to chalcogenide glasses, would of course be a dielectric constant to be 10 gives V5=4X10 3 V, i.e., formidable task, but there are similarities with the es- —4X109 \'/m for the threshold field! This is two orders tablished ideas of double-injection space-charge-current of magnitude, or more, greater than threshold voltages problems in conventional semiconductors [44] and cer- observed in chalcogenide glasses (Section IV-A), but tain comparisons may be profitable. Even here, however, clearly could be made to agree by taking a smaller value the only case leading to negative resistance (switching) for N ('—'10 rn-3). A smaller N also leads to difficulties, that has been analyzed in any detail is that of a semi- however [41], and one is still left with the prediction of of deep traps (recombination conductor with a single set a threshold field that increases with thickness in contrast centers). One feature that should nevertheless be gen- to the experimentally observed constant field. erally applicable is that the threshold voltage is a func- In its simplest interpretation the delay time t is the thickness (electrode separation) d. This tion of the square of time required to fill (or half fill) all the traps and/or is because in all space-charge dominated transport pro- recombination centers, i.e., to reach the condition repre- cesses (single or double carrier), the transition from one sented by Fig. 9(c) where the space-charge regions are regime to another is determined by the transit time of just beginning to overlap. For overvoltages (V— I7th) of the appropriate carrier becoming equal, at say, Vth, to a few volts, td is about 1 us. Again using a value of 1025 its lifetime r, i.e., [44], m 3 for the density of recombination centers, the current required to inject the equivalent amount of charge dur- (18) ing ti (1 s) is approximately 10 9 A/m! This should cor- VehM fiAT respond to Lucas' critical current jc, and again a dis- where 1A is the effective carrier mobility. Thus in this crepancy of several orders of magnitude is found. Lucas simple case, the threshold field increases with thickness, [41] remarks on the same problem but in slightly differ- which is not the relation observed in switching in chalco- ent terms. One could, of course, conclude that N was genide glasses (Section IV-A). One could conjecture much less, but as Lucas notes this would adversely about the temperature dependence of the threshold affect other features of the model, viz., short-screening voltage given by (18), but this could not be realistically lengths at the electrodes. Incomplete filling of the traps applied in the present context since the equation is sig- and/or recombination centers could be assumed, but nificantly modified by, for example, incomplete occupa- carried over into the ON state this would imply a trap- tion of the recombination centers at thermal equilibrium limited mobility and probably also a space-charge- (leading to a space-charge limitation on the prebreak- limited ON-state current, contrary to observations (Sec- down current [45a]) and by the effects of shallow tion V). A third alternative, mentioned by Lucas, is that trapping [45b]. The thickness dependence remains un- the current-carrying area is contracting (i.e., filament affected, however. formation) well before space-charge overlap occurs. The most directly comparable situation to have re- There seems to be little evidence at the moment for or ceived attention in conventional semiconductors seems against filament formation in the prebreakdown region, to be the pi n+ structure in silicon containing deep- but it is relevant to note that according to Vogel and donor (hole recombination centers) and deep-acceptor Walsh [47] the capacitance of a chalcogenide glass (electron recombination) levels with densities No and switch retains its geometric value up to (V/V5)'O.75- 0.80. In As2Se3 , deviations (negative) from the geo- Taking N= 10--21 m 3 as a typical value [43] and 7_1 = 10 -1 metric capacitance also occur, but even closer to the Lucas obtainsj 104 Aim2, which compares reasonably with experi- breakdown voltage, e.g., (V/Vh)—'0.95 [30]. Further- mental values. Her choice of TreI does not seem physically reasonable, however; it is likely to be one or two orders of ten smaller. In addi- more, above some critical frequency the small signal tion, a low value of L appears to have been used. As a result j,, capacitance remains constant as a function of bias estimated from (17) is likely to be several orders higher than the value quoted by Lucas, still taking V= lO m 3. virtually to T'th [47]. [48]. 360

118 IEEE TRANSACTIONS ON ELECTRON DEvicEs. FEBRUARY 1073

The most cogent evidence against charge injection phonon energy transfer near the energy maximum, how- models for switching is the absence of polarity effects, ever, and this implies some internal temperature in- and this has been widely debated at conferences and crease. Thus thermal effects are usually also linked with elsewhere. Shanks [49] and Balberg [50] have reported collective breakdown. that the delay time Id is unaffected by reversals of the Stratton [3] has calculated that the electron density polarity of the biasing pulse. Henisch and Pryor [51 I necessary for collective breakdown is 10 1-101 times have recently carefully repeated this type of experiment greater than the low-field electron density. In chalco- and they do find some polarity effects, particularly at genide glasses the presvitching I—V characteristics low temperatures (-200 K). The effects are not of suffi- imply that the maximum electron density is at the most cient magnitude, however, to suggest that the whole of a 102 times the low-field value, so collective breakdown is space-charge pattern has to be nullified and reestab- unlikely to be important in the initiation of switching. lished in the opposite direction on reversal of polarity; 2) Impact Ionization and Avalanche Breakdown: If i.e.. if the polarity is reversed at, say, 0.5 Id, the resultant the electron density is low, a high-energy electron may total delay time is greater than the original Id but is collide with an atom, instead of another electron, and rather less than 1.5 td. Nevertheless, these experiments ionize it producing a hole and two low-energy electrons. do suggest that at low temperatures some charge-injec- The two electrons will, in turn, be accelerated to high tion processes may have a more dominant role. This energies and ionize more atoms, thus the process can could be connected with the increased field dependence cascade to cause an electron avalanche. This process is, of the preswitching current observed in the "high-field" of course, well known in dielectric breakdown and has region and described in Section Ill-A, E; in typical been discussed in detail by many authors (see, for in- switching compositions this also becomes apparent only stance, [2] and [531). Impact ionization, or any other at temperatures ! ~ 200 K [ii], [21]. hot-electron effect, is not in itself sufficient, however, to produce negative resistance. As mentioned before, a C. Hot-Electron Effects positive feedback mechanism is required. Crandall [53a] A variety of effects related to hot-electron phenomena has investigated the conditions for CCNR and shows and dielectric breakdown could lead to the initiation of that a possible feedback mechanism is the increased an instability, negative resistance, and switching of the screening of the scattering potentials due to the in- form observed in chalcogenide glasses. Once more, how- creased carrier density. The electron scattering de- ever, there are no well-developed theories readily ap- creases, leading to an increase in the average electron plicable in the present context and it is possible only to energy. The hotter electrons cause more impact ioniza- speculate rather qualitatively. This section will, there- tion, generating more carriers and making the distribu- fore. be brief and limited to a discussion of two manifes- tion still hotter. Hence, a higher rate of impact ioniza- tations of hot-electron effects that are well understood tion can be sustained at a value of the electric field that in other contexts and provide some experimental and/or is lower than that required for impact ionization at low- theoretical data for comparison. It must be emphasized, carrier density. however, that such evidence as there is (see Section III One of the most important applications of avalanche and [38]) suggests that the onset of hot-electron effects breakdown is to p-n junctions. It provides a limit to in chalcogenide glasses occurs at fields > 10 V/rn, prob- diode reverse bias voltages or to transistor collector ably at least 3-5 X 10' V/m.These figures agree with Hind- voltages and has, therefore, been studied extensively. lev's theoretical estimates based on the random-phase There are several important characteristics of avalanche approximation [52a]. In typical chalcogenide glass breakdown in p-n junctions that are probably general switches, the threshold field at room temperature is usu- features of the phenomenon. ally < 10 V/rn. Thus hot-electron effects are unlikely The breakdown is localized and the small re- to be of first-order importance except in pulse experi- gions of avalanche are described as microplasmas. Light ments at substantial overvoltages, or at low temperatures. is emitted due to electron-hole recombination so the 1) Collective Breakdown: When the electron density spatial location and density of the microplasmas can be and the electron—electron collision rate is sufficiently studied, in p-n junctions at least [53b]. The microplas- high, the rate of energy exchange due to electron—elec- mas are thought to form at high-field spots associated, tron collisions is greater than that due to electron— for example, with crystalline defects, inclusions, non- phonon interactions. The electron energy-distribution uniformities in impurity concentration, or other in- function is Maxwellian with a mean temperature T homogeneities [54]. above the ambient temperature. The situation has a Rose [55] estimated that the temperature rise strong analogy with thermal breakdown, but in this in a microplasma was in the range of 15-40 K. A more case it is the steady-state electron temperature that rises recent treatment by Martirosov [56] puts it at —400K until at a critical value depending on current density. Equilibrium is no longer possible and breakdown The formation of microplasmas is a statistical occurs. There is always some increase in the electron-- process and this has two important consequences. 361

WEN AND ROBERTSON: ELECTRONIC CONDUCTION .ND SWITCHING No

If a high-voltage is applied, there is a time de- chalcogenide glasses which, so far as switching is con- lay before any avalanche occurs, but this time lag ap- cerned, are usually regarded as "typical" semiconduc- pears to be entirely statistical and not a parameter tors with a more or less large density of traps and/or unique to a given set of experimental conditions. Thus recombination centers [39]-[41]. in avalanche breakdown, the delay time can only be V. THE ON STATE defined statistically [57]. This contrasts with the situation as normally reported A. Experimental Features of the ON Slate in chalcogenide switches where it is usually implied that, There is a real lack of hard experimental data on the except possibly at small overvoltages, the delay time is ON-state parameters. Much of what has been published a uniquely defined parameter. Very recently, however, is essentially concerned with the same type of device Lee and Henisch have reported on significant statistical using basically the same composition, viz., the Ovonic variations of threshold switching in chalcogenide glasses threshold switch (OTS) of Energy Conversion Devices, [58]. According to Nield and Leck's [57] experiments, Inc., in a "DO-7 package" (see [6], [ 40 ] , [49], [31], the delay time for avalanche breakdown in silicon de- [62a], [62b]). It is difficult to be sure, therefore, creases approximately inversely with voltage. This is whether the data are typical or peculiar to that particu- rather slower than the variation observed in chalco- lar form of device. Neale's paper [62b] does give some genide glass switches (Section IV-A). information on switches in the form of a simple sand- The on-off fluctuations of microplasmas at wich of metal-chalcogenide-metal with crossover elec- fields below the overall breakdown field result in a very trodes and also on devices fabricated in the "pore-type" characteristic form of high-level current noise [59]. structure .4 Recent work often utilizes one or other of Current noise has been measured in As 2Sez glass at fields these structures [14], [631-[68], but again similar com- Of up to 0.5 X 10 V/rn with no evidence of any avalanche positions are usually investigated. In general, however, microplasma features [60], although preliminary mea- the main experimental facts are as follows. surements on a more conducting glass composed of In the ON state the dynamic resistance is essen- does As-Te-Ge-Si have indicated that the current noise tially zero (the I-V characteristic is practically vertical), This increase considerably at voltages close to Vth. but sometimes a small region of negative resistance is could be due to localized avalanche breakdown in re- observed as the device switches off. The conductance at gions of the sample where the field is much greater than the critical point (V,,, I) is usually about 10-3 ç -i the average value, but there are also other possible The holding voltage Vh is substantially indepen- causes. dent of temperature and thickness. The latter is taken d) An increase in ambient temperature reduces the as evidence that most of the holding voltage is dropped mean free path between electron-phonon collisions; near the electrodes and that there is only a small field electrons lose more energy to the lattice and a higher across the bulk of the device. field is required to produce an avalanche. This trend has The magnitude of 175 is often in the region of 1-2 been confirmed experimentally for avalanche breakdown V, but there are exceptions. Table III lists results from in abrupt silicon junctions where it is found that Vh a variety of sources showing that holding voltages as increases approximately linearly with temperature [61]. high as 20 V have been reported [64]. This seems to de- This contrasts with the decreasing threshold voltage in pend on electrode material, but there have been few, if chalcogenide glass switches. any, systematic studies on this point. It should be noted The conclusion is, therefore, that experimental and that all of the results in Table III, except the last entry, theoretical evidence exists for hot-electron effects, most refer to thin-film devices (<10 gm). Vt , in chalco- likely electron avalanching, at voltages Z The holding current I,, is relatively insensitive to genide glasses, and such effects may be present espe- temperature, usually decreasing slightly as temperature cially in pulse experiments with amplitudes somewhat increases. It is also independent of the electrode area. Comparisons with well-known ex- greater than Vth. These two points are taken to show that in the ON state, amples suggest, however, that they are not the domi- the current is carried in a filament of a semimetallic or nant mechanism giving rise to switching. Electron ava- degenerate-semiconductor nature. could also be significant in the lanching at the electrodes Pearson and ?vliller [70] and \Veirauch [71] have re- initiation process if appreciable internal self-heating ported evidence for conducting filaments on the ON state occurs, causing enhancement of the fields at the elec- from consequential thermal effects observable on the trodes [13], [14]. It is, of course, debatable to what ex- electrodes (i.e., after passing high currents). Uttecht tent such comparisons, e.g., with avalanche phenomena et al. [72] and Sie [73] have observed filament forma- in p-n junctions, are valid, but in the absence of defini- tive theory specific to transport in chalcogenide glasses 'The simple sandwich structure is often convenient for studying a comparative approach does at least offer some guide the preswitching region and perhaps even the threshold conditions, but it is not suitable for investigations of reversible switching. The to speculation and conjecture. It is also in the spirit of "pore" structure is designed to minimize field inhomogenemties and the current models of the electronic band structure of to reduce the device capacitance (62b].

362

12() IEEE TRANSACTIONS ON ELECTRON DEVICES, FEBRUARY 1973

TABLE Ill

Vh Ih "h 1h Electrodes Reference (Volta) t- PULFROM UL TEIC

1.3 0.6 0.8 C (49) VOL.Tft 3 0.3 0.9 INTO (62) ACROS9 OEVICC 12 0.1 1.2 No (62) 2.5 1 Au (69)

4 - - Au (69)

7. - - Au (69) 20- -7 -0.3 -2 Pt 2.8 0.4 1.1 Cu CURRENT ON•04AR.asa,s-rlc (A) 3.0 0.4 1.2 Brass (67) 1.8 0.3 0.6 V (67) 1.3 1.9 2.5 C (67) 10 20 0.06 1.2 Si (67) 1-3 1 1-2 Au, No (14) la (3.4)8 10 a 01 8 Ag a

These figures are for a typical thick" device, i.e., thickness cwARACTMamc .-...100 Am. F4 PUI.SE A$UMV.&1E O - -1 Note: The value of V5 has been taken at Is. 0 VOLTAGE (voI.Ts) tion, directly, in the surface of a chalcogenide glass switch (with planar electrodes). To the authors' knowl- Fig. 10. (a) Pulse form used to investigate transient ON state characteristics (from [611). (b) Typical transient ON state character- edge, however, there is no direct evidence for filament istic (from loll). formation in circumstances relevant to threshold switching, although, as will be mentioned later, it can he in- ferred. It follows that there is no experimentalinforma- transient ON characteristic (TONC) Fig. 10(a). Pro- tion on the temperature dependence of the radius of any vided that t, was short (0.1 ss), an extended ON-state filament, and hence conclusions about the electronic characteristic could be measured Fig. 10(b). This very nature of the conducting filament from the temperature fast transient response was explained in terms of the dependence of the ON-state current are speculative. capture of free carriers. Ridlev's [74] thermodynamic arguments are often When t, is increased beyond 1 s, gradual recovery of cited in the present context as support for the idea of the off-state features (1 7th, td, Rat) occurs. This is the current-filament formation. It should be recognized, more familiar recovery from a switching event and may however, that Ridley's model for S type (current-con- be expected to occur when the recombination rate of trolled) negative resistance predicts that a current fila- carriers exceeds their injection rate. On Prior and ment starts to form before the threshold voltage is Henisch's model this recovery is characterized by the reached and that the filament area grows as current in- recombination time of nonequilibrium carriers in traps. creases until, at the end of the negative-resistance region, it has expanded to fill the whole electrode area (or, B. Discussion at any rate occupies the same area as the prefilament In the injection mechanisms considered in Section current). IV-B, the ON state corresponds to Fig. 9(c). This dia- The holding current I,, is typically 0.1-1 mA (see gram is appropriate either to Lucas' [41] model or to Table III). The ON-state current can increase, at virtu- that of Henisch, Fagen, and Ovshinskv [39], [40]. \lott ally constant voltage, to several tens of milliamperes; [13], [42], [75] has also proposed the same band model under pulsed conditions, ON-state currents of up to 1 A for the ON-state without necessarily requiring that it is have been reported [49]. established by the same sort of injection process. Elec- The most useful technique so far established for trons and holes tunnel through the thin (e.g., about 5 examining ON-state behavior is that described by Prior nm) potential barriers at the electrodes directly into and Henisch [62a]. They applied a voltage pulse suffi- the conduction and valence bands and it is assumed that ciently large to switch a device and then superimposed a all traps and/or recombination centers in the mobility smaller pulse in the ON-state region to measure the gap are full. As'Mott [13] points out, the presence of I*I

OVEN AND ROI3ERTSON ELECTRONIC CONDUCTION AND SWITCHING deep traps is necessary in order to make the barriers conditions may be mainly controlled by thermal effects thin enough to be compatible with tunnelling (also see (though this need not apply to the rest of the ON state). Rose [76]). Except near the barriers [ 13 ], however, all VI. Sc.tlARv AND CoNcLusioNs traps are full and hence the oN-state Current is not trap or space-charge limited [13] [see Section V-A4)} and There is no doubt that there are significant electronic the quasi-Fermi levels for electrons and holes are above effects causing nonohmic conduction in the preswitching and below, respectively, the edges of the mobility gap. region of chalcogenide glass switches. The I- V charac- Hence the ON-state conductivity is high and there is teristics in the preswitching region can be fitted, super- only a small potential dropped across the bulk of the ficially, to simple models of space-charge limited cur- device. The holding voltage Vh is. therefore, the poten- rent, hopping conduction, or the Poole-Frenkel effect, tial required to sustain the quasi-Fermi levels at the but on the present evidence, at any rate, a closer ex- positions shown in Fig. 9(e), i.e., V h is approximately amination reveals inconsistencies in each case (Section equal to or a little greater than the mobility gap. This is III-B-D). There is, in fact, growing evidence that the the only quantitative prediction made by the present nonohmic effects in chalcogenide glasses could extend electronic models of switching. The mobility gap in the down to essentially zero fields and that a fundamentally type of chalcogenide glass used for threshold switches new approach may be required (Section III-E). There is is typically 1-1.3 eV and V h is often close to this but, also evidence of some undetermined hot-electron effects as shown in Table III, there are exceptions. contributing to the nonohmic behavior at "high" fields, An estimate of the filament diameter can be obtained but these are unlikely to be significant in the chalco- on the assumption that Fig. 9(e) does represent the genide glasses most commonly used for switching de- ON-state. Following Mott [13] we assume a carrier vices except perhaps in pulse experiments well above density of 10 m 3 and a mobility of 10 m 2/V.s, giv- the threshold voltage or in cases where there is a very ing an ON-state conductivity (o = neM) of approximately nonuniform field distribution across the device [Section 105 9-1 m'. Also assume [13] that the voltage across the III-E and Section IV-C2)]. Thus at the present time bulk of the device is about 0.1 V, i.e., a field of 10 V/m it is not possible to reach any firm conclusions about the taking 1 Am as a typical device thickness. The holding electronic mechanism(s) of the preswitching current. current is generally about I mA and hence, from the It is hardly surprising, therefore, that attempts to conductivity, the filament radius is in the region of account for the actual threshold conditions and switch- 0.3-0.4 Arn. There is evidence that the current density ing process in purely electronic terms are bound to be in the ON-state is constant [51], [62a]. With a current qualitative and speculative. This is not to question the of 50 mA the filament diameter would therefore increase fact that there is plenty of scope, in the chalcogenide to about 3 Mm; the maximum ON-state current quoted glasses, for electronic instabilities, but present theories for pulsed operation is about 1 A and, under the same are such that only the crudest quantitative comparisons assumptions, this would correspond to a filament di- can be made and these are only partially successful ameter of about 11 zm. These figures are to be com- (Section IV-B, Q. In the present circumstances the pared with the diameter of the total active area. For most useful approach seems to be to compare wherever switches constructed in the "DO-7" package, Neale's possible, the functional dependence of parameters such [62] figures give a maximum diameter of 8-10 itm; as the threshold voltage and delay time on, for example, "pore-type" structures usually have an active diameter temperature, field, and geometry with the predictions of about 20-50 sum. of related models for crystalline semiconductors where One problem always associated with small filament such comparisons are possible that they suggest that of size is the implication of very large power densities. For the most likely electronic mechanisms, none are actually a filament 1 Am long and 0.3-0.4 Am diam, a minimum dominant in the switching process (Section IV-B, Q. holding condition of 1 V, 1 mA gives a power density of To conclude, we draw attention again to Section II about 1016 W . m 3. This power may only be applied for and to the final comment of Section V-B. These empha- a short time period (e.g., 1 Ms) but even if 90 percent size the difficulty, in materials like the chalcogenide of the energy is dissipated out with the filament, the glasses, of disentangling electronic and thermal mech- remainder is sufficient to raise the filament temperature anisms and the inevitability of some thermal effects. As by several hundred degrees. This emphasizes one of the mentioned earlier, our view is that there must be some biggest problems of interpretation of experimental data: convergence of the two approaches. Indeed, even in in addition to electronic relaxation processes during purely electronic terms there is no reason why there recovery from a switching event, there are also likely should not be more than one contributory mechanism to be cooling processes with short (several micro- with different processes being dominant in different seconds) thermal time constants. Indeed, the relatively circumstances. It is only necessary to cite the closely constant values of minimum holding power (Ph = VhIh) related subject of dielectric breakdown [2], [3] to shown in Table III suggest that the minimum holding illustrate a phenomenon in which several mechanisms 364

122 IEEE rRANSACTIUNS ON ELECTRON DEVICES, FEBRUARY 1973 can compete and where the dominant process can [31] J. E. Hall, iii (8. p. 1251. 1321 S. Frank and A. E. Owen, private communication: results to be change drastically as a function of, for example, tem- published. perature and/or geometry. In the present subject there [331 H. Fritzsche. J. .Von-Crvst. Solids. vol. 6. p.49. 1971. B. Baglev, Solid Slate Common., vol. 8. p. 345. 1970. was, at one time, a tendency for unique causes exclusive R. M. Hill. Thin Solid Films, vol. 1. p.39. 1967. of all others to be propounded. Happily, that phase [36] J. L. Hartke, J. .4ppl. Phys., vol. 39, p.4871. 1968. J. G. Simmons, Phys. Rev., vol. 153, p.657. 1967. seems to have passed. J. Marshall and A. E. Owen. Phys. Slatus Solidi (a), vol. 12. p. 181. 1972. ACKNOWLEDGMENT [39] H. K. Henisch. .Sci. Amer., vol. 221, p.30. 1969. H. K. Her,isch, E. A. Fagen. and S. R. Ovshinskv, in (0. p. 3381. The authors wish to thank their colleague. Dr. J. M. 1. 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Viskakas, P. Mackus, and A. Smilga, Phys. Status Solidi, vol. A. D. Pearson and G. E. Miller, App!. Phys. Lett., vol. 14, p. 25, p.331, 1968. 280, 1969. [26) C. Vautier, D. Charles, and A. Colombani, Thin Solid Films, D. F. \Veirauch. App!. Phys. Lett., vol. 16, p. 72, 1970. vol. 3, p. 293, 1969. R. Uttecht et al., in 18, p. 3581. 127) L. Muller and M. Muller, in [9, p. 5041. [73] C. H. Sie, in [9. p. 3481. See, for example. R. M. Hill, Phil. Meg., vol.23, p.59. 1971. [74) B. K. Ridley, Proc. Phys. Soc. London, vol. 82. p. 954, 1963. B. T. Kolomiets and E. A. Lebedev, Sos. Phys. Semicond., vol. 1751 N. F. Mott, Festkorperproleme, vol. 9, p. 22. 1969. 1, p. 678, 1967. [76] A. Rose, Ber. Tag. mt. Prob. Mod. Get reidetchnol. Geireidechem., T. J. Renouf, private communication. vol. 29, p. 199, 1956.

365

REVUE OR PHYSIQUE APPLIQUft TOME 1 3 . DECEMRRE 19-1 S. PSUE 767

VARIOUS DEVICES.

THE CHARACTERISATION OF METAL-THIN INSULATOR-n-p 4 SILICON SWITCHING DEVICES J. BUXO. A. E. OWEN (), G. SARRABAYROUSE and J. P. SEBAA Laboratoire d'Automatique et d'Analyse des Systâmes du C.r4R.S.. 7, avenue du Colonel-Roche, 31400 Toulouse, France

Résumé. - Lea auteurs presentent des résultats experimentaux sur lea caractéristiqum de commu- tation des dispositifs MISS comportant une couche mince d'oxyde (< 50 A) lea rilsultats portent notamment sur lea effets thertniques. Is modulation apportée par un courant de base et enfin lea effets dynamiques. Lanalyse physique eat conduite 6 laide dun modéle i contre reaction doist lea ClCmenis sont decriEs. Un bon accord thCorie.expérience cot obtenu. Abstract. - Experiments are reported on the switching characteristics of MISS devices incorporating a thin (< 50 A) oxide layer, including the influence of a modulating base current, the effect of temperature and the dynamic performance. A quantitative analysis of a regenerative model of switching is briefly described and shown to give a good account of the experimental results.

I. Introduction. - There have been several recent hitherto on the essential characteristics of MISS reports of potentially useful bistable switching pro- switches, particularly on their temperature depers- perties in metal-insulator'n-p 4 (or ,p.nR) structures dence, the influence of a modulating base current on silicon (i.e. MISS devices) [1-6]. It is necessary for (through a third contact) and on the dynamic perfor - the insulator to be either thin enough (e.g. < 50 A) mance. We also describe the basic features of an to pass substantial tunnelling currents, or for it to be alternative theory which gives good agreement with otherwise semi-insulating (3), (5). Switching occurs, experiment and is capable of being extended to give at a critical voltage (the switching voltage V), when a complete description of both the static and dynamic the p-n (or n-p) junction is biased in the forward switching characteristics, including the ON-state and direction; with the opposite polarity the characteris- the negative resistance region. tics are similar to those ofa reverse-biased p-njunction. The switching voltage can be modulated through the 2. Device fabrication. - MISS switches were influence of an additional (third) contact to the inter- fabricated from Si n-epitaxial layers (doping level mediate Si layer (3], [4], and Yamomoto et al. [4] = 2 x 1015 cm -3 and 7 pm thick) on p'-substrates have shown that by utilizing this feature. MISS with a thin oxide layer thermally grown on the n-type Si devices could be applied to logic systems such as IC A) tic shift registers. Yamomoto and Morimoto (1] have also suggested that MISS devices could be light sensitive and hence used, for example. as optically' 0.8 modulated switches.

For the two terminal device, Simmons and El- S '° cviJ Badry [5] have developed two simple electrostatic models based on punch-through and avalanche mecha- 02 (at nisms appropriate, respectively, to lightly (< iO' cm -3) and heavily (> IO' cm - ') doped .0123'. intermediate Si layers. The equations predicted by (5) these models are, in fact, in reasonable agreement Cv) • c' Cc] with the results of El-Badry and Simmons [6]. However J•,j- let the experiments described in [1-4] have been carried 3) Out with Si of medium doping levels (Z iO cm- Flo. I. - a) Schematic illustration of the device structure. Epi- and neither these results, nor our own, fit the models ia.xial layer doping density : 2 a 10" cm-1; p - -substrate (5). 6 x lO cns'. Epitaxial layer thickness - 7(im: substrate of Simmons and El-Badry : 80 sins a -annulus This paper presents more detailed results than 200 urn. Diameter of thin oxide region 400 urn. b) Static switching characteristic (schematic). ') Experi- mental results for a typical device, also illustrating the effect of a () Permanent address Department of Electrical Engineering, base current (taken ftom oscilloscope traces). The curves to the mA. with I, University of Edinburgh. King's Buildings, Edinburgh EHO ilL. right and left oil8 0 correspond to steps in l of 0.05 Scotland. Grande-Bretagne. positive and negative respectively. 366

7b;. REVUE DE PHYSIQUE APPLIQUEE

by a low temperature process (oxidation at 700 oC for 30 or 60 mm in dry 0,). The structure is illustrated schematically in figure Ia. For modulation experiments an n-diffused contact was made to the n-layer; evaporated aluminium or gold contacts were made to the thin oxide and to the n - -region, and a gold contact was made to the p'-substrate. The diameter -_ of the thin oxide region is 80 jsm and of then - -annulus 400 pm.

3. Experimental results. - 3.1 STATIC MEASU- REMENTS. - Figure lb isaschematic illustration of the I-V curve as obtained on a typical transistor curve- tracer; similar results would be expected from a slow- ramp or a d.c. experiment. For the present purposes the p-substrate is called the emitter (E). the contact _200 _150 _lso _55 to the thin oxide the collector (C) and the ncontact T "ci to the intermediate Si layer the base (B). Figures la Ftc. 3. - The variation of switching voltage V5 with temperature and b then define the current and voltage directions for a typical device. The dashed curve was calculated from equation with the arrows in figure la indicating conventional (2) with the following values of the variables : (, - Z)-0.03 eV: ö = 20 A current. The device switches at the voltage V. with : 7.45 (dimensionless) : N a 2 n 1015 cm-3 a positive potential l'c applied between emitter and J. = 1.3 ii ID' A.cmt corresponding to a surface state density N = 8 x 10 tt cmeV;B, 10 collector. Typical experimental results, taken from ° cm 2 ;c,5 I0'cm.s. oscilloscope traces, are shown in figure lc. which also illustrates the influence of a base current: with 113 negative V5 increases and vice-versa. The different ing to the present results V increases as temperature traces in figure Ic correspond to steps of 0.05 mA in decreases and seems to saturate at about - 200 °C; IB for both directions from I = 0. Note that the so far, however, experiments have not been taken low-impedance ON-state characteristic remains un- to lower temperature to determine whether V. changed. remains constant or goes through a maximum. These Measurements have also been made of the static appreciable effects of temperature are in marked characteristics as a function of temperature with the contrast to the results of El-Badry and Simmons [6]; results, for another device, shown in figure 2. The they found essentially no change in V5, although they curves labelled (1) to (7) correspond to temperatures did observe a small effect on the ON-state characte- ranging from - 193.5 to 40 °C (see figure caption for ristic but in the opposite sense to that shown in figure 2. details). There is a significant change with temperature. 3.2 particularly in the switching voltage V and this is DYNAMIC MEASUREMENTS. - When a voltage shown explicitly for another device, in figure 3. Accord- pulse of magnitude greater than V is applied to an MISS switch, the current through the device remains low (essentially the OFF-state current) for a period z, after which it rises more or less instantaneously

to the ON-state current, i.e. there is a delay time (t0) before switching takes place. Figure 4 illustrates this for three pulses of different magnitude: as the applied pulse height increases (with respect to Vs) the delay time decreases. The shape of the current response appears to be controled by the internal switching mechanisms of the device and is independent from the. external circuit used for the measurements. In figure 4 the current seems to rise from its OFF to ON-state values in a two-stage process; this was a common but not universal feature of our measurements : for the range of pulse heights used, to is of the order 10-10 seconds. Kroger and Wegener (2) report delay times as short as a few nanoseconds or Voltage (V) less, but they applied pulses rather greater in magnitude FIG. 2. - Experimental static switching characteristics illustrating than those used for the present studies. the effect of temperature. For curve (I) 7 - 193.5 °C; (2) The base current also influences the delay time and 7 = - ISO "C: (3)7= - 100°C: (4) 7 = - SI "C; (5)7=0°C; figure 5 illustrates the combined effects of /s and pulse (6) 7 = 24C; (7) T* = 40 C. height on t. In figure Sb t ois plotted as a function of 367

CHARACTERISATION' OF METAL-THIN INSULATOR-n-p SILICON SWITCHING DEVICES 76

Time (ns) 2000 1500 1000 500 0 _500 _1000 _1500 20 0 50 100 150 200 250 300 13 In 15 a- 1? 0' 2 11 ;io '. 10 0• a'41 5 - 9 Ie E lal •30 "I a. ir: !i. izv 9./

12 10

10 •uiiii zz E E — 0.1 C '5. C I15.5V '13V 9v iiwauu C-) Ba' 2 -2000 -1500 -1000 -500 0 500 1000 1500 Base currenl I B (jiA I 0 0 50 100 150 200 250 300 Fro. 5.—a) The switching voltage V3 as a function of base current Is. b) The delay time to as a function of 15 for different voltage Time (rss) pulse heights.

FIG. 4. - Illustrating the influence of voltage pulse height on the switching delay time ti,. Curves (1). (2) and (3) correspond to pulses of increasing voltage.

1 (positive and negative) with pulse height as a parameter. For completeness figure So shows the E. variation of switching voltage V5 as a function of base current; 113 can be regarded as influencing to 004 through its effect on V. .• 4. Discussion. - It seems clear that the punch- H through and avalanche models of MOSS switching, developped by Simmons and El-Dadry (5], do not FIG. 6. - Schematic illustration of the energy-band structures apply to our own results nor, most likely, to the results of the OFF- and ON-states for the regenerative model of switching. of other workers (1-41. Yamamoto and Morimoto (1] The diagram also defines some of the parameters introduced in have already tentatively suggested the possibility the text. of a regenerative mechanism of switching related to the build-up of an inverted region in the Si at the Si- equations has been carried out and a continuous Si02 interface. In the following we give a brief des- description of the static I-V characteristic is obtained, cription of the quantitative development of such a including the negative resistance region. Preliminary model. The essential features of the model are given calculations also show that the regenerative model in figure 6 which shows the band structure of the is capable of describing pulsed operation (dynamic OFF- and ON-states: it also defines the parameters performance) but for reasons of space the present used in the following discussion. It is necessary to discussion is restricted to considering the switching solve the continuity equations for both the majority voltage V. and the ON-state characteristic. J and minority carrier current 19• taking into account Throughout the OFF-state characteristic the vol- the link between J. and J imposed by the presence tage Vsc is practically confined to the MIS part of the of the p is junction. The numerical solution of these structure. The continuity equation for the transport 368

770 REVUE DE PHYSIQUE APPLIQUEE of minority carriers has been discussed previously [7]. where Nss is the acceptor state density. c the acceptor (8]. and for the present case it is written as follows state capture cross section and r,,,, the thermal velocit y (10]. With reasonable values of the parameters involv- r (- i -- (i) 1 Jp = 7A1TCXP ix [_ kT ed, equation (2) gives a good approximation to the magnitude of V and its temperature dependence as x [1 - exp(— U1 )] can be seen from figure 3. The dashed line in figure 3 = a,,, T2 exp(— U8)[l - exp(— U,)] + .1,, was calculated from equation (2). with the variables set at the values given in the caption. It is possible to (1) obtain an even better fit between theory and experi- where 7 is the p n junction efficiency (9). J. the frac- ment by including, for example, the variation in the tion of the diffusion current that tunnels via the accep- width of the space-charge region in the Si at the Si- tor state density (10], a,, and a are tunnelling attenua- S'02 interface, but the more cumbersome computa- tion factors [7]. A is the Richardson constant, tion process involved has not yet been completed. T the temperature and k the Boltzmann constant. For the present purposes it is sufficient to note that The other parameters are as defined in figure 6. Equa- the model leading to equation (2) provides an ade- tion (1). together with the expression which relates quate description of the experimental data on V5 . the voltage across the oxide layer. V,,. to the applied It could also be extended to include the influence voltage ( V,, = (kT/q) U,,) [8]. gives a description of base current Is on V. of the OFF-state characteristic. As soon as the inver- The ON-state-can be analysed but a new boundary sion situation occurs i.e.: U, = U,,,,,, = U. (where U8,, condition for the diffusing minority carriers must is the saturation value of U81, at inversion), the be introduced because of the collapse of the barrier majority carrier driving current of the regenerative at the oxide-semiconductor interface. The appro- system. J.T. will tend to increase rapidly because the priate equation is inversion layer imposes larger field values across the oxide, thereby making I(4'M - x - J',,)/k T] decre- I (-X -:v)1 ase. The minority carrier current given by equation (1) AaznTSexP[_ kT ] will then have to adapt to the condition U8,, = Us , which remains unchanged once the inversion Situation x El - exp(— U,,)] E) Pw = is reached. As a result, the control of 4T is obtained - ( with much lower values of U,, in order to satisfy = Ama,,, T2 exp(— 17,,) + J,, equation (1), i.e. the system switches to the ON- where D9 is the diffusion coefficient of the minority state. By setting U8 ,, equal to U. in equation (1) we can derive the voltage V U1 (kT/q) at which the carriers, L the width of the intermediate (n) Si layer inversion situation is first established, i.e. the switch- and Pw the minority carrier density at the edge of the ing voltage. We obtain MIS space-charge region. A more complete account of the regenerative -1 '0 VS = model, including its application to the effect of a 2qNo r.5 52 base current, the negative resistance region and dyna- Y \/ ' 112 mic operation, will be published elsewhere. Clearly X — x — kTln' however, it is capable of providing a good approxima- I. N0[1 + J,,/A T2 e,,)] L jJ tion to our results and, possibly, to those of other workers. Since the models of Simmons and El-Badry + kTln N. also fit reasonably well to their particular results [5], [6], one must conclude, for the time being, that there where N, and N. are the densities of state in the valence are at least three operating mechanisms of MISS and conduction bands respectively, ND is the doping switches : punch-through, regeneration and avalanche level, and c,,, e, to are the insulator, silicon and vacuum mechanisms. Which one is dominant presumably perrnittivities. depends on device parameters such as the doping 7,, = qNss o V,, NV[E8 - ( (pM - x)] level andwidth of the intermediate Si layer.

References

(II YAMOMOTO T. and MORIMOTO. H.. AppL Phys. Lea. 20 (1972) (71 Buxo, L. EsTtvE, D. and SARR.ABAYROUSE, 0.. P111:. Status 269. Solid! (a) 37 (1976) K 105. [21 KRoccs, H. and WEGENER. H. A. R.. ibid. 23 (1973) 397. (8) BuxO. 3.. SaRIt...BAyROusE, G. and ESTEVE, D.. ibigL 42 (1977) (3( KROGEI,. H. and WEGENER. H. A. R.. ibid. 27 (1975) 303. 173. 141 YAMOM0T0. T.. KAWAMUR.A. K. and SHIMIZU. H.. Solid State 191 LISOMAYE8, 3. and WRIGLEY. C. Y., Semiconductor Device., Electron. 19 (1976 701. (Van t'!oo,rand Princeton) 1965. 151 SIMMONS. J. G. and EL.SAORY. A.. Ibid. 20 (1977) 955. (JO) SARRA8AYROUSE. 0., Buxo. J. and ESTEVE, D.. Phs. Siwu., 161 EL.BADRY. A. and SIMMONS, 3.0.. ibid. 20 (1977) 963. Solidi 46 (1978) 185. 369

Journal of Non-Crystalline Solids 32 (1979) 29-52 © North-Holland Publishing Company

THE THRESHOLD CHARACTERISTICS OF CHALCOGENIDE-GLASS MEMORY SWITCHES

A.E. OWEN * Laborato ire d'Automarique et d'Analyse des Sysrèmes, CNRS, 7 avenue du Colonel-Roche. 31400 Toulouse, France

J.M. ROBERTSON and C. MAIN ** Department of Electrical Engineering, University of Edinburgh, King's Building, Edinburgh E119 3JL, UK

Received 18 July 1978

A detailed study is reported of the threshold characteristics of memory switches made from a chalcogenide glass of composition Gel 5Te81 S2Sb2. Most of the devices were fabricated by microelectronic processing techniques compatible with integrated-circuit technology- The main experimental feature is the systematic variation of the factors which influence the thermal parameters of the devices, i.e. temperature, geometry (thickness and radius of the active region of the switch) and substrate material. Measurements are also reported of the temperature and field dependence of the isothermal conductivity of the chalcogenide glass. Analysis shows that the results can be described adequately by an electrothermal model which takes into account the full field dependence of the isothermal conductivity and of the thermal fringing in the substrate.

I. Introduction

The past 10-15 years has seen the field of "amorphous semiconductors" develop into a distinct field of solid-state electronics and physics. Much of the initial impetus arose from the great interest caused by electrical switching phenomena in chalcogenide glasses, although reports of a form of electrical switching in variety of materials, amorphous and crystalline, have appeared in the literature for at least several decades. The first references to a relatively crude type of switching in chalcogenide glasses were reported nearly 20 years ago [1-3], but Ovshinsky's paper of 1968 [4], which described the prototype of a potentially reliable device,

* Permanent address: Department of Electrical Engineering, University of Edinburgh, King's Buildings, Edingburgh EH9 3JL, UK. ** Present address: Department of Clinical Physics and Bio-engineering, West of Scotland Health Board, 11 West Graham Street, Glasgow G4 9LF, UK.

29 370

30 A.E. Owen et al. / chalcogen ide-glass memory switches probably marked the point at which the subject became of serious interest in solid- state electronics. Chalcogenide glass switches are normally classified into "threshold" (or monostable) and "memory" (or bi-stable) devices. Their general characteristics are well- curves are reproduced in known, but for convenience the familiar schematic I— V fig. la for the threshold device, and in fig. lb for the memory switch; VTH is the threshold voltage, curves I and 2 indicate the OFF- and ON-slates respectively and, in fig. la 'h is the holding current and Vh the holding voltage for the threshold ON. state. Whether threshold or memory characteristics are observed is mainly a matter of the composition of the chalcogenide glass. A typical threshold composition is As30Te48Si 12Ge 10 (the so-called STAG glass) [4], whereas memory materials often have a composition close to Ge 15Te81 X4, where X can be a variety of elements such as As, Sb, S etc. or even a mixture of these [5,6]. The reason why different compositions are usually more suitable, either for threshold devices or for memory switches, can be understood qualitatively in terms of their glass-forming tendencies. The prismatic diagram of fig. 2 shows, for example, the approximate glass-forming region in the four-component system Ge—Si—Te—As. The glass-forming regions in the two ternary systems Ge—Te—As and Si—Te—As have been determined experimentally [7,8], but in the four-component system it has been drawn in by a reasonable interpolation 'i'. The STAG threshold glass corresponds to point A in fig. 2; it is near the centre of a large glass-forming region and hence is unlikely to devitrify very readily, i.e. the STAG glass is expected to be relatively stable to crystallization. The memory glass Ge l 5Te81 As4 corresponds to point B; it is at the edge of a glass-forming region (as see footnote) and hence on the one hand it will readily crystallize at defined here - temperatures close to its glass-transformation range, but, on the other hand, it can be quenched to form a glass without too much difficulty. It is not the purpose of this paper to discuss memory-switching, but it should be noted that bi-stable operation (fig. lb) first involves switching to the threshold ON-state followed by the precipitation of a crystalline filament in the hot (;~ Tg) ON-state channel. Thus, SETTING to the permanent ON-state requires a current pulse of several milliamps for perhaps a millisecond or longer; to return to the OFF-state (RESET) requires a much larger but shorter (1-10 j.zs) and rapidly terminated pulse, in order to remelt the crystallized material and quench it into the glassy state [5]. Thus the choice of a material for memory-switching calls for a fine balance between the ease with which it crystallizes and the tendency to form a glass. A "good" threshold material is one which, other things being equal, is as "stable" as possible in its vitreous form.

* It must be recognized, of course, that 'glass-forming ability' is not well-defined notion. The boundaries of fig. 2 correspond to the preparation of at least several grams of a glass by normal quenching from the melt. Vapour deposition or special techniques for rapidly quenching small droplets of a melt would considerably expand the glass-forming region. 371

A.E. Owen et al. / Cizalcogenide-gloss memory switches 31

1h I ' ®

\. V V T H

S) ( 0 )

Fig. 1. Schematic I—V curves for (a) a chalcogenide glass threshold switch, and (b) a memory switch.

For example, another material used for threshold switching is the five-component composition As36Te39Si 17Ge7P1 [9]. It is difficult to relate this to the four- component diagram of fig. 2, but it is worth noting that the addition of extra components in order to stabilize a glass towards crystallization is a well-established trick of the traditional glass-maker's art and was probably discovered empirically long before such esoteric concepts as "configurational entropy" came on the scene. Ultimately, however, all glasses are thermodynamically unstable with respect to the crystalline state (or, at the most, metastable), and the distinction between threshold and memory materials is one of degree rather than of kind. In our experience, a memory switch can always be operated satisfactorily as a threshold device, provided the ON-state current is kept low, and a threshold switch can be made to function 372

32 A.E. Owen etal. / chalcogenide-glass inenzory switches

Ma Fig. 2. Prismatic phase diagram indicating the glass-forming region in the four-component system Ge—Si—Te—As. Point A corresponds to the composition As30Te40i 1 2Ge10, and point B to Ge l 5Te81 As4. bi-stably, albeit with a limited number of cycles between the OFF-state and the permanent ON-state (SET and RESET). The qualitative "explanation" of the difference between threshold and memory- switching materials in terms of their glass-forming tendencies does, of course, beg the important question - i.e. why are some materials much more easily prepared in a glassy form, and much less readily crystallized, than others? This is a complex problem involving structural, kinetic and thermodynamic factors and is beyond the scope of this paper. Nevertheless, what is known about glass-forming tendencies does provide a useful empirical correlation with switching characteristics.

2. Device structures and scope of the present work

The work to be described here was part of a project concerned primarily with memory-switching, and hence the composition Ge 15TeS2Sb2 was used for the majority of measurements (some related glasses in the Ge—Te—As system have also been studied but will not be reported on here). In most cases the switching devices were fabricated by standard microelectronic processing techniques in the so-called "pore" structure [10]. The active area of the switch was defined by etching a small pore (radii varying from 5 to 75 jim in the present work) through a suitable insulator. A few devices of larger area were also defined by the cross-over of ortho- 373

A.E. Owen eral. / C'halcogen ide-glass memory switches 33 gonally deposited upper and lower metal strip electrodes. The chalcogenide glass was deposited by rf sputtering, as also were the Mo electrodes. Evaporated Al contacts were provided between the lower Mo electrode and the substrate, and on top of the upper Mo electrode. The substrates were either borosilicate glass slides (Corning 7059) or silicon slices with a thin (0.4 pm) thermally grown oxide layer for electrical isolation. As mentioned in section 1, the establishment of a permanent ON-state in a memory switch is preceded by the threshold action, and it is this aspect of the work which is the subject of this paper. The threshold voltage has been measured as a function of temperature and the delay time tD (under pulsed operation) as a func- function of voltage, with systematic variations of thermal parameters such as substrate thermal conductivity and device geometry. From the interpretative viewpoint the aim was to determine to what extent the threshold characteristics could be quantitatively described by simple thermal or electrothermal models which take into account the thermal influence of the device geometry and substrate. Sections 3 and 4 give a brief account of, respectively, one-dimensional thermal and electro-thermal theories, and of the appropriate thermal conductance calculations. The experimental results and analyses are considered in subsequent sections.

3. Thermal and electrothermal switching

Figure 3 shows a typical I—V curve obtained from a pore device using a slow current ramp. The turnover and current-controlled negative differential resistance region (CCNDR) were observed, reproducibly, in all devices. A thermal theory attempts to explain this in terms of Joule heating, with switching following as an instability associated with thermal runaway. The cylindrically symmetric geometry analysed is shown schematically in fig. 4. A thin film of chalcogenide, thickness df, thermal conductivity Kf and specific heat capacity Cf is deposited on a substrate, with the parameters d, K and c, respectively. The sandwich electrodes are circular, radius a, and the substrate lower surface is in contact with a heat sink at the ambient temperature Ta. The heat transport equation is (1) v2(gr) ~ J7o=c(aT/at), where Je is the electrical current density. The following simplifying and self-consistent assumptions are made: The temperature distribution in the active region of the device is uniform, i.e. (aT/ar) 0 (0

34 A.E. Owen etal. / Chalcogenide-glass memory switches

Voltage IV) Fig. 3. A typical I—V curve for a Ge l 5Te81 S2 Sb2 pore device on a borosilicate glass substrate measured using a slow-current ramp. The turnover voltage is indicated by V and the switching voltage by Vy (see text).

Current and heat flow in the device are parallel and uniform. The electric field is uniform through the device. The electrodes are relatively poor heat sinks. While these assumptions seem reasonable for thin-film structures, their validity can in most cases be checked experimentally, or theoretically, as will be shown later. The first and third assumptions imply no current fIlamentation, which is probably the case prior to turnover, while the fifth is required to provide CCNDR, as electrodes held at ambient preclude turnover [11]. In the steady state, eq. (1) reduces to the heat balance equation:

(T— T a)=(VI/F)=g(V,f), (2) where T is the temperature of the active region, Ta the ambient temperature, F the overall thermal dissipation constant and g represents some function of V and I. If G is the electrical conductance the device current I, is 1= VG =f(V, 7). Turnover occurs when (af/a7)(ag/av) = Thermal runaway will occur at this point at a voltage V, for a constant voltage. (zero resistance) source. This is a limiting case of a load-line instability. The simplest case of field-independent activated conductivity gives the well- 375

A.E. Owen etal. / Chalcogenide-glass memory switches 35

/ Cho(coqernde film d. a I

d Substrate

- Heat sink -

Fig. 4. Cross-section illustrating the device geometry for thermal calculations. known thermistor results. Conductivity:

G = (lra2/df) ao exp(—L.E/k7). (3) Turnover temperature:

T Ta + (Tk/i1E). (4) Turnover voltage: 1/2 V 0 exp(E/2kT). (5) 6Ta(zE7ra2ao) kfdf Note that, assuming I' is constant, eq. (5) gives Vt at d 2 . While these results may apply to thick films and bulk samples (df 10 gm), in general they do not apply to thin films [12] ,even qualitatively. An electrothermal analysis takes into account the field-dependent conductivity of the active material. A number of functional forms for this dependence have been reported in the literature, but in many chalcogenides it appears to be given by [131.

a(T, e) = cFo exp[—(iE - ea(7)c)/kT] , ( 6) where the field parameter a(T) has the dimensions of length and is temperature dependent. That eq. (6) describes a bulk effect has been verified by using a wide range of interelectrode separations (0.3-25 gm). At high fields (e > el = 2.5-3 X 10 V/cm) a second, steeper field dependence becomes dominant in some chat- cogenides, i.e.

a(T, €) = co exp[—(AE - ea(T) e 1 )/kT] exp[eh(e - e 1 )/kT] , (7) where h is also a constant with the dimensions of length. The mechanisms responsible for the field-dependent conductivity have not yet been clearly identified, but such detailed knowledge is not required for an electrothermal analysis. Substituting eq. (6) into the heat balance equation gives, at turnover, turnover 376

36 A.E. Owen et al. / clialcogenide-glass memory switches

temperature:

T Ta + (Tk/iE'), (8) AE' =L1E—(ao Ve/df). (9) Turnover voltage: 2dfkT a r E ~o.ga kd-r' Vteao + log 1 . (10) Vt= e(ao _dTa)L: V (_LiEo0ira2 2dfEIi's] Note that V is now much less temperature sensitive, as

(aVt/aTa) < [df .E/eTa(ao - dT3)] and V d1 (F constant). A similar expression is obtained for the "second field regime" [eq. (7)], giving an even smaller temperature dependence. The behaviour predicted for V is in qualitative agreement with a number of observations on thin chalcogenide devices, but a quantitative prediction of the behaviour depends upon the value of F and its dependence upon device parameters, and this is considered next.

4. Thermal conductance calculations

The overall thermal conductance of the device of fig. 4 consists of the series combination of film—substrate--heat-sink conductances, Ff and F, respectively. The smaller of the two terms will control the overall dissipation, and hence V. For uniform device heating, and heat flux, the minimum value of Ff is obtained:

Ff(min) = (2Kf7ra2/df). (11) While for uniform, linear heat flow in the substrate:

rs = (K7Tt1/d). (12)

As Thornburg and Johnson point out E141 this is correct only for a - 00, because non-uniform heat flow, or fringing, can increase the effective thermal conductivity of the substrate. A simple approach to the problem of thermal fringing is to assume that the heat flow near the device is uniform over a hemispherical isotherm of radius a. Placing a mirror image of the heat-sink isotherm above the device produces a thermal flux pattern, in a "homogeneous" medium, which maintains the boundary conditions (T/Z) = 0. The effective thermal conductance F between this isotherm and the heat sink may be found by the method of images, where, in this case, an infinite series of images is produced on either side of the device. 377

A.E. Owen etal. / chalcogenide-glass memory switches 37

Summing the infinite series obtained, term by term (it is conditionally convergent), the result obtained is

T= [2K5ira/(l +(a/d5 ) log 2)] . (13) The effective conductivity of the substrate is increased over that for linear flow by the factor

[2(d1a)1(1 1- (a/d) log 2)] . (14) This factor can be very large for typical geometries (e.g. for a = 50 Jim, d 0.085 cm the factor is 35). For typical device geometries (a >> d), r'2KiTa. (15) The two limiting cases of a poorly conducting substrate (Tf>> F5) and a highly conducting substrate (F 5 >> I') may now be examined for the simple thermal model. In practice, the use of borosilicate glass and silicon substrates correspond, respectively, to these two cases. The discriminating criterion is given by (Kfa/K Sdf)> 1, F> r.

4.1. Poorly conducting substrates

The overall conductance F - F', and using eq. (15) (2kK,df 12 Vt 0.6 Ta Ea0a2) exp(E/2kTa). (16)

Note that this predicts a dependence on device diameter. The electro-thermal case will, of course, be less sensitive to this parameter. It can now be shown that the assumption of uniform device temperature is well justified in this case. At the film—substrate interface the heat flows from the film and through the substrate may be equated:

Kf [aT/az ].=d1 = (I'5/ira)2 [T(z) - Ta] z=df

The maximum temperature drop across the film can be estimated [14]:

[T(0) - T(df)} <0.5 df[aT/azl zd

Hence, the ratio of this temperature drop to that across the substrate is

(AT/T 5) <(K5df/Kfa).

Note that this ratio is much larger than that estimated by Hayes and Thornburg [151 for uniform heat flow, but for a typical device on a glass substrate (df '- 1 Am, a - 50 Am, K5 - K1) this ratio is still small (-0.02). 378

38 A.E. Owen et al. / Ghalcogenide-glass memory switches

Table 1

Material Thermal conductivity K Thermal capacity C Refs. 1 (\V cm'K') (i cm 3 K ) chalcogenide glass 2-5 X 10 1.0 [16], [171 borosiicate glass 1 X 10-2 2 [19] Si 1.5 2 [18]

4.2. Conducting substrates l' > r'f

The overall conductance I' - l.',

V(max) = 0.6 Ta(2kKf1I.EO 0 ) exp(LE/2kT a). (17) The turnover voltage is now independent of device diameter, although the "electrothermal" V will still be proportional to df. This expression is less satisfactory than that for poorly conducting substrates as there will now be a significant temperature drop in the film itself, and Ff may be larger than the computed value. For example, for a typical device on a silicon substrate, the ratio:

(Tf/1T s)max 1.5 (a = 50 pm).

However, the value derived for (Tf/zT 5 ) is a "worst case" result, and a number of factors will tend to reduce this ratio in practice. For example, the temperature dependence of the electrical conductivity will ensure that power dissipation in the cooler region near the substrate interface is greater than that in the upper, hotter region. It should be noted that since, in this case, the thermal resistance of the film itself controls r, then variations in the thermal properties of the surrounding medium would not be expected to affect switching behaviour. Table 1 lists approximate values of thermal properties of the materials used.

5. The conductivity of Ge l 5Te81 S2Sb2 glass

The temperature and field dependence of the isothermal conductivity of the Ge 15Te81 S2Sb 2 glass was measured in the same structures used for the switching experiments. The results are shown in figs. 5 and 6. The power dissipation was kept low in these measurements in order to avoid Joule heating - e.g. short (0.3 .is) voltage pulses were used at room temperature (note, in fig. 6, the different results for static and pulsed measurements at 296 K). The conductivity has the functional form of eq. (6) for € < e 2.5 X 10 V/cm, while € > €, it follows eq. (7). From the experimental data in figs. 5 and 6, the values of the parameters in 379

A.E. Owen etal. / Cizalcogenide-glass inemorY switches 39

U cI _9 -. 10

oil 10

_13 RI 4 5 6 7 10/ T (K) Fig. 5. The logarithm of the isothermal conductivity versus reciprocal temperature of the Gel 8 Te81S2Sb2 glass. The different points (0, • and o) correspond to devices with glass films of different thicknesses in the range 0.3-5 pm. The slope of the line gives an activation energy of 0.44 eV.

I I 10 I I

296K 296K (pulsed) I. 10 235K

E 10 162

> LI 10

C 0 U

10 ii i 25

10 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Electric field (V cm) x 10 Fig. 6. Field dependence of the isothermal conductivity of the Gel STc8l S2Sb2 glass, with temperature as a parameter. 40 A.E. Owen etal. / Cha1cogenide-glass memory switches eq. (6) and (7) are

a0 = 600 (ohm-cm)' and LE = 0.44 eV, a(T)—a 0 — dT, with a 0 =32A and d7Xl0 2 AK -' e, = 2.5X 10 5 Vcm', h85A.

6. Switching and 'turnover' in Get 5Te81 S2 Sb 2 devices

Static dc I—V measurements were made by standard voltmeter—ammeter methods, and also by a slow-ramp method employing the 150 V sweep output from a Tektronix 549 storage oscilloscope through a switched bank of limiting resistors. Scan times of 1-10 s were used.

6.1. Turnover voltage Vt versus temperature

Figure 7 shows the temperature dependence of V, for a device with a = 50 um on a glass substrate. The measured thermal conductance for this device is F =

200

100

so - >

> 3 20- - U 0 >0 10-

>U 0 5.

I-

I I I inn 200 300 400 Temperature (K) Fig. 7. Turnover voltage (V1) versus temperature (on a log-lineal scale) for devices on borosilicate glass substrates. The different points (o, ) are both for pore devices of radius a = 50 Jzm and thickness df = 0.95 jim. The solid line (1) correspond to Vt versus T calculated from the simple thermal model [eq. (5)], line (2) to the single-slope electrothermal calculation [eq. (10)] and line (3) to a calculation which takes into account both slopes in the field dependence (i.e. eq. (7)]. 381

A.E. Owen et al. / Chalcogenide-glass memory switches 41

0.33 mW K', a value obtained by comparing dc and pulsed I—V characteristics. Using this value for F, the turnover voltage was computed using the following: Simple thermal runaway - eq. (5). Single exponential field dependence - eq. (6) and (10). Total isothermal characteristics - eq. (7) and (10). At low temperatures, when V 7 V, the simple thermal prediction departs rapidly from the observed V, while the "single-slope" electrothermal model fits well, up to -25 V (corresponding to e l ). Computed curve (3), incorporating the second steep high-field regime, agrees well with the observed data over the whole temperature range. Calculation of the device thermal conductance, using eqs. (11) and (13) gives Ff =6.4mWK', F0.3mWK' Here, the substrate conductance controls cooling and is in very good quantitative agreement with the measured value. Note that F is -35 times greater than its value for linear heat flow. The device behaviour can be described the three 'regimes' Vt 25 V. Enhanced-slope electrothermal runaway. This is not merely an exercise in curve-fitting. It demonstrates that runaway (switching) under these circumstances may be fully described by Joule-heating considerations alone. 6.2. Turnover voltage versus temperature and film thickness Figure 8 shows V versus lIT for a range of film thicknesses (0.3 pm-4.6 j.zm, a = 50 Mm, glass substrate). The expected slope predicted by the simple thermal model is also shown (0.22 eV). It can be seen that this behaviour, corresponding to regime 1, is approached as df increases and as temperature increases, while for thinner films and/or lower temperatures, the temperature dependence is less, as predicted by the electrothermal model. Figure 9 depicts the film thickness dependence of V, at fixed temperatures. At room temperature and above, V a d' 2 approximately, while at lower temperature, Vt a d1. This behaviour is again consistent with the thermal analysis, tending to "simple" thermal turnover (regime 1) for higher temperatures and thicker films, and electrothermal turnover at lower temperatures and for thinner films. 6.3. Turnover power versus film thickness The data given above were for devices on a poorly conducting substrate (r < re). For this situation, the turnover power Pt may be written Pt = VtIt = F(T - Ta), P (2KS iraTk)/ [AE —(ea(flVt/dr)]. (18) 382

42 A.E. Owen etal. / C'halcogenid c-glass memory switches

330K 295K 250K

C' 0

a, 0 C I 11 I I Oh '' Z5 3.0 3.5 4.0 4.5 5.0 1 2 5 10 20 50 100 10/T (K) Turnover voLtage V (VI Fig. 8. Turnover voltage Vt as a function of reciprocal temperature for devices of thickness 4.6 urn (0), 1.7 urn (.), 0.95 urn ('s) and 0.30 urn (i) on borosilicate glass substrates. The dashed line has a slope corresponding to an activation energy of 0.22 eV.

Fig. 9. Device thickness versus turnover voltage for devices on borosilicate glass substrates, with temperature as a parameter. The dashed line shows the threshold voltage measured under pulsed operation V, at 295 K (see text).

Provided Vt <<(AEdf/ea(1)), the turnover power is independent of field parameters, and allo*s an easy check on I'. Note that in this case, r(-r) and hence Pt' should be independent of film thickness. Experimentally, P t was found to be between 5 and 6 mW, while the value calculated from eq. (18), is —5.8 mW. Fig. 10 plots Pt versus Ta for several film thicknesses. Although there is some scatter, the data are practically independent of film thickness; the curve through the data is obtained from eq. (18).

220 240 260 280 300 320 340 T0 (K) Fig. 10. The turnover power Pt versus ambient temperature Ta for devices on glass substrates. The different points (o, ., and A) correspond to devices of different thickness in the range 0.3-5 urn. The solid line was calculated from eq. (18).

383

A.E. Owen etal. / cizalcogenide-glass memory switches 43

6.4. Turnover voltage versus device radius

6.4.1. Borosilicate glass substrates Figure 11 plots V versus pore radius, varying from 7 V for a 75 pm radius, to —22 V for a 5 pm radius pore. For a simple thermal model, Eq. (16) predicts, V a a 112 , and hence fig. 12 plots V versus a 1 ,including data for a 500 pm crossover device. In the region where a simple thermal model is appropriate, a good linear fit is obtained. The intercept for (1/a) = 0 should give V (00), i.e. Vt for a device of infinite radius and corresponding to linear heat flow, for which ['s(oo) = (K/d5). From fig. 12, V (0o) 2.5-3 V, while using Fs() in eq. (5) gives V(°°) 2.2 V, in reasonable agreement. Once again the thermal fringing analysis gives good agreement with the observed turnover, even though the expression for fringing conductance r' is a simple approximation. It is pertinent to compare the values for r obtained from the data for turnover power and eq. (18), and the values predicted by the fringing analysis. This is shown in fig. 13. The agreement is reasonable for the larger devices, but for small- area devices the actual thermal conductance is somewhat higher than that predicted. This may be due to the approximation used, and the contribution of the electrodes.

6.4.2. Silicon substrates An identical range of "variable-radius" pore devices were fabricated on Si substrates incorporating a thin insulating layer of Si0 2 (-0.4 pm). The results, V versus pore radius, are shown in fig. 11. Higher turnover voltages are obtained, and

2 >

a 0

> 0 C

25 50 Pore radius (pm) Fig. 11. Turnover voltage Vt as a function of pore radius a for devices of thickness df 1 jim. The points o are for borosilicate glass substrates and A are for silicon substrates. 384

44 A.E. Owen et al. / C'halcogenide-glass memory switches

500

400

300 > 200

100

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 -1 3 a_ i I cm x 10 ) pm on glass sub- Fig. 12. 1't2 versus reciprocal of pore radius for devices of thickness df 1 strates.

Vt is much less sensitive to pore radius than for devices on glass substrates. In this case I', the device—substrate conductance, controls cooling. Including the thermal resistance of the 0.4 pm SiO 2 layer, in I", r5 =47mWK, r'f =4.5mWK (for df = 1 pm, a = 50 pm, d5 = 0.02 cm). According to eq. (17), Vt is independent of device radius, and the computed value at 295 K is V (thermal) = 49 V, while V (electrothermal) = 24 V. For small pore diameters the measured V - 24 V, but is slightly less for larger devices. The power density (mW/cm2) at turnover in these devices is independent of pore radius, which is to be expected if the film itself controls cooling, while for the glass

_i00- -

0• 10—

1 10 0.01 0.1 1 Thermal conductance 1mW IC I Fig. 13. The variation of thermal conductance with pore radius for devices of thickness df 1 pm. 0,• - glass substrates, - silicon substrates. The solid lines were computed from eq. (18). 385

A.E. Owen etal. / Chalcogenide-glass memory switches 45

CV Cr

> 0 C = I.-

- 14O 160 180 200 220 240 260 280 300 320 340 Temperature (K) Fig. 14. The turnover voltage V as a function of temperature for devices of thickness df zts 1 zm. Borosilicate glass substrates: • - pore radius 10 jim, o - pore radius 50 xim, A - crossover device (500 Mm X 500 Mm).Silicon substrate: x - pore radius 50 Mm. substrate the power density increases with decreasing pore radius. Also plotted in fig. 13 are the device thermal conductances, obtained from the turnover power and eq. (18), and the theoretical F (mm). The agreement is good, giving raa2 as predicted.

6.5. Turnover voltage versus temperature and device thermal parameters

It is now appropriate to consider how the V versus T dependence is affected by varying the pore radius. Fig. 14 shows V versus T for devices on glass - a 500 .un crossover device, 50 Mm and 10 gm radii pore devices and for a 50 Jim radius pore device on silicon. The features evident in this figure are: (1) at low temperatures, in regime 3, Vt is relatively insensitive to thermal parameters; and (2) the transition to regimes 2 and 1 occurs at lower temperatures for devices with inefficient cooling. It is clear that very different results may be obtained with devices of different radii, even when the film thickness is the same - a fact which has not often been considered.

7. Surface temperature measurement

A surprising aspect of the work reported is that the results imply that the metal electrodes do not nesessarily provide the major heat dissipation path. Substrate and film parameters alone can predict turnover behaviour, if "fringing" effects are 386

46 A.E. Owen etal. / C'halcogcnide-glass memory switches

included. As the thermal conductivity of the electrode material is typically lO times higher than the film, or glass substrates (e.g. for Mo, K 1.4; for chalcogenides Kf 2-5 X 10-3 W cm -1 K'), it might seem as though the electrodes could be treated as isothermal infinite heat sinks, held at ambient. However, in the absence of field effects this precludes the observed turnover and CCNDR [ii], an argument sometimes used against thermal models, and the assumptions mentioned above must be checked. Electrodes held at ambient? As the electrodes in this work are typically deposited on a substrate of poor thermal conductivity this assumption is incor- rect - i.e. the electrodes must heat up with the device. Infinite heat sink? The electrodes will probably have a smaller thermal capacity than the substrate. Electrodes are isothermal and good lateral thermal conductors? Initially, this seems to be the case, as the electrode material has a very high thermal conductivity. The point in question is whether the thermal conductance of the electrodes is comparable to that of the substrate (not the film). It is impossible to treat the two elements - electrode and substrate - separately, as they are strongly coupled along the electrode length. This is the key to the problem. Whichever element dominates the cooling will control the temperature distribution around the device, and quite different patterns are to be expected if the substrate, or electrodes, are dominant. Where electrode conduction dominates, the temperature gradient along the electrodes will be much less than that along the substrate, and the tangent to the isotherms will change direction rapidly at the electrode pattern edges. Where substrate conduction dominates, the isotherm pattern should be almost circularly symmetric. To check this experimentally, a thin layer of cholesteric temperature-sensitive liquid crystal was deposited on top of the pore devices by spinning. The material used indicated isotherms in the temperature range 36-40 °C, by changing colour through the spectrum from red to blue. The device (top electrode) temperature and lateral temperature distribution were monitored during I—V curve measurements. It was found that at turnover, the electrode temperature immediately above the device was uniformly —'36° C, i.e. —17° C above ambient, as predicted by the thermal model. This implies that the electrodes do heat up, and that the device temperature (up to turnover) is uniform. Beyond turnover, in the CCNDR region, the 36-40°C isotherms shifted outward from the device, and were nearly circular, implying that substrate conduction dominates, as suggested earlier.

8. Slow-ramp switching in Ge l 5Te81 S2 Sb2

Devices could be made to switch in the course of slow-ramp (10 s sweep) I—V measurements. On glass substrates this occurred well into the CCNDR region, as shown in fig. 3, while on silicon substrates it occurred at, or close to, turnover. For 387

A.E. Owen et al. / C'halcogenide-glass memory switches 47 devices on glass substrates, switching occurred at almost constant current (large series R), to the ON-state curve, and the point at which this occurred (Vsw, ') was reproducible from device to deviëe, and in the same device after "resetting" ('reset - 100 mA j.ts). It should be pointed out that the whole I—V characteristic could be reproduced, including Vt and V (see fig. 3), through many such operations. It is evident that such a switching event is not a simple device characteristic such as load-line instability, but may depend implicitly on material parameters. There are a number of possibilities, e.g. Uniform heating to a critical temperature, such as Tg (395 K for Ge l 5Te81 S2 - Sb2) above which structural rearrangement may occur. Sudden contraction of the current distribution to filamentary form. A combination of (1) and (2). The behaviour of devices on silicon is significantly different with switching occurring near turnover. The power densuy at switching is independent of device area (implied by fig. 13), i.e. —103 W cm 2 , and the average current density is roughly the same as in devices on glass substrates at switching. This situation approximates more closely to the "well-heat-sunk" device, in which CCNDR should not occur, with uniform device current.. It is possible that the switching at turnover and the apparent inaccessibility of a CCNDR region is due to sudden channelling of the current distribution. An examination of the form of the thermal conductance reveals how this might occur in devices on silicon, but not in devices on glass substrates. I'(per unit area) on Si = (2Kf/df), r (per unit area) on glass = ( 2Kg /a)t If a is the effective radius of a hot area (rather than the whole device), then for glass substrates the cooling efficiency (per unit area) increases as a shrinks, but does not change for silicon substrates. This would make the current distribution on glass substrates less prone to a channelling instability.

9. Pulsed characteristics The static and slow-ramp measurements give information which allows assess- ment of the thermal behaviour of devices under pulsed conditions. Fig. 15 shows a simple thermal equivalent circuit of a device from which a number of thermal time constants may be derived, each relevant to a particular regime of operation. Values for a 100 urn diameter pore on a glass substrate are as follows: Tff = (Cf 1Ff) = (cd?/2K) = 1As, (19) (20) Tsf = (C/Ff) = (cddf/Ka) = 60 ms, (21) Tss = (cs = (Cds2IKs) = 2 5, 388

48 A.E. Owen et al. / C'halcogcnide-glass ,ne,nory switches

rs ii

Fig. 15. Thermal equivalent circuit.

where Tff is the short-pulse device cooling time constant, i.e. before significant substrate heating, Tsf the substrate heating time constant and T s the substrate cooling time constant.

9.1. Pulsed threshold voltage Vt , on glass substrates

For short pulses, before the glass substrate is significantly heated, the heat generated in the device is dissipated in the substrate via the device thermal conductance l'. For Tff < tpe < TS, the sample is in "quasi- thermal equilibrium", and V, may be obtained from the relevant steady-state turnover equation, but using rf in place of r. For virgin devices, this will result in a pulse threshold voltage V 1,, greater than the dc turnover voltage V, and, in fact, equal to the dc turnover voltage V for similar devices on silicon substrates. In addition, devices which display simple thermal turnover (regime 1) at room temperature under "dc" conditions, will now be operating in an electrothermal regime. In fig. 9, the dashed line shows the pulsed threshold voltage versus film thickness for the same range of thickness as the dc tests, at room temperature (pulse width 10 ps). The two points to note are (1) V 1., a df, while Vt ad' 12 and (2) for a 1 j.zm film, V, = 19 V V (Si substrate). After a number of threshold operations, "forming" occurred, and V, fell to a lower value, often close to V. By applying a suitable 'reset' pulse (150 mA, 10 its), the original V, could almost be recovered for the next pulse.

9.2. Switching delay time tD and forming

For very short voltage pulses, such that t, < r, all of the input energy raises the device temperature. Eq. (1) may be integrated to find, say, the time taken to raise the device temperature to some critical value T. Ignoring the thermal conductance, then,

Tc Cf exp[(E — ea(fle)/kT] dTt. (22) -p--00 f Ta Assuming that above room temperature, a(7) [in eq. (22)] tends to a constant value A.E. Owen et a1. / Cizalcogenide-glass memory switches 49 of a' = 10 A, and also that (AE - ea(T)e)>> T gives the result

Cf df tD — — (T exp [(AE - ea'e)/kT] - T exp [(E - ea'e)/kT] }. (23) Vi-- ao The second term may be ignored if (T - Ta)> 50° C, giving

Cf df2 k Ta2 tD exp(E/kT3) exp (24) (- dfkTa) This is plotted as a function of V, in fig. 16, for 0.1 ps 1 Ps.

It was found experimentally that during "running-in" V 1, would drop by a factor of 2-3 below its virgin value, while tD, for a given voltage dropped considerably more (up to two orders of magnitude). Also shown in fig. 16, are the measured values of td versus v, in several "time-regions" of the 'forming' sequence. The curve closest to the computed curve was obtained during the first few operations, where some "forming" must have already occurred, since the computed tD is an underestimate for tD> 1 ps. The lower curves were obtained after a number of threshold operations (-100, and —10). It can be seen that while V tp drops from 20 to 8 V, tD (at 25 V) drops from 4 to 0.15 ps, a considerable shift. A simple explanation for the extreme sensitivity of tD to forming is based on the electrothermal model, and Thomas and Bosnell's [20] suggestions regarding forming in threshold devices of STAG glass. Forming in STAG devices is thought to involve the precipitation of small crystallites of Te from the amorphous matrix. The

Forming E

r 0. CD 1'

5 10 20 50 100 Pulse voltage (V) Fig. 16. The variation of delay time tD with pulse voltage for a pore device of radius a = 50 .im and thickness df 1 J.Lm on a glass substrate. The solid line on the right was calculated from eq. (24) and the arrow indicates how the experimental curves (o, 6 and A) shift to the left as forming proceeds. 390

50 A.E. Owen et aL / C'/zalcogenide-glass memory switches data for Ge 15Te81 S2Sb2 [21] suggest that Te is also the major crystallization product (Ge—Te crystallization being suppressed), and since the material is near the edge of a glass-forming region this forming mechanism is probably even more appropriate. It is assumed that the result is to reduce the effective "electrical thickness" of the film, dfe, in the region affected (which may be a relatively narrow channel). It is also probable that the effective film thickness (say, dft) for thermal considerations, e.g. heat capacity, is not changed. If pulsed switching is in the electrothermal regime, then, from eq. (10),

V, (formed) cedj'e . While for the delay time, from equation (24), tD (formed) d e exp(—ea' V/kTadfe). (25)

This assumes that the resistivity of the amorphous matrix is not much changed on forming and that the field is uniform in the matrix. It can be seen that the presence of the field exponential in eq. (25) can lead to a large reduction of tD during forming. It should also be noted that eq. (24) is independent of pore radius and therefore the argument should also apply to "forming" within a small channel. Using the data given above, a reduction of V, from 20 to 8 V (factor 0.3) should be accompanied by a reduction in tD (for a 25 V pulse), by a factor '-0.03—'-0.12 us, in reasonable agreement with the measurements. (Note that since the computed "reduced" tD < 1 j.is, the value obtained should be valid.)

10. Conclusions

The threshold characteristics of chalcogenide glass memory switches under typical operating conditions (quasi-static or pulsed) can be quantitatively or, at least, semiquantitatively described by an electrothermal model. It is necessary to take into account the influence of thermal fringing on the effective thermal conductance of the substrate, and the full field .dependence of the isothermal conductivity of the chalcogenide glass. The latter is crucial and there is no doubt that the field dependence arises from electronic mechanisms which as yet are not properly understood. However, from the viewpoint of the initiation of switching (i.e. the threshold conditions), the electronic mechanism is immaterial; the field dependence simply serves to deliver energy to the device more rapidly than would otherwise happen. it must be emphasized that these conclusions do not necessarily apply to the threshold ON-state of these devices. Here, there may be significant electronic factors, although it is obvious from the way in which the permanent ON-state is formed (precipitation of a crystalline filament) that appreciable heating is involved during the memory setting pulse. Even so, in some of our experiments on the esta- 391

A.E. Owen etal. / Clialcogenide-glass ineniory switches 5 1 blishment of the permanent ON-state there are indications that non-thermal factors may also play a role, but this will be reported elsewhere.

Acknowledgements

Most of the material in this paper is based on reports made to the UK Science Research Council (SRC) over the period October 1972 to May 1974 and we are grateful to the SRC for research grants which made the work possible. The authors also thank Messrs. D. Reynolds and G. Borzucki for their technical assistance in preparing the materials and devices. The first author (A.E. Owen) is very grateful for the hospitality exended by D. Esteve, J. Buxo and their colleagues of the Laboratoire d'Automatique et d'Analyse des Systèmes (Toulouse, France) while he was on sabbatical leave in their laboratory (September 1977—September 1978) and during which period this paper was written. Finally, but most importantly, we acknowledge with sincere thanks our debt to Prof. Sir Nevill Mott. Our group at Edinburgh was one of the first in the UK to concern itself with amorphous semiconductors; its research has been concerned with chalcogenides, transition-metal oxide glasses and other oxides, and it has covered various aspects of the subject from fundamental optical and transport properties to applications in switching devices, etc. Over many years we have been fortunate to have the benefit of innumerable informal discussions and much correspondence with Sir Nevill; whatever the topic he has never failed to respond with helpful comments and criticisms - always perceptive and thought-provoking, but always impartial to his own point of view. He has, we know, given the same consideration to many other groups in the UK and elsewhere. The debt which the subject owes to Sir Nevill is not simply for his own incomparable contribution, but also for the energy and enthusiasm with which he has helped and encouraged groups like our own.

References

lii SR.Ovshinsky, Electronics, 32(1959)76. (21 A.D. Pearson et al., Advances in Glass Technology (Plenum Press, New York) (1962). [3] B.T. Kolomiets and E.A. Lebedev, Radiotech. Electron. 8(1963) 2097. (4] S.R. Ovshinsky, Phys. Rev. Letters 21 (1968) 1450. [5] H. Fritzsche, Chap. 6 of Amorphous and Liquid Semiconductors, ed. J. Tauc (Plenum Press, New York) (1974). 161 W.D. Buckley and S.H. Holmberg, Solid State Electron. 18 (1975) 127. Ill A.R. Hilton et al., Phys. Chem. Glasses, 7, 105 (1971). [8] J.A. Savage, J. Non-Crystalline Solids 11 (1972) 121. 19 1 K.E. Petersen and D. Adler, AppI. Phys. Letters 11 (1975) 625;J. App!. Phys. 47 (1976) 256. 392

52 A.E. Owen et al. / C'halcogenideg1ass memory switches

I 10 R.G. Neale, J. Non-Crystalline Solids 2(1970)558. H. Leuder and E. Spenke, Phys. Z. 36 (1935) 766; R.W. Keyes, Comments on Solid State Phys.4(1971) 1. G.V. Bunton etal., J. Non-crystalline Solids 6(1971) 251. [131 J.M. Marshall and G.R. Miller, Phil. Mag. 27(1973)1151. D.D. Thornburg and R.I. Johnson, Thin Solid Films 20 (1974) 177. T.M. Hayes and D.D. Thornburg, J. Phys. C. 6(1973)450. W.W. Sheng and C.R.Westgate, Solid State Commun. 9 (1971) 387. A.H.M. Shousha, J. App!. Phys. 42 (1971) 5131. S.M. Sze, Physics of Semiconductor Devices (Wiley, New York, 1969). Handbook of Chemistry and Physics (Chemical Rubber Co., Cleveland, Ohio, 1974). [201 J.R. Bosnell and C.B. Thomas, Solid State Electron 15 (1972) 1261. [21] R.T. Johnson et al., Mater. Res. Bull. 9 (1974) 667. 393

New amorphous-silicon electrically programmable nonvolatile switching device Prof. A.E. Owen, P.G. Le Comber, G. Sarrabayrouse and W.E. Spear

Indexing terms: Switching. Semiconductor devices and materials

Abstract: The paper reports preliminary data on the characteristics of a new electronic switching device based on amorphous silicon structures. The device is polar, and is switched from OFF to ON (WRITE) or ON to OFF (ERASE) by voltages of opposite signs; the threshold voltage for the WRITE operation is 4-8V, depending on the device, and for ERASE it is - iv. The OFF and ON resistances are typically lMfl, and in the range 50-300 a, respectively. Experimental devices have been switched through 10' WRITE and ERASE cycles, and the prospects are that this could be considerably increased. Particularly notable features of the new memory device are its extremely fast transition times (100 ns or less for both the WRITE or ERASE operations) and the very low energy involved in the switching transition ( 10' I). Present results indicate that the threshold voltages are only slightly dependent on temperature.

Introduction and device fabrication 50 92 and the electrical properites of the specimen have been permanently modified by this forming procedure. In this paper we report the first results on a novel electrically The specimen now functions as a nonvolatile memory programmable nonvolatile semiconductor memory device device, and Fig. 2 shows a typical set of static (DC) charac- fabricated from amorphous silicon (a-Si) [1] . The observations teristics measured point-by-point. Immediately after forming, reported here indicate that the new a-Si memory is potentially the device is in its ON state so that small positive and negative superior, in terms of speed, retention time, operating voltages, voltages trace out curve ab; ON-state currents of lOmA or stability etc., to the metal-nitride-oxide-semiconductor more are generally observed. On increasing the reverse potential (MNOS) or floating.gate.avalanche-metal.oxide-semicOfldUCtOr (i.e. a negative voltage applied to the p-doped region) a reverse (FAMOS) devices currently used in semiconductor memories threshold voltage VThR is reached, beyond which the device for nonvolatile, programmable storage [2]. switches to an OFF state with a resistance of the order of 1 M12, All of the experiments were carried out on a-Si junctions represented in Fig. 2 by the characteristic cd. The reverse deposited by the glow-discharge technique and doped from the threshold voltage VThR = 1 V for the particular batch of gas phase [3]. Stainless-steel substrates were used, and the specimens described. The OFF state is stable for voltage swings 0.5 and l.Opan. A number of total thickness lay between of ± 4V. If now the forward potential is increased beyond a different juction configurations have been investigated, and value of VThF, the forward threshold voltage, the device most showed the possibility of electrically programmable switches back into its high-conductivity state ab, in some cases switching to varying degrees. In its simplest form the device through an intermediate state such as e or!. consists of a p. and an n-layer deposited onto conducting The above cycle has been repeated up to lO 5 times without stainless-steel substrates. It was generally found, however, that observable changes in characteristics or threshold voltages. a third undoped quasi-intrinsic (1) layer tended to stabilise Devices set in ON and OFF states have been monitored for at device performance. After completion of the a-Si deposition least several weeks, and there has been no detectable change in a series of gold (Au) or aluminium (Al) dots (approximately either characteristic. With the present design the threshold 1 mm in diameter) was deposited onto the surface of the voltages for stable switching to the ON or OFF state are of specimens, and the top contact was completed either by a opposite polarity, although in a few devices some form of probe or a thin wire attached to the metal dots with silver paste. switching could be observed with both positive and negative voltages. The actual values of VThF and VThR depend on a Forming and static characteristics number of variables, including the thicknesses of the various For most of the configurations investigated it was found that layers, which need further investigation. the first switching operation is unique; all following cycles As noted above, the OFF-state resistance is of the order of occur, reproducibly, at a considerably lower threshold voltage. I M12 in the present devices, which have effective areas of This first operation is therefore called 'forming', and it seems to about 0.01 cm 2 (defined by the top evaporated contacts of be an essential precursor for subsequent operation of the I or 2mm diameter). The ON-state resistance is determined by device as an electrically programmable and nonvolatile memory. the current allowed to flow through the device during the Fig. I illustrates the forming process for a typical p-n-i structure. switching transient (and so by any series resistance). This can Curve a in Fig. 1A is the initial static I/V characteristic in the become as low as 1012, but it is preferable to prevent the conventional forward direction (i.e. the electrode adjacent to ON-state resistance falling below about 20012 to achieve the p-layer is made positive with respect to the other electode). reproducible cycling of the device. Curve b represents the initial reverse characteristic. When the Preliminary experiments on the effect of temperature on applied forward potential is increased to values between VThF and VThR indicate only small changes in either threshold 20 and 25 V, a rapid rise in current takes place and the device voltage with temperature (e.g. in a particular experiment, 3V at is brought into a highly conducting formed state, represented VThF varied from 4V at room temperature to by curve c in Fig. 1 B. The resistance is now of the order of 450 K). The threshold voltages are also unaffected by illumination from filament lamps. Paper 18331, first received 18th January and in revised form 8th February 1982 Dynamic characteristics Prof. Owen is with the Department of Electrical Engineering, University of Edinburgh. King's Buildings, Edinburgh EH9 3JL, Scotland. Dr. The dynamic responses of a typical p-n-i a.Si switch through Le Comber and Dr. Spear are with the Carnegie Laboratory of Physics, the OFF-'ON and ON-OFF transistions are illustrated in University of Dundee, Dundee DDI 4HN, Scotland. Dr. Sarrabayrouse Figs. 3a and b, respectively. In both cases a 10 V pulse of 100 ns is with the Laboratoire d'Automatique et d'Analyse des Systemes du CNRS, 7 Avenue du Colonel Roche, 31400 Toulouse, France duration superimposed on a IV pulse of approximately 0.5 pa fEE PROC. Vol. 129, Pr. I, No. 2, APRIL 1982 0143-71001821020051 #04 $015010 51 394

duration is applied to the device and, as before, the positive oscilloscope, and they demonstrate that either transition is polarity (Fig. 3o) means that the p-layer of the p-n-i structure completed within lOOns. is positively biased. The purpose of the lv pulse is to 'read' A particularly notable feature is that, for both switching the state of the switch; it is not a holding pulse to maintain the transients, the device current responds essentially instan- ON state, and separate experiments clearly established that the taneously ta the voltage signal, i.e. on the time-scale of these 1OV, lOOns pulse switches the device into a nonvolatile experiments, at least, there is no delay time involved in the conducting state. This latter point is also evident from Fig. 2 response of the switch. During the switching pulse the current which shows that the ON-state characteristic passes through through the device may reach 100 mA or more, and it rises and 'the origin. The characteristics shown in Fig. 3a and b were falls too rapidly to be clearly recorded in the present experiments. The relatively slow current decay on the trailing taken from switching transients recorded on a fast storage edges of the current transients is determined by the time constants of the measuring circuit. It is estimated that the

1 4 energy absorbed during either transition is extremely low, typically in the range 10-6 to 10 8 J.

Fig. 2 Complete static current/voltage characteristics for formed p-n-i- a.Sl device, showing forward and reverse threshold voltage, VThF and VThP., respectively Preliminary experiments have been carried out on the effect of varying the height and duration of the switching pulse. As far as the pulse height is concerned the important factor is probably the overvoltage, i.e. the amount by which the pulse voltage exceeds the static threshold voltage, defined in Fig. 2. For the devices reported here Vrh, (see Fig. 2) is typically 5 15 25 voltage y 4 to 6 V, and it has been observed that if the height of the 8 V, the pulse duration must be Fig. 1A Initial static current/voltage characteristics in forward switching pulse is reduced to direction (curve a) and in reverse direction (curve b) increased to about 400 ns to ensure that the device is switched into a permanent ON state. It is not clear at the moment, however, whether this effect is the direct result of the lower pulse height, or a consequential reduction in the current during the switching transient.

4 Discussion The mechanisms underlying the switching phenomena described above are unknown at present, and it does not seem profitable to speculate about their possible nature on the basis of the above preliminary observations. More detailed experimental results are required to establish the critical features and parameters of the device, e.g. the relationship between switching performance and the thickness of the various layers. It is worthwhile, however, to draw some comparisons with other related switching devices. First, it is relevant to note that, although threshold switching has been observed in a-Si [4], this is the first report of memory switching in an a-Si device. Also, threshold switching is well known in related structures fabricated on monocrystalline silicon, i.e. in the so-called MISS device which has the struture g'-n4 or np4 [5.8]. The Mayer in these devices is usually an SiO2 film thin enough to pass significant tunnelling currents (i.e. < 40 A) but it may be significant, in the present context, that MISS divices can also be fabricated in 'all-Si' form, using polycrystalline Si as the i-layer [8b]. MISS devices switch to a 0.2 U.4 UD voltogeV nonpermanent ON state when the p4.n (or n'p) junction is forward biased, i.e. the same polarity that causes memory Fig. IS Current/voltage characteristics in formed conducting state (curve c) switching in the a-Si structures reported here. In the MISS lEE PROC. Vol. 129, Pt. I, No. 2, APRIL 1982 52 395

device the switching action is associated with minority-carrier for the MISS device well-defined voltage-dependent delay times injection and accumulation at the interface of the Mayer, of at least 100 n are commonly observed. normally leading to punch-through to the injecting contact In the field of amorphous semiconductors much attention which causes the device to switch ON. It is conceivable that has been given over the past 10-15 years to memory switching in the a-Si structure the initiation of nonvolatile memory devices fabricated from multicomponent chalcogenide glasses switching involves similar processes, but it must be noted that in which the reversible memory action is associated with the the OFF-'ON transition time, required to set the a-Si device growth and destruction of a crystalline filament [9-121. The into a permanent ON state, is at least an order of magnitude switching phenomenon in the a-Si structures reported here is faster than the delay time observed in MISS switches for clearly very different, at least operationally. The most obvious threshold switching. Moreover, as mentioned earlier, on the difference is the completely nonpolar character of switching in time-scale used in the present experiments there is no observable chalcogenide glass devices, in contrast to the marked polarity- delay time in the switching action of the a-Si device, whereas dependence of the a-Si memory switches. More importantly, perhaps, the switching times for the a-Si device are much faster lOOns (lOOns or less for either the OFF-ON or ON-OFF transition, compared with at least several milliseconds in chalcogenide devices) and the energy involved in the switching process is considerably lower (1 izJ or less compared with I mJ or more). Also, chalcogenide glass devices require voltage pulses of magnitude 25-30 V (for a device 1 .Lm thick) to establish the ON state, and very often they need 100 or more 'forming' cycles before resonably stable operation is achieved. This again contrasts with the operation of the a-Si memory switch, in which (for a total device thickness of 1;Lm) there is a single forming step with a threshold voltage of about 30 V, and for all subsequent operations the forward threshold (VThF) is 4 to 6 V. The closest parallel to the a-Si devices described in this paper seems to be the observation of memory switching in heterojunctions of n-type ZnSe grown epitaxially on p-type (single- crystal) Ge substrates, reported by Hovel and Urgell [131 - The ZnSe-Ge heterojunction devices are polar and the transition times for the OFF-ON and ON-vOFF operation are both in the region of 100 n or less. Similar but not so well- substantiated memory switching characteristics have also been -6 E briefly reported in devices fabricated by forming Schottky contacts on n-type GaAs and Si (single crystal) [14] - Hovel and Urgell have tentatively and qualitatively explained switching in the heterojunction by a model involving the filling and emptying of traps in the ZnSe, with the formation of a current filament in the ON state. Even in this case of superficially similar characteristics however, there are notable differences. Most 100 n significantly, the polarity required for switching in the ZnSe-Ge heterojunctions is the opposite to that found in the a-Si devices and the OFF-ON threshold voltage for the ZnSe.Ge switch decreases substantially with temperature (from about 1 V at 200K to less than 0.1 V at 400K), whereas VrhF for the a-Si devices investigated so far is at the most only weakly temperature dependent. The MNOS and FAMOS devices which, as mentioned in Section 1, are currently used in the semiconductor industry for nonvolatile memory applications, are both basically field-effect transistors which rely on charge injection and storage in the gate insulator(s) for their nonvolatility [15, 161. They are therefore inherently slow, the WRITE and ERASE voltages are relatively large (e.g. 20-30V) and they degrade significantly after cycling through 10 to 106 WRITE/ERASE (ON/OFF) operations. Finally, it should be noted that in all switching devices, 4 except those of the field-effect type mentioned latterly, it is almost certain that the ON state, whether permanent (nonvolatile) or temporary (as in a threshold switch), involves the formation of a current filament. The present experimental evidence, although indirect, suggests that a conducting filament -2 is also formed in setting the a-Si switch into its ON state. No information is available at the moment on the size of the Fig. 3 Dynamic switching characteristics of p-n-i a-Si device, using filament, but it is worth noting that if it is assumed to be 10 V, 1 00 n pulse superimposed on 1 V. — 0.5 pa pulse (drasm from I 1Am in diameter, a fairly typical value, the observed ON-state oscilliscope traces) conductance implies a conductivity in the region of 10 4 — a OFF-'ON transition (p-layer positively biased) S m for the filament 'material', i.e. a conductivity typical t, ON-'OFF transition (p-layer negatively biased) 105 lEE PROC. Vol. 129, Pt. I, No. 2, APRIL 1982 396

of semimetals. It could perhaps be relevant that conductivities YAGUE, A., and SEBBA, J-P.: 'Inversion-controlled switching mechanism of m.i.s.s. devices', lEE Proc I, Solid-State & Electron approaching 104 S m have recently been observed in doped Devices, 1980, 127, (3), pp. 119-125 microcrystalline films of Si [17]. 7 SIMMONS, J.G., and EL.BADRY, A.A.: 'Switching phenomena in metal.insulatorn/p structures; theory, experiment and applications', Acknowledgments Radio & Electron. Eng. 1978, 48, pp. 215-226 5 8a KROGER, H., and WEGENER, H.A.R.: 'Bistable impedance states The authors thank J.J. Delima (of the Department of Electrical in MIS structures through controlled inversion', AppI. Pays. Len., Engineering, University of Edinburgh) and S. Kinmond (of the 1973, 23, pp. 397-399 8b KROGER, H., and WEGENER, H.A.R.: 'Controlled-inversion Carnegie Laboratory of Physics, University of Dundee) for transistors', Appl. Phys. Lett., 1975, 27, pp. 303-304 valuable assistance with the experimental work. 9 OVSHINSKY, S.R.: 'Reversible electrical switching phenomena in disordered structures', Phys. Rev Lett., 1968, 21, pp. 1450-1453 References 10 COHEN, M.H., NEALE, R.G., and PASItIN, A.: 'A model for an amorphous semiconductor memory device', J. Non.Cryst. Sot., 1 OWEN, A.E., SPEAR, W.E., SARRABAYROUSE, G., and LE 1972, 8-10, pp. 885 -891 COMBER, P.G.: 'Improved semiconductor device'. British Patent 11 STEVENTON, A.G.: 'An interpretation of "lock-on" in amorphous Application 8124248, 7th Aug. 1981 chalcogenide switches' in STUKE, 3., and BRENIG, W. (Eds,): 2a JOHNSON, W.S., KUHN, G.L., RENNINGER, A.L., and PERLE- 'Amorphous and liquid semiconductors, Vol. 1' (Taylor & Francis, GOS, G.: '16.K EE-PROM relies on tunnelling for byte-erasable London, 1974, pp. 675-680 program storage' s Electronics, Feb. 28th 1980,53, pp. 113-117 12a OWEN, A.E., and ROBERTSON, J.M.: 'Electronic conduction and 2b UCHIUMI, K., and MAKIMOTO, T.: '16-K EE-PROM keeps MNOS switching in chalcogenide glasses', IEEE Trans., 1973, ED-20, in the running', ibid., Feb. 24th 1981, 54, pp. 154-156 pp. 105-122 2c POSA, J.G., and BERESFORD, K.: 'Technology up-date; semi- 12b OWEN, A.E., ROBERTSON, J.M., and MAIN, C.M,: 'The threshold conductors and memories', Ibid., Oct. 20th 1981, 54, pp. 117-138 characteristics of chalcogenide-glass memory switches', I. Non.Cryst. 3 SPEAR, W.E., and LE COMBER, P.G.:'Electronic properties of sub- Sol., 1979, 32, pp. 29-52 stitutionally doped amorphous silicon and germanium', Phil. Mag., 13a HOVEL, H.J.: 'Switching and memory in ZnSe-Ge heterojunction', 1976, 33, pp. 935-949 AppL Phys. Lett., 1970, 17, pp. 141-143 4a DEY, S.K., and FONG, WJ.T.: 'Conduction processes and threshold 13b HOVEL, H.J., and URGELL, JJ.: 'Switching and memory charac- switching in amorphous Si films', I. Vac. ScL Technol., 1979, 16 teristics of ZnSe-Ge heterojunction', J. AppI. Phys, 1971, 42, pp. pp. 240-243 5076-5083 4b DEY, Sit.: 'Electrothermal model of switching in amorphous 14 MOSER, A.: 'Bistable switching in metal-semiconductor junctions', silicon films', ibid., 1980, 17, pp. 445-448 Appl. Pays. Lett., 1972, 20, pp. 244-245 So YAMOMOTO, T., and MORIMOTO, H.: 'Thin-m.i.s.-structure Si 15 CHANG, JJ.: 'Theory of the MNOS memory transistor', IEEE negative-resistance diode', Appl. Pays. Lett., 1972, 20, pp. 269-270 Trans., 1977, ED-24, pp. 511-518 Sb YAMOMOTO, T., KAWAMURA, K., and SHIMIZU, H,:'Silicon 16 FROHMANN-BENTCHKOWSKY, D.: 'Memory behaviour in a p.n4nsulator (p.ts4.M) devices', Solid-Stare Electron., 1976, 19, floating-gate avalanche-injection MOS (FAMOS) structure', AppL pp. 701-706 Phys Lett., 1971, 18, pp. 332-334 6a BUXO, 3., OWEN, A.E., SARRABAYROUSE, G., and SEBAA, J-P.: 17 SPEAR, W.E., WILLEKE, G., LE COMBER, P.G. and FITZGE. 'The characteristics of metal-thin insulator-n-p silicon switching RALD, AG.: 'Electronic properties of microcrystalline silicon Pays. Colloque C4, devices', Rev. Phys. Appi., 1978, 17, pp. 767-770 flints prepared in a glow discharge plasma', I. 6b SARRABAYROUSE, G., BUXO, 3., OWEN, Al., MUNOZ - 1981, 42, (10), pp. 257-260

54 lEE PROC.. Vol. 129, Pt. I, No. 2, APRIL 1982 397

_'73 J.urn&I of NonCryslIhne Solids SQ & 60 1983) 1273-1280 North-lkilland Publishing C..inpany

MEMORY SWITCHING IN AMORPHOUS SILICON DEVICES

A.E. OWEN, P.G. LE COMBER' , W.E. SPEAR and J. HAJTO

• Department of Electrical Engineering. University of Edinburgh. King's Buildings, Edinburgh EH9 3JL, Scotland, U.K. Carnegie Laboratory of Physics. University of Dundee. Dundee 001 4KM, Scotland, U.K.

Recent experimental observations on high-speed memory switching in p-n-i structures of amorphous silicon are described. Particular emphasis is given to the first switching operation of a virgin device which is effectively a unique forcing process for all subsequent operations. There isa charcteristlC delay for forming, varying over ten orders of magnitude from - 10' to - 10 -e8 S. Evid nce Is presented to show that forming is a charge controlled process which occurs at a constant field across the n-layer, but details of the switching mechanisms in the a-Si p-n-1 devices remain obscure.

1. INTRODUCTION Widespread interest in the phenomena of threshold (monostable or volatile) and memory (bistable or non-volatile) electrical switching in chalcogenide glasses provided the impetus for much of the polneering research in the field of amorphous semiconductors in the late 1960s and early 1970s. 1.2 There are still some controversial features but by and large generally acceptable models for both threshold and memory switching in chalccsgenide glasses are now reasonably well established. 3,4 By contrast, and despite the almost unprecedented growth of research and development on amorphous silicon (a-Si) since the mid-1970s, practically nothing has been reported on electrical switching in that material. Some cursory observations of threshOld switching in homogeneous films of evaporated a-Si, with very tentative evidence for memory behaviour, were described by Feldman and I400rjani contemporaneously with some of the early literature on switching in chalcogeflide glasses. 5 There seems to have been no further activity until the work of Dey and Fong who observed threshold switching in 6.7 homogeneous films of evaporated a-Si with titanium contacts. Much more recently, den Beer has also reported threshold switching in a-Si, but specifically in hydrogenatedro n-I-n + structures (i stands for "intrinsic - ). To the authors knowledge, the only other relevant paper, again on hydrogenated a-Si, is the slightly prior publication of Gabriel and Adler 9 who searched unsuccessfully for switching in homogeneous films with molybdenum They concluded that unlike the chalcogenide glasses, hydrogenated contacts.

Of Japan 0022.309318310000_00001S03.00 0 1983 North-Holland/Physical Society 398

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a-Si does not have the fundamentally requisite properties for reversible switching behaviour. The present paper is concerned with studies of the electrical switching behaviour of heterogeneous structures of a-Si which evolved from a collaborative project between the amorphous semiconductor groups at the Universities of Dundee and Edinburgh in the U.K. A preliminary description 10 of this work has already been published and in the following sections a more detailed account of experimental observations will be presented with emphasis on the initial switching operation of freshly prepared (and previously unswltched) devices. It Is believed, on the basis of the present evidence, that the first operation is a charge controlled process. The research Is still in its early stages however and the switching mechanisms remain obscure.

DEVICE STRUCTURE AND FABRICATION A variety of configurations has been Investigated but the majority of the observations to date, and all of the results reported in this paper, have been obtained on devices having a-Si doped and intrinsic (i) layers in the sequence pt-n-i. The a-Si was deposited by the glow discharge technique, with gas phase doping, as described by Spear. Stainless steel substrates were generally used and the total thickness of the deposited a-Si layers was between 0.5 and 1.0 LAm. After completion of the a-Si deposition a series of gold (Au), aluminium (Al) or nichrome (NICr) dots (up to approximately 1 me in diameter) was evaporated onto the surface of the samples and the top contact was completed either by a probe or by a thin-wire attached to the metal dots with conducting silver paste.

STATIC CURRENT-VOLTAGE CHARACTERISTICS OF VIRGIN DEVICES Typical current-voltage (I-V) characteristics for a freshly prepared (unswitched) device are illustrated in figure 1(a) in both the forward and reverse directions (the forward direction is defined such that the substrate, and hence the pt -region, is positively biased). It must be emphasised here that these measurements were taken "by hand", point-by-point, in a manner which required a few seconds for each point to be measured. The significance of this remark will become apparent in the next section. In the forward direction there is a region of ohmic behaviour over an appreciable voltage followed by an abrupt change to a markedly non-ohmic region until, at the point indicated by the arrow, the device was unstable and it was impossible to continue with point-by-point measurements. The change from ohmic to nonohmic behaviour is more clearly apparent in the conductivity vs. field plot of 399

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FIGURE 1(a) Static current-voltage characteristics of a virgin (unformed) device. The positive+ quadrant corresponds to the p _ layer positively biased.

FIGURE 1(b) The data for 300C in figure 1(a) plotted in the form log.conductivity vs. field, and Illustrating the change from ohmic to nonohmic behaviour. I V.

of figure 1(b). As ambient temperature increases the onset of non-ohmic behaviour moves to lower voltages. In the reverse direction there Is also an initial ohmic region (and this part of the I-V curve is symmetrical) but the change to non-ohmic behaviour is much more gradual and eventually the device breaks down.

4. FORMING - STATIC CHARACTERISTICS As noted in the previous section, during point-by-point measurements under forward bias, at room temperature, the a-Si p+-n1 device tends to become unstable when the applied bias is about 25 V. At higher temperatures the instability occurs at lower voltages, as indicated by the arrows in figure 1. On attempting to increase the voltage still further the device switches into a low resistance ON-state. Typical I-V characteristics for both polarities in the ON-state are shown in figure 2; the I-V curve is ohmic, it extrapolates through the origin (i.e. the ON-state is permanent) and it is slightly asymmetrical. Note that the current is now measured in milliamps and the voltages across the device are small. On increasing the voltage In the forward direction the ON-state current continues to increase apparently indefinitely, subject only to any current limiting resistor, and the device is eventually destroyed, presumably by Joule heating. In the reverse rtIU

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FIGURE 3 Complete static current-v2ltage characteristics of a formed a-Si p -n-i device, showing the forward and reverse threshold voltages, VThF and VThR respectively. direction however another instability is observed and at about -1 V (typically) the device switches back Into a high resistance OFF-state. The OFF-ON transition may now be repeated by biasing in the forward direction but on the second and all subsequent switching operations the forward threshold voltage occurs at a much lower voltage than the first operation e.g. at - 5 V conpared with the 25 V observed under the conditions obtaining for the measurements shown in figure 1. The first OFF-OH transition, occurring at a relatively high voltage, seems therefore to be unique and by analogy with the usage of switching in chalcogenide glasses, it is referred to as "forming". + The formed a-Si p -n-i device may be cycled through ON and OFF states by a sequence of biasing in forward and reverse directions with critical points at VThF and, in the reverse direction, VThR. A complete and typical characteristic obtained on a curve tracer is illustrated in figure 3. On occasions the device appears to go through a number of intermediate OH-states during the OFF-ON transition and this is indicated in figure 3. In addition, there is often an observable and appreciable region of negative resistance in the reverse biased OFF-state characteristic of a formed device (also indicated in figure 3). 401

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10 FIGURE 5 Illustrating the variation of the -S forming voltage, V, on applying a FIGURE 4 voltage pulse, with the thickness of The delay time for forming, t0, as a the n-layer. function of applied voltage, at three different temperatures. See text for details.

S. FORMING - DYNAMIC CHARACTERISTICS The forming process, i.e. the first OFF-ON transition, does not occur instantaneously when a voltage step or pulse is applied to the a-Si p-n-i device. Initially there is a delay time. t, during which the device current remains essentially constant at the OFF-state value appropriate to the voltage across the device. Only after this delay does the current begin to increase and it then rises essentially instantaneously to its ON-state value. The forming delay time is an extremely sensitive function of the applied voltage and typical data, obtained at three temperatures, are illustrated in figure 4. The forming delay time varies over nearly ten orders of magnitude from a few hundred seconds at low forming voltages to about 10 ns at high voltages. In particular, at a temperature dependent critical voltage VCR there is virtually a discontinuous change in t0 as a function of bias. The voltages VCR indicated in figure 4 are roughly coincident with the voltages at the points of instability marked by-the arrows in figure 1; V CR also corresponds to the forming voltage which would be obtained in an experiment on a virgin device with a curve tracer. Note in figure 4 that above and below V CR the delay time tends to a value which seems to be approximately independent of both voltage and temperature, for the particular results illustrated t 0 is in the range 102_103 s for V < VCR and in the range 10-100 ns for V > VCR. 402

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i 4. 1t:.iTt

es L .WLC 223 ° 3', - 0) 1 I I I 20 4.0 60 pUSI HIIONT IV)

FIGURE 7 The ratio of charge to n-layer FIGURE 6 thickness (Q/dn) as a function The OFF-ON switching transient of a of forming potential. formed device, drawn from an oscilloscope trace. TOP: Applied bias; vertical scale 2 V per division, horizontal scale 20 ns per division. BOTTOM: Device current; vertical scale approximately 6 mA per division, horizontal scale 20 ns per division The results plotted in figure 4 for V VCR correspond of course to voltages less than the point of instability indicated in figure I. There does appear to be a lower limit to the forming voltage however and present results indicate that the limiting voltage coincides witi the bias at whiCh the I-V characteristics change from their ohmic to non-onmiC behaviour (see figure 1 and section 3). Several experiments have shown that virgin devices fall to switch (form) even if held for many hours at a forward bias only just below the non-ohmic region. In other words, forming occurs at any forward bias within the non-Ohmic region of the 14 characteristics but at voltages below the point of instability to is comparatively lone. It must also be emphasised again that the device current remains constant at its OFF-state magnitude during the delay time, even when t 0 is 130 s or more. Preliminary experiments have been carried Out to determine the effect of increases device geometry on the forming voltage VF. It was found that v devices linearly with the thickness of the n-layer and data for typical pfl1 are plotted in figure 5; V in this case was measured by a curve tracer and as already mentioned therefore it coincides with the volt3ge VCR indicated fl Note that the results do not extrapolate to zero voltage for zero figure 4. n-layer thickness. 403

!, 1 I. 1 I i, lung ui a,'!' 'liii .'l .1 t. sh. , it t1 d,

6. DYNAMIC SWITCHING OF FORMED DEVICES The principal experimental features of the pulsed operation of formed a-Si A representative diagram drawn pt -n-i devices have already been described. transition on applying a fast voltage from an oscilloscope trace of the OFF-ON ramp is shown in figure 6. The main points to note are as follows: When biased with a pulse in the forward direction the device switches ON provided the pulse height exceeds the static threshold voltage VThF as defined in figure 3. On the time scaie of 1 ns or less, there is no observable delay time in the response of a formed device. The device current follows the applied voltage instantaneously on this timescale. Provided the pulse is long enough the ON-state i s permanent and the pulse duration required for switching to a memory state increases as the pulse In typical cases a permanent ON-state is height decreases towards VThF. obtained with pulse durations of a few tens of nanoseconds and magnitude 5 '1 in excess of VThF. Similarly, on tasing in the reverse direction with a pulse of height V_hR the device switches from OFF -ON and again there is no observable delay in response. The ON-state appears to be truly permanent. No detectable changes have been observed in devices stored in their ON-state for a year cr more.

7. DISCUSSION The authors earlier paper on the new a-Si p-n-i device focusses mainly on the properties of the formed switch, particularly its dynamic response, and the operational characteristics were compared witn tnose of other switching devices. 10 The switching mechanismS in the new device ,-emalfl obscure and much At this staqe therefore there is more detailed research needs to be done. little to add to the problem of memory switching in formed devices but recent results do provide some insight intO the forming process. It should be noted first tflat the charge which flows tnrougr or into the device during the delay time (i.e. during forming) is an approximately constant In fact, as illustrated in function of pulse height for voltages VCR. figure 7, the ratio of charge to n-layer thickness (t,'d) seems to be roughly independent of both pulse height and the thickness of the n-layer (sample 403 has an n-layer thickness of 215 nm, sample 349 of 445 nm and 223 in figure 7 By contrast both the current and the power vary markedly with of 780 nm). the pulsing conditions. These observations suggest very strongly that forming is an electronic in terms process associated with a critical charge density in the n-layer. 404

1280 A. E. Qt'eii etal. / Memory switching in dmnrphous silicon dc'gcts of electrons the critical charge is .. 1.2 x 10 19 electrons cm-3. Moreover, the results in figure 5 imply that forming occurs at a constant field across the n-layer. For the particular devices Shown the field is 2 x 10 V cm- 1, corresponding to a geometric charge which is rather less than the critical charge. Typically, the ratio of the critical charge to geometric charge varies from about 30 in sample 403 (dn 215 nm) to about 100 in sample 223 io_l cm2 V 1 s, the (dn 780 nm). Finally, assuming a carrier mobility of transit times across the n-layers are in the range of 1-4 ns for the samples shown. This is not inconsistent with the delay times for V VCR (see figure 4).

ACKNOWLEDGEMENTS The authors are grateful to the Venture Research Unit of British Petroleum International Ltd. for a grant supporting the research described in this paper. We also thank Dr. F.F. Carasco, Mr. M.C. Holland (both of Dundee University) and Mr. W.K. Choi for assistance with the experimental work and for enthusiastic help in preparing this paper.

REFERENCES S.R. Ovshinsky. Phys. Rev. Lett. 21, (1968). 1450. A.E. Owen and J.M. Robertson. IEEE Trans. ED-20, (1973). 105. 0. Adler, K.H. Hemisch and N.F. Mott. Rev. Mod. Phys., 50. (1978), 209. A.E Owen, J.M. Robertson and C.M. Main. J. Non-Cryst. Sol.. 32, (1979), 29. C. Feldman and K. Moorjani. J. Non-Cryst. Sol., 2, (1970). 82.

S.K. Dey and W.T.J. Fong. J. Vac. Sd. and Technol., 16. (1979), 240.

S.K. Dey. J. Vac. Sci. and Technol.. 17, (1980), 445. M.C. Gabriel and D. Adler. J. Non-Cryst. 501., 48, (1982), 297. W. den Boer Appl. Phys. Lett. 40. (1982), 812. A.E. Owen, P.G. Le Conker, G. Sarrabayrouse and W.E. Spear. lEE Proc. (Part I. Solid-State Electron 0ev.), 129, (1982). 51. W.E. Spear. Adv. in Phys.. 26, (1977), 811. aJ5

SEMICOI4oqj4Cr0SS AND SEMIMETALS. VOL. D. PART 0

CHAPTER 15 Electronic Switching in Amorphous Silicon Junction Devices

P. G. LeComber A. E. On

CARNEGIE LABORATORY OF DEPARTMENT OF ELECTRICAL PHYSICS ENGINEERING UNIVERSITY OF DUNDEE WIVER3fTY Of EDINBURGH DUNDEE. SCOTLAND EDINBURGH SCOTLAND

W. E. Spear I. Hajto

CARNEGIE LABORATORY OF DEPARTMENT OF ELECTRICAL PHYSICS ENGINEERING UNIVERSITY OF DUNDEE UNIVERSITY OF EDINBURGH DUNDEE. SCOTLAND EDINBURGH. SCOTLAND W. K Choi DEPARTMENT OF ELECTRICAL ENGINEERING UNIVERSITY OF EDINBURGH EDINBURGH. SCOTLAND

INTRODUCTION ...... 275 PREViOUS WORE. ON ELECTRICAL SWITCHING IN AMORPHOUS SiLICON ...... 277 (U. Davwg STRUCTURE AND FABRICATION ...... 279 STATIC CURRENT-VOLTAGE CHARACTERISTICS OF VIRGIN DEVICES...... 279 FORMING: STATIC CHA1ACTUTICE ...... 281 FORMING: DYNAMIC CHAIACTUTI ...... 282 DYNAMIC SWITCHING OP FORMED DEVICES ...... 284 286 D!SCUSMON OF POSSIBLE SWITCHING MECiSAN8MS . REFERENCES ...... 288

1. IntroductIon Electronic switches are solid-state devices that can be changed from a nonconducting OFF state to a conducting ON state by an appropriate electrical signal. The importance of such devices in the development of solid-state digital electronics has been enormous and is likely to remain so in the foreseeable future, especially with the increasing demand for memory elements. 275 Capp* C 1954W A=&=k PI. I. AD rWM of ,oducbos a HEN O.I2.752110.X 406

276 P. G. LECOMBER et al. The present article is concerned with two-terminal switching devices. Generally, these have one or the other of the two types of current - voltage (I- V) characteristics shown schematically in Fig. la,b. In FF4 lathe device Switches from its OFF to its ON state at a critical threshold voltage V, but if the ON state conditions fall below a critical holding point (lb. Vb), the device reverts spontaneously to its OFF state. Devices of this kind are called threshold switches; they are nonpermanent, or "volatile," and they always revert to the OFF state in the absence of an appropriate bias. In FF4 lb there is again a critical switching voltage for the OFF to ON transition, but both ON- and OFF-state characteristics extrapolate through the I- V origin. Devices of this kind are therefore permanent, or nonvolatile, and they are called memory switches.. Memory devices can remain in either the ON or OFF state more or less indefinitely, whether or not a bias is applied and the ON to OFF transition is usually triggered by a current pulse. By the early 1970s many examples of threshold and memory switching had been reported in homogeneous thin films of a variety of amorphous materials, including simple oxides, transition-metal oxides, elemental selenium, and boron. By far the most important materials, however, were the chalcogenide glasses in which, depending on composition, reproducible characteristics of the kind illustrated in Fig. la(threshold switching) or Fig. lb (memory switching) may be obtained (Ovshinsky, 1968; Owen and Robertson, 1973). A substantial specialist literature on electrical switching

(0)

I1%

I V - vT

FIG. I. Current - voltage ctia,acteriicS for (a) threshold switching and (b) memory switch- in& 407

15. ELECTRONIC SWITCHING IN a-Si JUNCTION DEVICES 277 in the chalcogenide glasses has developed, and although there are still some controversial features, generally accepted models of at least a semiquantita- tive kind are now reasonably well established for both types of switching (Adler et al.. 1978; Owen et at. 1979). This article is concerned with recent studies of a rather different switching behavior in heterogeneous structures of amorphous hydrogenated silicon, which evolved from a collaborative project between the authors' groups at the Universities of Dundee and Edinburgh in the United Kingdom. A preliminary account of the work has already been published (Owen et at. 1982), and in the following sections a more detailed description of the experimental observations will be presented. Before proceeding, however, it is worth noting that, by contrast with the chalcogenide glasses; and despite the almost unprecedented growth in research and development on amorphous silicon (a-Si) since the mid-1970s, very little has been reported on electrical switching in the latter material. To put the recent observations in context it is pertinent therefore to review briefly the relatively few previous reports of switching in a-Si.

II. Previous Work on Electrical Switching in Amorphous Silicon Some cursory observations of threshold switching in homogeneous films of a-Si, with very tentative evidence for memory behavior, were described by Feldman and Moorjani (1970) and Mooijani and Feldman (1970) contemporaneously with some of the early literature switching in chalcogenide glasses More detailed experiments on the same structures were reported later (Feldman and Charles, 1974; Charles and Feldman, 1975). The authors studied vacuum-evaporated films of a-Si in the range 0.3-2.0 pm thick, fitted with titanium electrodes. They also, incidentally, made similar observations on evaporated films of germanium, boron, and boron plus carbon. As threshold switches these a-Si structures had threshold voltages Vof5-10 V,OFF resistances inthe range 1-30kZ and anON resistance of about 100 CL In common with the chalcogenide alasses there was a delay time before switching of 20- 50psec or more (at room temperature) and the actual switching time was at least several microseconds. Feldman et at did not, however, report any initial "forming" process, unlike the situation in chalcogenide glass switches (see also Part V). As already noted, there was some tentative indication of memory switching, but that feature was apparently not substantiated. Feldman and Charles (1974) interpreted the results in terms of a simple and qualitative electrothermal model involving the formation of a conducting filament; a similar, more quantitative model has been developed for switching in chalcogenide glasses (Owen et al.. 1979). The work of Feldman and his colleagues, which originated in the early 278 P. G. LECOMBER et al.

1970s, seems to be the only investigation of switching in a-Si until the later studies of Dey and Fong (1977, 1979) and Dey (1980). These authors report results very similar to those of Feldman etal. They studied thin films of a-Si in the range 0.3- 1.5 pm thick, deposited by electron-beam heating in a vacuum evaporator. Titanium contacts were again used, either in the form of evaporated films or as probes. Dey and Fong reported only threshold switching, with 1- V characteristics similar to those in Fig. Ia; they did not mention any evidence for memory behavior. In contrast to Feldman et at, however, Dey and Fong did observe forming effects; that is, the initial threshold voltage was relatively large but it decreased to a more or less constant value after a number of switching cycles. In Dey and Fong's devices the threshold voltage varied systematically from about 6 V for the thinner films (0.3 pm) to about 9 V for the thicker films (' 12pm). The delay time before switching was in the range 2-60 psec, varying in a systematic way with pulse height, pulse duration, and repetition rate, again in a manner very similar to switching in calcogenide glasses (Adler et at. 1978; Owen et at. 1979). Dey and Fong also interpreted their results in terms of a simple one-dimensional electrothermal model, but one developed a little more quantitatively than that by Feldman et al. ft should be noted that both Feldman etal. and Dey and Fong used a-Si films deposited by vacuum evaporation. This probably accounts for the relatively Low OFF-state resistances which they both found (100-100 kfl). It is now well established that vacuum-evaporated a-Si is a very different material from the hydrogenated form of a-Si obtained, for example, by the carefully controlled glow-discharge decomposition of silane (e.g., Spear, 1977). Three papers concerned specifically with switching in hydrogenated amorphous silicon by Gabriel and Adler (1982), den Boer (1982), and Owen et al. (1982) appeared almost concurrently early in 1982, each reporting very different effects observed in different a-Si structures. Our own work (Owen et al.. 1982), including recent results, is described in detail in the following sections. Den Boer studied n'- i- n structures of a-Si prepared by the glow-discharge decomposition of SIRI ("1" stands for "intrinsic" or undoped mate- ial). The n' layers were 50nm thick and prepared byadding 1%PH 3 tothe S1H4 gas flow the i layer in different devices ranged in thickness from 2.5 to 5 pm. Den Boer found that the n4 -i-n devices functioned as threshold switching devices with nonpolar characteristics similar to those sketched in Fig. Ia. For the first switching cycle the threshold voltage was in the range 40-100 V, but for all subsequent operations it was only 10-35 V, depending on the i layer thickness (as the i layer thickness increased the threshold voltage also increased). The OFF-state resistance of j... switches 409

15. ELECTRONIC SWITCHING IN a-Si JUNCTION DEVICES 279 was about 1 M(, and the ON-state resistance about I W. There was again an observable delay time before switching, ranging from a few microseconds when the applied voltage was about 8 V greater than VTb to about a millisecond for voltages within I V of V. The • - I threshold switches could be cycled through at least 10' stable switching operations. Den Boer also compared structures with chromium or a combination of chromium and n4 contacts (i.e.. Cr- ,i4 -i-Cr and Cr-i-Cr). The former had rectifying characteristics while the latter switched but were very unstable. Gabriel and Adler (1982) prepared their a-Si films by sputtering from a polycrystalline silicon target in an argon - hydrogen plasma. In all cases their devices were notionally homogeneous thin films of intrinsic a-Si: H with molybdenum contacts. The samples were fabricated under a wide range of deposition conditions in two sputtering systems, and although results from some of the devices were rendered rather doubtful because of contamination problems, in no case did Gabriel and Adler observe any evidence of reversible switching. They concluded that, in contrast to the chalcogenide glasses, a-Si does not have the electronic and structural properties required for reversible switching,

III. Device Structure and Fabrication We turn now to the work on electronic switching carried out in the authors' laboratories. Although a number of different a-Si multilayer structures have been investigated, all the results discussed in the following refer to p - n - idevices deposited in this sequence by the Slow-discharge technique, with gas-phase doping. Stainless steel substrates were generally used and the total thickness of the deposited a-Si layers was between 0.5 and 1.0 pm. After completion of the a-Si deposition a series of gold (Au), aluminum (Al), or nichrome (NiCr) dots (up to approximately I mm in diameter) was evaporated onto the surface of the samples, and the top contact was completed either by a probe or by a thin wire attacbed to the metal dots with conducting Over paste.

IV. Static Current - Voltage Characteristics of Virgin Devices Typical current-voltage (I- V) characteristics for a freshly prepared (unswitcbod) device are illustrated in Fig. 2a in both the forward and reverse directions. (The forward direction is defined such that the substrate, and hence the p4 region, is positively biased.) It must be emphasized here that these measurements were taken "by hand," point by point, in a manner that required a few seconds for each point to be measured. (The significance of 410

280 P. G. LECOMBER et al.

'1 a I

0 10 20 30 V (Vi charscteristics ate vein (unformed) p - n-i device The Fmo. 2. (1) Static current -voha positive quadrant corresponds to a positive bin applied to thef layer. (b) The data for 30'C in (a) phoned in the form of Mg conductivity vasis vohage and Wuntin$ the flfP from ohmic to noncbmic behavior.

this remark will become apparent in the nezt sections.) In the forward direction there is a region of ohmic behavior over a limited voltage range followed by an abrupt change to a markedly nonobmic region until, at the point indicated by the arrow, the device is unstable and it is impossible to continue with point-by-point measurements. The change from ohmic to nonohmic behavior is more dearly apparent in the effective conductivity versus applied voltage plot of Fig. 2b. As the ambient temperature increases, the onset of nonohmic behavior moves to lower voltages. In the reverse direction, corresponding to a negative potential applied to the 0' side there is an initial ohmic region which is symmetrical for positive and negative voltages. However, the change to nonohmic behavior is much more gradual in the reverse direction, leading to the eventual breakdown of the device. 411

IS. ELECTRONIC SWITCHING IN a-Si JUNCTION DEVICES 281

V. Forming: Static Characteristics As noted in the previous section, during point-by-point measurements under forward bias, the a-Si e - n - i device tends to become unstable when the applied bias is about 24 V at room temperature. At higher temperatures the instability occurs at lower voltages, as indicated by the arrows in Fig. 2a. On attempting to increase the voltage still (Wiher, the device switches into a low-resistance ON state. Typical 1- V characteristics for both polarities in the ON state are shown in Fig. 3; the 1-. V curve is ohmic, it extrapolates through the origin (i.e., the ON state is permanent), and it is slightly asymmetrical. Note that the current is now measured in milliamperes and that the voltages across the device are small. On increasing the voltage in the forward direction the ON state current continues to increase apparently indefinitely, subject only to any current-limiting resistor, and the device is eventually destroyed, presumably by Joule heating. In the reverse direction, however, another instability is observed, and at about —1 V (typically) the

4 C

.tM( (V) Fzo. 3. Current-voltage dia,acteris*ia of a device in the ON state. at different tempera- turm in the forward and revene directions. The forward direction coiwods to e byer positively biased. (•) forward bias. 80C; (A) forward bias. 30C; (0) reverse bias WC (A) reverse bias 30C. 412

282 P. G. LECOMBER et al. device switches back into a high-resistance OFF state. The OFF-ON transition may now be repeated by biasing in the forward direction, but on the second and all subsequent switching operations the forward threshold voltage VTw occurs at a much lower voltage than the first operation, e.g., at 5 V compared with the 25 V observed under the conditions obtaining for the measurements shown in Fig. 2. The first OFF-ON transition, occurring at a relatively high voltage, seems therefore to be unique and by analogy with the usage of switching in chalcogenide glasses, it is referred to as "forming." The formed a-Si p-n-i device may be cycled through ON and OFF states by a sequence of biasing in forward and reverse directions with critical points at VnF and, in the reverse direction, V. A complete and typical characteristic obtained on a curve tracer is illustrated in Fig. 4. On occasions the device appears to go through a number of intermediate ON states during the OFF-ON transition, and this is indicated in the figure. In addition, there is often an observable and appreciable region of negative resistance in the reverse-biased OFF state characteristic of a formed device, denoted by N in Fig. 4.

VI. Forming: Dynamic Characteristics The forming process (i.e., the first OFF-ON transition) does not occur instantaneously when a voltage step or pulse is applied to the a-Si p4 -n-i device. Initially there is a delay time 1D during which the device current remains essentially constant as the OFF-state value appropriate to the voltage across the device. Only after this delay does the current begin to increase, and it then rises almost instantaneously to its ON-state value. The forming delay time is an extremely sensitive function of the applied forming

4 10

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. j : VT Ao. 4. Complete static current-voltage characteristics of a formed a-Si p'-n-i device, showing the forward and reverse threshold voltages, VTw and V. respectively. 413

283 15. ELECTRONIC SWITCHING IN a-Si JUNCTION DEVICES voltage I',, and typical data, obtained at three temperatures, are given in Fig. 5. The forming delay time varies over nearly 10 orders of magnitude, from a few hundred seconds at low forming voltages to about 10 nsec at high V. In particular, at a temperature-dependent critical forming voltage Vc L there occurs virtually a discontinuous change in 1D• The forming voltages V indicated in Fig. S are approximately the same as the voltages at the points of instability marked by the arrows in Fig. 2a; VcR also corresponds to the forming voltage obtained in experiment with a curve tracer, operated in ac mode at a frequency of 50 Hz. It can also be seen in Fig. 5 that above and below VcIt independent of both voltage and temperature; for the particular results V < VcR and ties between 10 illustrated, (D is in the range 1()3 - UP sec for and 100 nsecfor V> V. The results plotted in Fig. 5 for V < VcR correspond of course to voltages less than the point of instability indicated in Fig. 2a. There does appear to be a lower limit to the forming voltage, however, and present results indicate that the limiting voltage coincides with the bias at which the I- V characteristics change from their ohmic to nonobmic behavior (see Fig. 2b). Several

I-

-6

-8

0 10 20 30 40 VF (Vi FIG. 5. The delay time for toriiun$ to a function of applied funning voltage at three different Iempertures (•) 30C, (0) 80C, (+) 160C. 414

284 F. G. LECOMBEa et al. experiments have shown that virgin devices fail to switch (form) even if held for many hours at a forward bias only just below the nonohmic region. In other words, forming occurs at any forward bias within the nonobmic region is of the I- VcharacteflstiCs, but at voltages below the point of instability 1D comparatively long. It must also be emphasized again that the device current remains constant at its OFF-state magnitude during the delay time, even when 1D13 100 sec or more. Preliminary experiments have been carried out to determine the effects of device geometry on the forming voltage V,. It was found that V, increases devices linearly with the thickness d of the n layer. Data for typical p - n - i are plotted in Fig. 6. In this case V, was measured by applying a voltage ramp and its value coincides with the voltage Vca indicated in Fig. S. It can be seen that the results do not extrapolate to zero voltage for zero n layer thickness. which flows through or into the device during the The charge Q - f I di, forming delay time, has been determined for V,> Vca. Figure 7 shows that V, and d,, in this range of V, the ratio QJd, is approximately independent of for n layer thicknesses between 215 and 780 am. This could mean that forming occurs when a critical volume charge has accumulated in the n region.

VII. Dynamic Switching of Formed Devices The principal experimental features of the pulsed operation of formed eral. (1982). A a-Si p4-n-i devices have already been described by Owen representative diagram drawn from an oscilloscope trace of the OFF -. ON

20 'p

1.0 12 0.2 0.4 0.6 0.6 n-LAYER TMCKNESS 1101 V, on the thickness of the "layer. FIG. 6. The dependence of the forming voltage 415

IS. ELECTRONIC SWITCHING IN a-Si JUNCTION DEVICES 285

0.s 'I

:F 20 40 60 SO 0 VF (V) FIG. 7. The ratio of charge ton layer thickneas (Q/d.) as a function of f0ratiOg potential. d-2I5 nm(z),445 am(0),7SOnm(0). transition on applying a fast voltage ramp is shown in Fig. 8. The main points to note are as follows: (i) When biased with a pulse in the forward direction, the device switches ON, provided the pulse height exceeds the static threshold voltage V as defined in Fig. 4. (u) On the time scale of - I nscc or less, there is no observable delay time in the response of a formed device. The device current follows the applied voltage instantaneously on this time scale. (iii) Provided the pulse is long enough, the ON state is permanent and the pulse duration required for switching to a memory state increases as the

>0 3

(ci) Time (b) Time FIG. S. The OFF-ON switching transient of a formed device drawn from an oscilloscope tznco (a) applied bias vertical scale. 2 V per divmoo, horizontal scaler 20 neec per division; (b) device current, vertical scale: approximately 6 mA per divreon, horizontal scale 20 usc per division. 416

286 P. G. LECOMBER et al. pulse height decreases toward V. In typical cases a permanent ON state is obtained with pulse durations of a few tens of nanoseconds and magnitude -5 Vin excess ofVy . Similarly, on biasing in the reverse direction with a pulse of height > V the device switches from ON -. OFF and again there is no observable delay in response. The ON-state appears to be truly permanent. No detectable changes have been observed in devices stored in their ON state for a year or mote.

VIII. Discussion of Possible Switching Mechanisms The mechanisms underlying the switching phenomena in the a-Si devices are not understood at present; clearly, more data will be required to explain these exceptional properties. In the following we therefore draw some comparisons with other related switching devices and only briefly speculate on possible mechanisms. Although there is no observation of memory switching in analogous crystalline Si (c-Si) devices, threshold switching is well known in c-Si pr" -n-i, n-p-i, and related structures (Yamamoto and Monmoto 1972, Yamamoto El al.. 1976; Buxo et al.. 1978; Sarrabayrouse El al.. 1980; Simmons and El-Badry, 1978; Kroger and Wegener, 1973, 1975). The Mayer in these devices usually a Si02 film (:940 A), thin enough to P 5 appreciable tunneling currents, but it may be significant that these metal- insulator- semiconductor- semiconductor (MISS) devices can also be fabricated in an "all-Si" form using polycrystalline Si as the Mayer (Kroger and Wegener, 1975). These devices switch to a nonpermanent ON state when the f-n (or n4 -p) junction is forward biased, which is the same polarity producing the memory ON state in the a-Si structures. In the MISS device the switching action is associated with minontycaiTier injection from the p-n junction and accumulation at the interface of the i layer, normally leading to punchthrough to the injecting contact, which causes the device to switch ON. The values of the threshold voltage are similar to those observed in the a-Si forming process, and it is conceivable that for this operation the processes are similar. The experimental results described in Part VI, which suggest that the forming process is likely to be charge controlled, would not be inconsistent with this model. For the c-Si MISS p4 - n - i threshold device the values of threshold voltage are predicted (Simmons and El-Badry, 1978) to depend on (d - W)2, where da is the thickness of then layer and Wis the width of the depletion region, and for low donor concentrations this dependence is supported experimentally. In contrast, the results in Fig. 6 show that in the a-Si devices the forming voltage varies linearly with da . It is difficult to decide at present 417

15. ELECTRONIC SWITCHING IN a-Si JUNCTION DEVICES 287 whether this disagreement suggests a different mechanism or whether it arises from our attempt to extrapolate from a model developed for a crystalline threshold device to an amorphous memory junction. An alternative model, based on a regenerative process, has also been suggested for the c-Si MISS devices (Sarrabayrouse et at. 1980), taking into account a carner multiplication mechanism at the "inverted" Si-Si0 2 interface. The model correctly accounts for a number of MISS properties; for example, it predicts a threshold independent or d., which agrees with the observations on f-n-i MISS structures when the n layer is more heavily doped. But this model is also in disagreement with the data for the a-Si devices shown in Fig. 6, and just as for the "punchthrough" model, it is therefore difficult at the present stage to decide whether the regenerative model could be relevant to the understanding of a-Si memory switching. In the field of amorphous semiconductors much attention has been given over the past 10-IS years to memory switching devices fabricated from multicomponent chalcogenide glasses in which the reversible memory action is associated with the growth and destruction of a crystalline filament (Ovshinsky, 1968; Cohen et at. 1972; Steventon, 1974; Owen and Robert- son, 1973; Owen et al.. 1979). Although it is very Likely that in the a-Si devices some form of filament formation (not necessarily crystalline) is taking place in the OFF-ON transition, the switching phenomena are clearly very different from those in the chakogenides, at least operationally. The most obvious difference is the completely nonpolar character of switching in chalcogenide glass devices, in contrast to the marked polarity dependence of the a-Si memory switches. More important, perhaps, the switching and setting times for the a-Si device are much faster(— 10 nsec for either the OFF-ON or ON-OFF transition, compared with at least several milliseconds in chalcogenide devices) and the energy involved in the switching process is considerably lower (I pi or less, compared with I mi or more). Also chalcogenide glass devices require voltage pulses of magnitude 25-30 V (for a device 1 pm thick) to establish the ON state, and very often they need 100 or more "forming" cycles before reasonably stable operation is achieved. This again contrasts with the operation of the a-Si memory switch, in which (for a total device thickness of 1pm) there is a single forming step with a threshold voltage of about 30 V, and for all subsequent operations the forward threthold(V) is4-6 V. The closest parallel to the a-Si devices described in this paper seems to be the observation of memory switching in heterojunctions of n-type ZnSe grown epitaxiafly on p-type (single-crystal) Ge substrates, reported by Hovel (1970) and by Hovel and Urgell (1971). The ZnSe-Ge heterojunction devices are polar and the transition times for the OFF-ON and ON-OFF operation are both in the region of 100 nsec or less. Similar, but not so 418

288 P. G. LECOMBER et al.

well-substantiated, memory switching characteristics have also been briefly reported in devices fabricated by forming Schottky contacts on n-type GaAs and Si (single crystal) (Moser, 1972). Hovel and Urgell (1971), have tentatively and qualitatively explained switching in the heterojunction by a model involving the filling and emptying of traps in the ZnSc, with the formation of a current filament in the ON state. However, even in this case of superficially similar characteristics, there are notable differences. Most significantly, the polarity required for switching in the ZnSe-Ge heterojunction is the opposite to that found in the a-Si devices, and the OFF-ON threshold voltage for the ZnSe-Ge switch decreases substantially with temperature (from about 1 Vat 200K to Less than 0.1 Vat 400 K) whereas VTv for the a-Si devices investigated so far is at the most only weakly temperature dependent In addition, the memory state of the ZnSe-Ge heterojunctions is generally lost within a few weeks, whereas no change in the characteristics of the a-Si memory devices has been observed after storage over a period of 18 months. To conclude this chapter it is perhaps worthwhile to note that the switching phenomena observed in the a-Si memories are not the only nanosecond processes known for this material. Drift mobility studies, which show that electron transit times across about 1-pm-thick films are of the order of 10 nsec or so, have been known for over a decade (LeComber and Spear, 1970; Spear, 1983). More recently, it has been demonstrated that hydrogenated amorphous silicon can be used to modulate light at subnano- second speeds (Phelan et al, 1981; see also Chapter 13 by Phelan of this volume). In pulsed laser annealing of a-Si it has been suggested that the electron-hole plasma generated by the laser could produce rapid second. order phase transitions (van Vechten et al.. 1979). The challenge and excitement in understanding the a-Si memories Lies in discovering whether the origins of the fast switching processes are electronic, sinactural, or both.

AcowLEDOMENT The authors ale gi*tdui to the Venture Rrth Unit of British Petroleum International

PLC for a grant supportui$ the I aroh deaceibnd in this paper.

REFERENCES Adler, D., Henisch K. H., and Molt, N. F. (197$). Rev. Mad. P*s. 50. 209. jBu*o. L. Owen, A. L, Sarmbsi'Ouiej 0.. and Sebsa, J. P. (197$). Rev. Phys. e4pçt 17, 767. Chat, H. K. and Fe111. C. (1975). ). 4pp'. ?*ys. 46, $19. den Doer. W. (1982). Appt Phys. Lea. 40, $12. den Doer, W. (1983). lntezfereoce Effects and Space-Chaise-Limited Conduction in Amot- plious Silicon Devic&' D. Tech. Thetis, Technische Hceschool. Drift, The Netherlanth. September 1983. 419

15. ELECTRONIC SWITCHING IN a-Si JUNCTION DEVICES 289

Cohen. M. H.. Neale, R. G.. and Paskin. A. (1972). J. Non-Cryst. Solids 8/10, US. Dey, S. K. (1980). 1 Vac. Sci. Technol. 17.445. Dey. S. K.. and Fao.. W. 1.1. (1977). Proc. lat. Vacuum Co.*gress. 71h. andlnt. Corf Solia Surfaces. 3rd. Vienna, 1977. Vol. 3. Dey. S. K., and Foss. W. T. J. (1979). J Vac. Sd. Technol. 16, 240. Fe ld man .C.. and Charles, H.K.(1974). Solid Common. IS,55I. Feldman, C.. and Mooijani. K.(1970). J. Non-Cryst. Solids 2. 82. fGabeiel. M. C.. and Adler, D. (1982). 1 Non-Cryst. Solids 48,297. Hovel, H. J. (1970). AM. Phys. Len. 17, 141. Hovel, H. J., and Urell, J. .J.(1971). I. App!. Pluys. 42, 5076. Ksoer. H.. and Weencr. H. A. R. (1973). App!. Phys. Left. 23, 397. Kroer, H.. and Weoer. H. A. R. (1975). App. Phys. Len. 27,303. LeComber. P. G.. and Spear. W. E. (1970). Phys. Rev. Lest. 25, 509. v0oosjani, IL and FeMivn, C. (1970). J. Non-Cryst. Solids 4,248. Moser. A. (1972). App!. Phys. Lest. * 244. Ovshinsky, S. R. (1968). P*ys. Rev. Lest. 21, 1450. Owen. A. E.. LeComber P. G., SariibeyTouse G., and Spear. W. E. (1982). lEE Proc. (Pan! Solid State Electron Devices) 129, SI. Owen. A. E., and Robertson. I. M. (1973). IEEE Trans. Electron Devices. ED-20, lOS. Owen, A. E. Robertson. J. M.. and Main, C. (1979).). Non-C'yst. Solids 32.29. Phelan, R. J. Jr., Larson. D. P... and Werner, P. E. (1981). App!. Phys. Lest. 38, 596. Sarrab.yrouse, G.. Buio, 3., Owen, A. E., Munoz-Yigue. A., and Seboa. 3.-P. (1980). IEI Proc. (Pan I. Solid State & Electron Devices) 127.(3L 119. Simmons, J. G.. and E1-Badiy. A. A. (1978). Radio and Electron. Eg. 48,215. Spear, W. E. (1977), Adv. Phys. 26,11 IC. Scent. W. E. (1983). / Non-C". Solids 59/60,1. Stevenson, A. G. (1974) in Siuke, I., and &eni. W. edL 'Amorphous and Liquid Semioun ducwn.' Vol. I. p.675. Taylor and Fracas. London. Van Vechien. J. A.. Tsu. P.., and Sat. F. W. (1979). Phys. Lest. 74A, 422. Yamomoso. 1.. Kawamura. K.. and Shimizu. H. (1976). Solid State Electron. 1,70l. Yamomoto. 1.. and Monmoto. H. (1972). App!. P1,ys. List. 20, 269. 420

Inorganic resists based on photo-doped As-S films A.P. Firth, P.J.S. Ewen, A.E. Owen and C.M. Huntley Department of Electrical Engineering, University of Edinburgh, Edinburgh, EH9 331, U.K.

Abstract In recent years there has been considerable interest in inorganic resist systems based on the photo-doping of amorphous chalcogenide films, the majority of the research being devoted to Ge-Se films. This paper presents a detailed investigation of inorganic resists based on the photo-doping of Ag into As-S films. It is shown that high resolution patterns can be produced in such resists using holography or optical lithography and that they are compatible with wet-chemical or plasma etching. Structural studies using Raman spectroscopy indicate that for best resolution the composition of the As-S film should be close to As33 S67 since on photo-doping it will yield a single-phase homogeneous material. A possible mechanism for the photo-doping process is described based on a tarnishing-type photo-chemical reaction. It is shown that the actinic radiation initiating the photodissolution effect is absorbed primarily in the photo-doped layer, close to the interface with the undoped As-S region. Introduction

Interest in inorganic resist systems based on amorphous chalcogenide semiconductors stems from their potential to extend photolithography into the submicron region.' In terms of resolution capability and linewidth control the performance of these systems matches that of organic resists and they are compatible with industry-standard processing techniques, such as plasma-etching and spin-coating. 2 They can also be used as resists for X-ray, e-beam and deep-U'! exposure, with an expected resolution capability of 0.3 um in the latter case. Because chalcogenide glasses can be deposited by vacuum evaporation very thin, uniform films are obtainable, which makes these materials particularly suitable for multilevel resist techniques for producing chromium photomasks and also for ion-beam lithography. The most extensively studied chalcogenide resist system is A92Se/Ge-Se in which the basis of image formation is the metal photo-dissolution effect. Resists based on the many other photo-induced phenomena which occur in chalcogenide glasses 0 have yet to be fully investigated. 4 In the present paper we report a study of resists based on the photodissolution of Ag into As-S films. The first section is concerned with the resolution capabilities and etching properties of these resists and the following section discusses the influence of As-S film composition on resist performance. The final section describes in qualitative terms a possible mechanism for the photo-dissolution process. Ag/As-S resists The resolution capability of the Ag/As-S resist system is demonstrated in Figures 1 and 2, which show patterns produced by the photo-dissolution of Ag into As-S films evaporated onto Si wafers. Figure 1 is an electron-micrograph of a grating produced in a film of composition As30 S70 by conventional holographic techniques 5 using 488 run (optical) illumination from an Ar-ion laser. The grating spacing is ' 0.3 m. Figure 2 is a micrograph of a test pattern containing a range of line-widths and line-spacings and was also generated in an As30 S,o film but using a UV wafer stepper (Eaton Optimetrix 8010); line-widths and spacings down to 0.6 gm were readily resolved. The thicknesses of the As30S7o and Ag films were 3000 and 1000 A respectively. Typical exposure times on the wafer stepper for an incident intensity of 150 mW/cm 2 were 3-4s in the case of the film thicknesses quoted above. These results are comparable with what can be achieved with the more widely studied Ag/A52S3 resist. The Ag/As30S70 system is compatible with both plasma and wet-chemical etching. The most satisfactory wet etchants were found to be iron (III) nitrate solution (2.5 g/100 ml of methanol) for the Ag film and a saturated ammonia solution in methanol for the As-S film. 6 These etchants do not attack photo-doped material and give etch rates of 100- 300 A/s. In most instances it was found that aqueous solutions caused the films to flake, although this effect can be reduced by annealing the films. Plasma etching was done in an IPC 2000 Series plasma system using CF. gas. Photo-doping using an Ag layer above the As-S film was found to be preferable for plasma-etching since surplus Ag remaining on the surface after photo-doping can be easily removed before etching is commenced. When the Ag layer is below the As-S film it is exposed to the plasma when

160 / SPIE Vol. 539 Advances in Resist Technology and Processing // (19851 421

the As-S is etched off, which causes it to flake. Ootimization of the As-S film composition Most studies of Ag/As-S resists have used amorphous A52S3 as the chalcogenide film. The compound A52S3 is commercially available in bulk glassy form and it has the additional advantage that as a prototype amorphous material it has been extensively studied, so that a considerable amount of information on its properties exists. 7 However, it is not obvious that A52S3 is necessarily the optimum resist composition and there is a need to investigate other As-S compositions. The glass-forming region in the As-S system extends from 5 to 43 atomic percent and any composition in this range can be deposited as an amorphous film. Bulk samples of these glasses for evaporation sources are easily made. One of the basic questions regarding the photo-dissolution process is how the photodissolved Ag is incorporated in the As-S film? Is it accommodated interstitially, does it form a new homogeneous compound or does it produce a phase-separated mixture of new compounds? The compounds which might form depend on the composition of the As-S film and of the Ag source. Thus film composition influences the photo-dissolution process and hence, through it, resist performance. The principal compounds of the As-S-Ag system together with the glass-forming regions are shown in the phase diagram 8 of Figure 3. Also drawn are five tie-lines: four linking the compositions A52S3 1 As36 S62, As33570 and As20 S90 with Ag and one between A52S3 and Ag2S, which can also be used as a source of photo-dissolved Ag. 9 When elemental Ag is used as the source to photo-dope one of these four As-S films the resulting overall composition during photo-dissolution must move along the appropriate tie-line terminating at the Ag vertex. None of these four tie-lines passes through any of the known ternary compounds and only the tie-line for As30 S70 passes through the central glass-forming region. Thus of these four compositions only As 3 0 S70 can yield a homogeneous photo-doped film. Raman results obtained in the present study and shown in Figure 4 suggest that photo-doped as-evaporated As30 S70 films are indeed homogeneous, having a structure similar to that of a bulk glass within the central region of glass formation and close to the Ag-As30 S70 tie- line. Spectra A and B of Figure 4 are for an As30 S79 film before and after photo-doping respectively; spectrum C is for a bulk glass of composition A930As22S4a and is clearly similar to the photo-doped spectrum B. Photo-doped films of As2S3, As36 S62 and As20 S90 on the other hand yield spectra indicative of a phase-separated structure: photo-doped As2S3 and As39 S62 consist of a mixture of amorphous As and an amorphous Ag-As-S phase while photo-doped As20 So is a mixture of AgzS (not necessarily stoichiometric) and an amorphous Ag-As-S phase. The existence of phase-separated regions in the photo-doped film may have important technical consequences since it could ultimately affect edge definition and resolution. In the case of binary (undoped) As-S compounds, for example, phase-separation occurs for As contents greater than 43 atomic percent and results in the formation of 1 gm diameter pockets of crystalline As.S. embedded in an amorphous As-S network. Such inhornogeneities in photodoped As-S resist films would be undesirable. The optical and chemical sensitivity of the resist may also be influenced by the phase separation which appears to be occurring in some of the photodoped As-S compositions. It has recently been shown" that the amount of Ag which can be incorporated into an evaporated As-S film on dipping in AgNO3 solution is at its lowest for the composition As33 S67 , which lies at the centre of the range of compositions whose tie-lines to the Ag vertex pass through the central glass- forming region. This result might be due to the fact that additional Ag can be incorporated at the boundaries between phase-separated regions so that minimum Ag uptake occurs for those As-S compositions yielding homogeneous films. Thus, as the optical and chemical sensitivity of the resist will depend on the amount of Ag incorporated during photodissolution they may be significantly influenced by the composition of the As-S film. The tie-line between A52S3 and Ag2S passes through the ternary compounds AgA5S2 and A93AsS3, which suggests that a homogeneous photo-doped phase will be produced if Ag2S is used as the Ag source. This has been confirmed by Raman experiments carried out in the authors' laboratories, the photo-dissolution product being A93AsS3. An e-beam resist based on Ag25/As2S3 has been developed by Singh et al. 11 and has been shown to be capable of producing 0.1 m lines and edge-to-edge spacings down to 0.03 urn.u The sensitivity of this resist, however, is very low (4 x 10_3 _10_2 C/cm2 ). The photo-dissolution mechanism in the Ag/As-S system An important objective in the study of the photo-dissolution effect is to identify where the actinic radiation which initiates the process is absorbed. The spectral dependence of the photo-dissolution rate suggests that the basic optical excitation occurs in the amorphous film but as the effect can be stimulated by light of energy less than the band- gap energy of the As-S glass some investigators conclude that the actinic photons are

SPIE Vol. 539 Advances in Resist Technology and Processing 11(1985) / 161 422

absorbed in the Ag film. A third possibility is that the actinic radiation is absorbed in the photo-doped region between the Ag and the undoped As-S 12 ; our results support this and show, in addition, that absorption takes place primarily near the interface between the doped and undoped regions.

Figure 5 is a diagram of the sample geometry used in experiments designed to investigate the lateral migration which can occur when photo-dissolution is carried out on a conducting substrate. A film of gold (which does not significantly photo-dissolve into As-S films) was first deposited over a section of the surface of a glass slide and a band of Ag 5 mm wide and 600 mm thick was evaporated on top. Finally a 60 am thick As-S film of composition A52S5 was evaporated over the whole surface. The thickness of the Ag band is considerably larger than that of the As-S film in order to provide a reservoir of Ag for lateral photo-dissolution over the substrate surface.

By focussing the light source through a microscope objective it was possible to illuminate small areas of the sample. Normal photo-dissolution occurred when the region above the Ag band was illuminated whereas illumination of the film above the gold had no effect. However, when a photo-doped region and a neighbouring undoped region above the gold were exposed simultaneously the Ag migrated into the undoped region. Figure 6 is a micrograph of a photo-doped strip that was drawn out by tracking the illuminated spot across the film over the gold layer. To produce the strip only a segment of the interface between the photo-doped and undoped regions had to be in the illuminated area - it was not necessary to illuminate the photo-doped region between this interface and the Ag band or the Ag band itself. Hence th actinic radiation is being absorbed in the photo-doped region close to the interface with the undoped region.

The photo-doped strip could be drawn out indefinitely, provided the Ag source was not consumed. Figure 7 shows the time dependence of the distance advanced by the tip of the photo-doped strip as the illuminated spot was tracked stepwise across the As-S film over the gold. The plot is linear, indicating a constant speed of dissolution, and its slope gives the rate of advance as 1 am per minute.

The lateral photo-dissolution occurs only over the conducting gold-coated part of the substrate and this observation suggests that a supply of electrons is necessary for photodissolution to proceed. Ordinarily, the electrons would be supplied by the Ag source and would have to diffuse through the reaction products to reach the interface but in the present experiment electrons can be supplied by the gold and they do not have to diffuse further than the thickness of the As-S layer.

Recent measurements in the authors' laboratory of the change in transmissivity of Ag/As-S systems during photo-dissolution indicate that a single, highly-absorbing photodoped layer builds up in the As-S film, its thickness increasing as the square root of exposure time. This, together with the structural information and lateral dissolution results, suggestsa mechanism involving .a solid state photo-chemical reaction analagous to tarnishing. When Ag is exposed to S a tarnishing reaction occurs which results in the formation of an A92 layer on the Ag surface. This reaction is well understood 13 and it involves Ag ions and electrons from the Ag diffusing through the Ag2S reaction product to the Ag 2 S/S interface and there reacting with S, so that the Ag2S layer thickness continues to increase although the Ag and S are separated. Similarly, the photodissolution mechanism may involve Ag ions and electrons from the Ag source diffusing through the photo-doped layer to reach the undoped As-S region, where they react to produce a homogeneous compound or mixture of compounds. This mechanism is illustrated schematically in Figure 8. The Ag-As-S reaction product is likely to be a mixed conductor' (i.e. ionic and electronic) so that electrons as well as Ag+ ions will be able to diffuse through it and indeed the speed at which they diffuse may govern the photo-dissolution rate since the photo-doped region advances much more quickly for the case of lateral photo-dissolution, where electrons are supplied by the conducting substrate.

The role of illumination in the process is probably to stimulate the diffusion of Ag ions through the Ag-As-S region to the interface with the undoped As-S. Why this occurs is not clear but a similar effect is observed in bulk Ag-As-S glasses 15 and crystalline A9 3 AsS 6 (proustite) and results in the formation of a silvery deposit on the illuminated surface. Illumination with light of energy greater than the band-gap energy of the As-S film will also create electron-hole pairs in the undoped region. Electrons in As-S glasses are trapped immediately at defects, possibly 3-fold co-ordinated S atoms. Thus at the interface between the photo-doped and undoped regions the electrons available for neutralising the Ag 4 ions may be associated with S atoms, thus facilitating the formation of Ag-S bonds. The existence of two illumination-dependent processes, one in the undoped As-S region and one in the photo-doped Ag-As-S region, would explain why the process has a spectral dependence which resembles the absorptance profile of the As-S film but can still be stimulated by below-band-gap radiation.

162 / SPIE Vol. 539 Advances in Resist Technology and Processing// (1985) 423

If the proposed mechanism is correct then the speed at which the photo-doped layer builds up will be governed by the rate at which Ag ions from the Ag source diffuse through the photo-doped region to reach the interface with the undoped As-S. This implies that the thickness of the photo-doped layer will increase as the square root of the exposure time so that unreasonably long exposure times will be required to photo-dope thick films. Conclusions Inorganic resist systems based on the photodoping of Ag into amorphous As-S films have been investigated, particularly with regard to the influence of film composition on resist performance. It is shown that submicron resolution is achievable with AS30 $70 films and that such resists are compatible with plasma and wet-chemical etching. Structural studies using Raman spectroscopy suggest that As-S compositions such as As30 S70 , which are close to As33 S67 , will yield the best resolution, although they may not be as optically or chemically sensitive as As2S3. Experiments on the photo-dissolution of Ag into As-S films indicate that the actinic radiation initiating the effect is absorbed primarily in the photo-doped layer, close to the interface with the undoped region of the As-S film. A possible mechanism was described based on a tarnishing-type photo-chemical reaction. Acknowledgements The authors are grateful to the Edinburgh Micro fabrication Facility for the use of their UV wafer stepper and ancillary equipment. This work was supported by the Science and Engineering Research Council. References Tai, K.L., Ong, E. and Vadimsky, R.G., "Inorganic Resist Systems for VLSI Micro- lithography", Proc. Electrochem. Soc., Vol.82-89, pp.9-35, 1982. Chern, G.C. and Lauks, I., "Spin Coated Amorphous Chalcogenide Films: Structural Characterisation", J. Appl. Phys., Vol.54, pp.2701-2705, 1983. Tanaka, K., "Photo-Induced Phenomena in Chalcogenide Semiconductors", in Amorphous Semiconductor Technologies and Devices 1982, edited by Y. Hanakawa, Ohmsha/NorthH011afld, pp.227-244, 1981. Mednikarov, B., "Evaporated As2S3 - Reproduction Fidelity for Microelectronics", Solid State Technology, Vol.27, pp.177-179, 1984. Hariharan, P., Optical Holography, Cambridge University Press, Chapter 12, 1984. Shirakawa, T., Shimizu, I., Kokado, H. and Imbue, E., "Relief Image Formation in Ag Chalcogenide Glass Sensors", Photogr. Sci. and Eng., Vol. 19, pp. 139-142, 1975. Mott, N.F. and Davis, E.A., Electronic Processes in Non-Crystalline Materials, Clarendon Press, Second Edition, Chap.9, 1979.

S. Kawamoto, Y., Agata, M. and Tsuchihashi, S., "Structure of Glasses in the Systems A5 2 S3T12S and As 2 S 3 -Ag2S", J. Ceram. Assoc. Japan , Vol.82, pp.502-507, 1974. Ewen, P.J.S., Taylor, W.T., Firth, A.P. and Owen, A.E., "A Raman Study of Photo- Chemical Reactions in Amorphous AszS3 on Polycrystalline Ag2S Substrates", Phil. Mag. B, Vol.48, pp.1,15-L21, 1983. Petrova, S., Simidchieva, P. and Buroff, A., 'PhotodesactivatiOn: Influence of the As/S Ratio", Proceedings of the Conference "Amorphous Seniconductors-84", edited by E. Fahri-Vateva and A. Buroff, Bulgarian Academy of Sciences, pp.256-258, 1984. Singh, B., Beaumont, S.P., Bower, P.G. and Wilkinson, C.D.W., "Inorganic Electron Resist System for High Resolution Lithography", Appi. Phvs. Lett., Vol.41, pp.889-891, 1982. Janai, M., "The Kinetics of the PhotodissolutiOn of Silver in Amorphous As2S3", Proc. Electrochem. Soc., Vol.82-89, pp.238-253, 1982. Stone, F.S., "Lattice Defects in Ionic Crystals", in Chemistry of the Solid State, edited by W.E. Garner, Butterworth Scientific Publications, Chap.2, 1955. Koebel, H., Ibl, N. and Frei, A.M., "Conductivity and Kinetic Studies of Silver- Sensing Electrodes", Electrochim. Acta, Vol.19, pp.287-295, 1974. Maruno, S. and Kawaguchi, T., "Metal Photosurface Deposition in As-S-Ag Glasses", J. Appi. Phys., Vol. 46, pp.5312-5314, 1975. Smolenskii, G.A., Sinii, I.G., Prokhorova, S.D., Kuz'minov, E.G. and Godovikov, A.A., "A New Phase Transition in Proustite", Soy. Phys. Crystallogr., Vol.27, pp.82-85, 1982.

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Figure 1 - Electron-micrograph of a grating produced holographically by Ag photo-doping of an As 30 S70 film.

Figure 2. Micrograph of a test pattern produced in an As 30 S7 film by Ag photo-doping using a UV wafer stepper.

As 20 S 80 2000 uJ I- As30S70 2 80

A92S3 N)

\ 601

\ AgAsS 3 a A9AsS2 —A92S 1000

glass-forming 20 region

As 20 40 60 80 Ag at. Z Ag -> 0 0 250 500 Figure 3. Phase diagram of the Ag-As-S system showing the principal FREQUENCY (cm) ternary compounds and the glass- forming regions (cross-hatched Figure 4. Raman spectra of films of areas). Tie-lines are discussed composition A530 S70 . Spectrum A in the text. is prior to photo-doping with AS and Spectrum B that of the photodoped film. Spectrum C is for a bulk glass of composition A930 As 22 S

164 / SPFE Vol. 539 Advances in Resist Technology and Processing II (1985) 425

(a)

_-. 2nd layer Ag —6001

glass slide 5t layer Au 5O0A b --- - } 1

a- --- -a - top layer As2S 5 600 A I1 section along/ a—a / Ag

photodopedlayer

Section along A g Figure Micrograph of a photo-doped strip drawn out in an As-S film above - - - - .- - a gold layer. The Ag source is Au layer at the left. Dimensions: (b) 1 cm : 100 em.

Figure 5. The sample geometry used in the lateral photo-dissolution experiments: (a) plan view, a c (b) cross-sections at indicated regions. 400 Ag-As- S

F-. Ag + As-S 3- '-S Ag—.- e— 0, C a; b d C 0

3 Figure 8. Diagram showing the formation of a 0 photo-doped layer of Ag-As-S (0 material as a result of a • 0 tarnishing-type reaction involving the diffusion of Ag ions and 0 300 electrons. a-b is the interface 111umintiori timQ (miris). between the Ag and the photo-doped Figure 7. Distance travelled by the photo- layer and c-d the interface dissolution front (dissolution between the photo-doped layer and length) against time. The the undoped region. Interface straight lime is a linear fit c-d advances through the As-S to the data points and has a region as the square root of the gradient of u 1 im per minute. exposure time.

SPIE Vol. 539 Advances in Resist Technology and Processing // (1985) / 165 426

Journal of Non-Crystalline Solids 77 & 78 (1985) 1373-1382 North-Holland, Amsterdam 1373

THE SWITCHING MECHANISM IN AMORPHOUS SILICON JUNCTIONS

P.G. LECOMBER*, A.E. OWEN, W.E. SPEAR*, J. HAJTO, A.J. SNELL*, W.K. CHOI, M.J. ROSE* and S. REYNOLDS

*Carnegie Laboratory of Physics, University of Dundee, Scotland. Department of Electrical Engineering, University of Edinburgh, Scotland.

Extensive new results have been obtained on memory switching in a-Si p+ni junctions. It is shown that the ON-state has its origins in a highly conducting filament less than ljuit in diameter. The physical mechanisms that could play a role in the switching operations are discussed.

INTRODUCTION Threshold and memory switching in the chalcogenide glasses greatly stimulated research on these materials in the late 1960s and early 1970s 1 ' 2 . In spite of the wide-ranging work on amorphous Si (a-Si) in recent years, memory switching phenomena were not observed and this led to a general belief that homogeneous films of this and other tetrahedrally bonded amorphous materials do not possess

the physical properties required for switching behaviour 3 . However, some three years ago we demonstrated that heterogeneous junction layers of a-Si could be made to exhibit reliable, fast, polarity dependent memory switching 46 . The present paper is concerned with recent results on these specimens. As prepared, the a-Si layers, generally with a p+ni structure, require one forming operation after which they are in a non-volatile low resistance ON-state. They can be switched back (ERASED) to a non-volatile high resistance OFF-state by the application of a negative potential to the p layer and switched ON again (WRITE operation) by a positive voltage. This cycle can be repeated many time 6 . The particular exciting aspect of these devices is their remarkable switching speed. Pulses of a few volts in height and a few tens of nsecs 5,6 duration are sufficient for both the WRITE and ERASE operations. In the first part of this paper our latest results, mainly for formed devices, will be presented. This will be followed by a discussion of possible physical mechanisms that could be playing a role in the switching operation.

SPECIMEN PREPARATION The samples were prepared by the RE glow discharge decomposition of silane or appropriate silane gas mixtures. The majority of the results have been obtained on specimens having a p+ni structure where i denotes an undoped layer. We have also observed similar results for n+pi and related structures and recently

0022-3093/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) 427

1374 P. G. LeC'o,nber etal. / Switching inechanisin in amorphous silicon junctions

Gangopadhyay et a1 7 have reported memory switching in pFi n + samples. The samples were deposited onto Corning 7059 glass substrates at 300°C and generally prepared with Cr bottom electrodes and Al or Cr top contacts, although a number of other metals have been investigated. The active area of the samples, defined by photolithographic techniques, ranged from about 10 7cm 2 to 10 3cm2 . The work has also been extended to specimens in which the a-Si i-layer was replaced by an insulator. The results for these samples showed the same general features as those reported for the pni layers and will be described in subsequent publications.

3. STATIC EXPERIMENTAL RESULTS A number of dc experiments have been carried out on formed devices in an attempt to provide additional information primarily about the nature of the ON- state. The current-voltage characteristics of recent pni specimens had the same general features as those reported previously 46 , although by carefully controlling the forming process their ON-state resistances were generally kept at a somewhat higher level (—lko).

3.1. Area Dependence of ON- and OFF-state Resistances Fig. 1 shows the area dependence of the OFF-state resistance, ROFF, for samples of different area fabricated from a single pni deposition run. These results were obtained on specimens that had been previously switched many times. Within the experimental scatter, ROFF clearly scales with the reciprocal of the area A demonstrating that in the OFF-state the current flows throughout the whole area of the specimen.

10h

ROFF RON (IL) (n)

-S - S - - •-•. ali • . S S . • S S

-3 1°a 10-0 10 , id iü A (cm 2) A (cm 2)

FIGURE 1 FIGURE 2 Area dependence of ROFF. Area dependence of RON. 428

P. G. LeC'o,nber etal. / Switching mechanism in amorphous silicon junctions 1375

In complete contrast, the values of the ON-state resistances, RON, for the same specimens show no area dependence at all as can be seen from fig. 2. This result can be understood only if the ON-state has its origin in a highly conducting filament, less than a few).Lm in diameter, that extends at least part of the way through the specimen thickness. This result, although not entirely un- expected 4 , will obviously be of central importance in developing a model for the switching process. A direct proof of the existence of a filamentary ON- state is provided by the results in section 4. 3.2. Temperature Dependence of the ON- and OFF-states The temperature dependence of both the ON- and OFF-state conductance has been measured over the temperature range from about 230K to 400K. Results for a typical pni specimen are shown in fig. 3. The OFF-state conductance GOFF varied slowly with temperature, increasing by less than a factor of three between 23OK and 36OK. The ON-state conductance was even less temperature dependent, increasing by only 10% over this temperature range. The insensitivity of these device parameters to temperature is also observed in other properties. For example, the magnitudes of the WRITE and the ERASE voltage, measured under pulsed conditions, both increase by only a factor of 2.5 as the temperature is decreased from 400K to 200K Clearly the general insensitivity of the switching parameters must also be a feature ofany model proposed to explain the switch- - ing behaviour. - - - - - - - - 3.3. MagnetoreSiStance Measurements of the ON-state transverse magnetoresi stance measurements of the ON-state of a number of specimens- L__nave uueniL------La, I- eu.,4 uu,. 1 room temperature up to fields of GON 0.51. Within the experimental (n { error,&C/f was proportional to and fbund to be positive. The - values of Ar/(rB2) ranged from (0.5 to 2.0) x 10 4f 2 . Experi- ments on phosphorus doped a-Si after thermal crystallisation also gave io- a positive magnetoresi stance with 00FF6 t he same A.(/(eB2) dependence. Unfortunately, no results on 2 I homogeneous undoped glow discharge 10-61 T-JOL___L 2-5 30 35 40 45 a-Si films have been reported. 10/T (K1) it is therefore difficult to FIGURE 3 draw any definite conclusions Temperature dependence of (a) OFF- and (b) ON-state conductances about the amorphous or crystalline 429

1376 P. G. LeComber er al. / Switching mechanism in amorphous silicon junctions nature of the ON-state filament from the present results. However, it is probably correct to associate the 82 dependence of the memory ON-state with a longer mean free path than is normal in amorphous solids. Using conventional crystalline theory the magnitude of the measured A/(fB 2 ) would lead to an effective mobility - 100cm2 Vs in support of this suggestion.

4. OBSERVATION OF ON-STATE FILAMENT In order to learn more about the ON-state a series of experiments have been carried out in which the surface of the a-Si device was covered with a thin layer of thermochromic liquid crystal. By passing current through the device in the ON-state it is possible to observe the current path from the changes produced by Joule heating in the reflected colour of the liquid crystal (LC). The results for two 20jam diameter structures, viewed through a high power optical microscope, are given In fig. 4. Fig. 4(a) shows the LC surface in the absence of any current flow through the specimen and fig. 4(b) shows a sample with current flowing in the ON-state. The change in the LC appearance produced by Joule heating within the filament can be dlearly seen. Fig. 4(c) represents another device in which the filament has been made more visible by passing a larger current through it. We estimate from a number of measurements that the ON-state is associated with a filament of maximum diameter 0.5?m. The result therefore has interesting implications for the understanding of the switching process. The visual observation of the current filament has also enabled us to establish that switching a device OFF and ON again produces the current filament in the same place and this implies that the switching processes are not destructive. In another series of experiments the specimens were covered by a thin layer of a liquid crystal, 4-cyano-4 altylbiphenyl, which undergoes a nematic-liquid phase transition at 35.3°C. The phase boundary may be observed quite easily in

( a ) (b)

FIGURE 4 Photographs of LC surface covering 20)un diameter memories. The arrows in (b) and (c) denote the positions of the current filaments. 430

P. G. LeComber etal. / Switching mechanism in amorphous silicon junctions 1377 hysteresis. If the sample temperature is fixed using a thermostatically controlled stage, the difference in temperature between a region of local heating (t a temperature Th) and the surrounding film (at I f ) may be determined. The results indicate that no observable temperature rise occurred as a result of applying electrical power to the pore in either the unformed stage just prior to forming, or in the formed OFF-state just prior to switching. However, as described above, in the ON-state local heating which results on applying continuous power could be clearly seen. The effect of changing the RMS power applied to a 50im diameter pore was studied using a continuous train of 300ns pulses. The stage was maintained at 21°C, thus the phase boundary represented the locus of points (35.3-21) = 14.3°C above the film temperature. These isotherms were circular and in the particular case studied were symmetric about the centre of the pore. A plot of the phase boundary diameter d 1 vs. RMS power P is shown in fig. 5. Although there is considerable scatter it can be seen that the relationship between d 1 and P is substantially linear for d 2jzm. Below this the accuracy of the measurements is limited by the resolution of the microscope used. A linear dependence oil d 1 on P is obtained as a solution of the heat conductivity equation for an idealised system of this kind, in which the source of local heating is assumed to be much smaller than d 1 . Thus this data indicates that the diameter of the ON-state filament must be much less than 2)&m, in agreement with the above experiments. 5. DYNAMIC BEHAVIOUR I - In an earlier publication the existence of a voltage dependent delay time had 40 S.

d .. been established for the forming operation. (.pm) • S In the course of our recent work we have

30 S.-- observed a number of other time dependent effects and these will be described in the • S following. 20 .-- 5.1. Current Instabilities in Unformed

S Structures S S Current instability phenomena have been 10 beLow those required -• •i - observed at voltages .• I for forming, in unformed a—Si pni structures with thin i-layers. Typical . . • I - 2 4 6 results obtained under pulsed operation

PRMS (MW) are shown in fig. 6 for three pulse heights FIGURE 5 decreasing in amplitude from (i) to (iii). Diameter of 35°C isotherm as a function of RMS power. 431

1378 P. G. LeC'omber er al. / Switching mechanism in amorphous silicon junctions

V (v

U I.0 t U 2 6 b b t (us) fhi (us) (m (m.

(Ps) (Ps) FIGURE 6 Voltage and current responses of an a-Si memory prior to forming. It can be seen that the current increases after a delay time such as r(11) . At first sight the results in fig. 6(a) for the unformed a-Si memories appear to be similar to those observed in crystalline Si MISS devices 9 . However, the a-Si pni device does not remain in a high conductivity state whilst the voltage is maintained, as is the case for the MISS. The current increases fairly rapidly and then decays over a somewhat longer time-scale, resulting in an asymmetric current pulse as shown in fig. 6(b). However, it is worth emphasising that these current instabilities were observed at voltages just below those required for forming. If the voltage pulse was left on for many tens of microseconds then, some time after the first, a second current pulse would propagate through the sample, and then a third, etc. As the applied voltage V approached the forming voltage VF these current pulses appeared to merge until at V = VF the current level remained high as the device formed. These current instabilities therefore appear to be an important precursor to the forming process. 5.2. WRITE and ERASE Delay Times We have recently observed a delay in the WRITE operation which is a strong function of the WRITE pulse.magnitude as shown in fig. 7. Note that these delay times td are significantly faster than the forming delay times reported previously5 ' 6 . If the results in fig. 7 are expressed in the form td = t o exp (-V/V0 ) then t o = 335ns and V 0 = 4.5V. Similar results have been obtained for all the specimens investigated although the V 0 values ranged from about 0.5V to 13V.

432

P. G. LeC'onber et al. / Switching mechanism in amorphous silicon functions 1379

00 - Experiments to measure 0 any ERASE delay time were td unsuccessful implying that (ns) 60 —>! any delay was less than the rise time of the meas- 40 Dia(jm) uring circuit, i.e. typic- °25 ally less than 5ns. 200 60 - \10 X 125 + 200 S\ In ' 320 FIGURE 7 0 5 10 15 Dependence of WRITE delay time on forward bias V (Volts) pulse height.

6. DISCUSSION OF POSSIBLE SWITCHING MECHANISMS The work described above contained a number of important new results. Prob- ably the most crucial of these to an understanding of the physical processes underlying the switching mechanism, was the proof of the existence of a filament in the memory ON-state. Filamentary conduction has been observed previously in a wide range of materials. These include single crystal silicon, gallium arsenide, zinc telluride, gallium arsenide phosphide and polycrystalline silicon, all of which can show current-voltage characteristics associated with threshold switching 10 . This applies as well to the amorphous chalcogenide glasses 1 ' 2 in which both threshold and memory behaviour can be observed. In the following we shall begin by discussing the filament formation and then describe some of the models that have been used for these materials and discuss their relevance to the switching in a-Si. 6.1. Filament formation During the forming process which, as has been demonstrated, leads to a current filament, the metal-insulator barrier of the device will be under reverse bias. This suggests that the forming is likely to be associated with extremely high local fields across the thin a-Si i-layer which may approach 106Vcm. Under these conditions tunnelling of field emitted electrons from the top of the electron distribution in the metal electrode becomes the dominant transport mechanism, injecting appreciable electron densities into the semiconductor. The current instabilities described in section 5.1 would certainly be consistent with such a model. How is the reproducible filament produced during the forming process? A possible answer is suggested by the extensive work on thin film composite 433

1380 P. G. LeC'omber etal. / Switching mechanism in amorphous silicon junctions materials in which small isolated metallic particles are dispersed in a dielectric medium. This work has given a great deal of insight into the tunnelling process between isolated metal particles and its dependence on average particle size and separation, applied fields, etc. It is feasible that the high fields and current densities developing locally during forming could lead to enhanced diffusion of metallic particles from the electrode into the thin insulator. Such a region would become the preferred current path carrying the electron current in the ON-state. Further experimental work is required to confirm such a model; the observed field and temperature dependence in the ON-state is certainly consistent with results established in the previous work on these systems. 6.2. Possible Mechanisms for Memory Switching • The above considerations refer to the initial formation of the filament but cannot as such explain the subsequent memory switching. We should now like to critically discuss a number of possible mechanisms to explain this behaviour. 6.2.1. Thermal models At first sight this model 12, used to explain the behaviour of the amorphous chalcogenide memories, might appear to offer a basis for explaining the a-Si switching processes. In the chalcogenides the ON-state Is associated with a filament of crystalline material which is formed after sufficient power has been applied to the layer to melt asmall area of the material. Switching OFF is achieved by burning out this filament using a number of relatively short high- power pulses and allowing rapid quenching to reform the highly resistive amorphous phase. However, there are a number of important differences between the a-Si and the chalcogenide memories: (a) it has been established that the a-Si memories do not form or WRITE at constant power; in general forming occurs at much lower powers ( .41O 8J) than in the chalcogenides (lO - 10 3J); (b) the forming, WRITE and ERASE operations for the a-Si memories are generally polarity dependent; (c) no rise in the temperature of the a-Si specimens can be observed immediately prior to switching; and (d) the a-Si WRITE and ERASE times are many orders of magnitude faster than those for the chalcogenides e.g. for the WRITE operation 10 8s compared with 10 3s. 2 For all these reasons we do not believe that the crystalline/amorphous thermal model is applicable to a-Si. 6.2.2. Models based on Trapped Space Charge In many respects the behaviour of the amorphous Si layers appears tobe closely related to that of crystalline Si MISS structures in that both show fast polarity dependent switching, both show current instabilities during some stage of the forming processes and both have high conductance states associated with current filaments. The major difference of course is that the MISS structures are threshold switches which always revert to the OFF-state when the power is 434

P. G. LeComber et al. / Switching mechanism in amorphous silicon junctions 1381 removed, whereas the a-Si layers have the additional advantage (and complexity) of non-volatile memory behaviour. It is nonetheless possible that the memory switching in the a-Si devices is closely related to the mechanisms proposed for the MISS layers. Essentially two models have been used to explain the MISS behaviour 13 . These are generally referred to as "punch-through" and the "avalanche-mode' and both rely for switching on establishing high fields across space charge barriers in the films. In addition, in both models the low impedance state is produced by injected charge causing inversion of the Si at the Si/insulator interface. It is tempting to suggest that the "permanent" memory of the a-Si layers may be produced by a similar mechanism in which the charge is trapped in deep gap states at the insulator-semiconductor interface for which the probability of release is very small. However, the a-Si layers retain their memory without any observable change in their properties for as long as we have measured them, namely, for over one year at room temperature and 24 hours at 95°C. Using the thermal release time from deep mid-gap states as a measure of the persistence of the trapped space charge, then from the well-known expression for the average thermal release time we estimate that the capture cross-section of these centres should be less than 10 8cm2 . Although extremely small, such values would be consistent with Coulomb repulsive centres identified in crystalline materials 14 . However, the problem is that recombination of the trapped distribution through tunnelling or diffusion may well invalidate the above estimate by leading to a much faster decay of any trapped space chrge distribution. All one can say at present is that a model in which the observed memory is associated with a trapped space charge cannot be excluded but, in view of the remarkable non-volatility of the memory states, may not be the correct explanation. 6.2.3. Other suggestions It should be remembered that in the random network of the a-Si layers significant amounts of hydrogen are incorporated. The possibility therefore exists that memory switching may be associated with some atomic motion of hydrogen in the material. For instance, it has been reported that in ambient sensors containing Pd/a-Si Schottky barriers, hydrogen plays an important role in lowering the barrier 15 . Also the polarity dependence of the threshold voltages for the a-Si memories could be understood on the basis of field assisted hydrogen diffusion. Like all the possibilities mentioned above, this remains at present somewhat speculative and further work is required to identify the most likely memory switching mechanism.

7. CONCLUSIONS A number of new results, including the observation of a filamentary ON-state, 435

1382 PG. LeComber et al. / Switching mechanism in amorphous silicon junctions are reported in this paper. These provide important information relating to the physical processes underlying the operation of the a-Si memories. At present we believe that the initial stages of memory formation are likely to be associated with high fields and/or trapped charges in a manner analogous to that responsible for the threshold switching in crystalline Si MISS devices. However, neither the nature of the ON-state current filament nor the physical mechanisms responsible for the permanent memory of the a-Si layers, have been established with any certainty at the present time.

ACKNOWLEDGEMENTS We should like to thank S. Kinmond and A. Carrie for their help with the specimen preparation. The financial support of the B.P. Venture Research Unit for some of the work described in this paper is gratefully acknowledged.

REFERENCES S.R. Ovshinsky, Phys. Rev. Lett. 21 (1968) 1450.

A.E. Owen and J.M. Robertson, IEEE Trans. ED-20 (1973) 105.

M.C. Gabriel and D. Adler, J. Non-Crystal. Sol. 48 (1982) 297.

A.E. Owen, P.G. LeComber, G. Sarrabayrouse and W.E. Spear, Proc. lEE 129 (1982) 51.

A.E. Owen, P.O. LeComber, W.E. Spear and J. Hajto, J. Non-Crystal. Sol. 59 and 60 (1983) 1273.

P.G. LeComber, A.E. Owen, W.E. Spear and J. Hajto, in: Semiconductors and Semimetals, Vol. 21D, eds. R.K. Willardson &A.C. Beer (Academic Press, 1984) pp 275 - 289.

S. Gangopadhyay, J. Geiger, B. Schröder, H. Ribel and S. Iselborn, in print.

E. H. Putley, The Hall Effect and Related Phenomena (Butterworths, London, 1960).

J. Buxo, A.E. Owen, G. Sarrabayrouse and J.P. Sebaa, Revue de Physique Appl. 13 (1978) 767.

A.M. Barnett, in: Semiconductors and Semimetals, Vol. 6 eds. R.K. Willardson and A.C. Beer (Academic Press, 1970) pp 141 - 200.

e. g. B. Abel es, Ping Sheng, M. D. Coutts and Y. Arie, Adv. Phys. 24 (1975) 407.

A.E. Owen, J.M. Robertson and C. Main, J. Non-Crystal. Sol. 32 (1979) 29.

J.G. Simmons and A.A. El-Badry, Radio and Electronic Engineer 48 (1978) 215.

A. Rose, Concepts in Photoconductivity and Allied Problems (Krieger, N.Y.,l978)

A. DAmico and G. Fortunato, in: Semiconductors and Semimetals, Vol. 21D, eds. R.K. Willardson and A.C. Beer (Academic Press, 1984) pp 209 - 237. 436

J Phys. D: Appi. PhyS. 21 (1988) S78-S8IECOOSA 88. Printed in the UK

A Zakery, C W Wingert, P J S Ewen, A P Firth and A E Owen Department of Electrical Engineering, University of Edinburgh, Edinburgh EH9 3JL, UK Received 3 May 1988

Abstract A theoretical analysis of transmissive gratings of the type likely to be produced by the metal photodissolUtlofl effect in chalcogenide glasses has been carried out and used to predict diffraction efficiencies for a typical chalcogenide system, namely As30S70 glass photodoped with Ag. This was done to assess the potential for using these devices as diffractive elements for the 8-14 pm IR region. For bulk diffractive devices with a rectangular modulation profile, predicted efficiencies in the multiwave regime were over 90 0/6 and in the volume regime up to 1000%, irrespective of profile. Surface relief gratings with a rectangular profile yielded a maximum efficiency of at least 73%. The refractive index of Ag-doped and undoped As0S70 glass as a function of wavelength was measured for use in the grating analysis and was found to be approximately constant above 1 pm, the value depending on the amount of Ag photodoped into the glass. For AsS70 containing approximately 27 at% Ag the refractive index at 2pm is 2.8 compared with 2.2 for undoped AsSio, and this large difference is expected to be retained beyond 10 um.

grating performance, we also present some refractive 1. Introduction index measurements for a typical chalcogenide system Chalcogenide glasses exhibit a wide variety of pho- that might be used to fabricate these gratings in practice. toinduced phenomena that enable them to be used as namely As-S glasses containing photodissolved Ag. The optical imaging or storage media (Owen et a! 1985). metal photodissolution effect is probably the most useful The use of these glasses in optical imaging has been of the various photoinduced phenomena as far as these extensively studied over the past decade because of applications are concerned because it produces the their potential applications as photoresists for vlsi largest change in the properties of the chalcogenide, particularly the refractive index and etch resistance. lithography (Tai eta! 1982). Their use as optical storage media, particularly for holography, has also received considerable attention (Bordogna and Keneman 1977), though little work has been done on modelling the 2. Refractive Index measurements performance of gratings produced in these materials. Chalcogenide glasses are also well known ta-trans- Samples were prepared by evaporating onto Corning mitting materials, having pass bands (depending on 7059 glass substrates first a layer of Ag and then a layer composition) from the visible to beyond 15 pm (Savage of As30S70 glass, there being no break in the vacuum 70 was 1985), so that, by making use of the photoinduced between evaporations. The composition AsS- phenomena that occur in these glasses, it should be chosen for these measurements because earlier studies possible to produce diffractive elements for use at IR have shown that it is around the optimum composition wavelengths. Such elements have several potential uses. with regard to resolution and sensitivity and because it 0 film for example in beam combining, filtering and spectral exhibits negligible photodarkening. The AsS1 for all samples but analysis, and also have advantages over conventional thickness was approximately 5000 A refractive elements (typically Ge lenses) as regards the thickness of the underlying Ag layer was different IR by varying weight. cost and ease of manufacture (Close 1975). for each sample. varying from 500 to 1500 A; This paper is chiefly concerned with a theoretical the thickness of the Ag layer in this way, it was possible analysis of the performance of chalcogenide gratings. to control the amount of Ag that photodissolved into and hence the Ag concentration in the particularly with a view to their use in the ia. Since the AsS70 refractive index is one of the main parameters governing final photodoped material. The maximum possible Ag concentration is around 33 at% for AsS60 (Janai 1981) t Permanent address: Royal Signals and Radar Establishment. but may vary with As-S composition. Photodissolution Great Malvern. Worcs WRI4 3PS. UK.

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Chalcogenide gratings formed by metal photodissolution

5.2 The grating is assumed to be phase-modulated, lossless, unslanted and uniform with depth, with a rectangular ., u ., Aszi modulation profile. This situation approximates mask ., 720SILVER 5.0 ' C, C o I0O0S!t. exposure in chalcogenides (Owen eta! 1985). A coupled C. SILVIR C wave approach is used (Moharam and Gaylord 1981, S 0 2.0 C C 1982). The grating is taken to be in the form of an infinite parallel-sided slab of thickness d in the y—z plane, with 2.5 •' C C a normal in the x direction. The input and output surfaces of the slab are bounded by homogeneous

2.4 regions. An infinite, monochromatic plane wave, polar- ised perpendicular to the grating vector and of ampli-

tude A 0 , is incident upon the slab. 2j . ,,00 ,ioo In the slab (0 < x < d) the grating parameters may VPVDGTh r. generally vary as a function of y and z, but the slab is Figure 1. Refractive index versus wavelength for 5000 A assumed to be locally plane (Syms and Solymar 1981). thick As30S films photodoped with various amounts of Ag. The Ag concentration in the films was controlled by varying The rectangular modulation profile can be written as a the thickness of the initial Ag layer and these thicknesses Fourier series: are indicated In the figure. e(r)=e+ecos(ik.r) (1) was achieved by illuminating samples with a 200W with mercury lamp. The transmission of the samples was measured as a = e,,,,,, + Aep Elf = (2/ix)Ae sin(4ur) (2) function of wavelength using a Perkin-Elmer Lambda where e is the bulk relative dielectric constant of the 9 spectrophotometer and the refractive index n was slab and e is the phase modulation of the ith harmonic calculated from the fringes. Figure 1 shows the resulting of the grating profile. K is the grating vector (with IKI = values of refractive index obtained over the wavelength 2ir/A, where A is the grating period), Ac = - Emin range 500-2000 am for undoped AsS70 and four pho- and p is a fill (mark/space) parameter. todoped samples containing, various amounts of Ag. The key coupled wave equations describing propa- The curves are - similar in shape but are displaced to gation inside the grating can be written as the set (for. higher values of n as the Ag content is increased. For M = —N,. . . , —1, 0, 1..... N) phase gratings an important quantity is the difference An between the refractive indices of the doped and K1 1 d 2A m dA m + —jmQ(m + P)A m undoped material, and from the figure it is seen that cos2 0 for the most heavily doped sample, which contained approximately 27 at% Ag, An > 0.5 over this wave- (3) length range. Single-oscillator fits were obtained for iI K1 each of the curves and extrapolation of these to longer wavelengths showed that for the most heavily doped where Am is the amplitude of the mth diffraction order, sample An >0.5 at 10.6 pm. x. = 9e: /4e, fi = 2xVe/A is the propagation con- Although the optical constants of As—S—Ag glasses stant in the grating, A is the free-space wavelength have not been reported previously, those for two of the of the radiation, C = icx/cos 00 is a modulation crystals in the system have been measured: smithite parameter, Q = K 2/2flic 1 is the volume parameter (AgAsS2) and proustité (Ag3AsS3) were both found to (Solymar and Cooke 1981), P = (sin 00)2fi/K is a Bragg be weakly absorbing from 0.65 to 12 pm and had an parameter and 00 is the propagation angle of wave zero. approximately constant refractive index in the IR, the Thus each diffraction order A m is coupled to the m ± i values being 2.33 for smithite (Wehmeier et a! 1968) orders A m +i and A m_i by the ith coupling coefficient x. and 2.52 (n5) and 2.73 (n0) for proustite (Hulme et a! The coefficients of A m are a measure of the mismatch 1967). As the structure of the As—S—Ag glasses is in phase velocities of the diffracted orders. believed to contain the same structural units as smithite, The diffracted amplitudes A m are obtained by solu- it is likely that for them also n remains constant above tion of these equations, subject to grating boundary 2pm so that extrapolation of the oscillator fits to longer conditions (matching tangential electric and magnetic wavelengths is valid. fields) at x = 0, d. Both the cases of bulk and surface relief gratings can be analysed in this way. However, it is possible to simplify the calculations by neglecting 3. Theoretical analysis second derivatives in (3). This is valid if e/e 4 1, generally a figure of —0.2 being the limit. For bulk and In this section we analyse the performance, in particular surface relief gratings in photodoped chalcogenides, the efficiency, of both bulk and surface relief gratings. 0.1 and 1.0 respectively.

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A Zakery et a!

So for the bulk case (3) becomes (for m = cations though, operation in the volume regime is —N.....1,0,1.....N) usually desired. Note, however, that this may not always be possible—the high refractive index of the chal- —jmQ(m + P)A,,, cogenide may require operation at lower values of Q. dC Using a bulk square wave grating with refractive indices of 2.22 (undoped) and 2.42 (photodoped region) +j(A, +1 +Am_i)0. (4) and at (, Q) = (r/2, 5) requires a grating thickness of K1 3.6A—around 38Mm at A = 10.6 pm. No backward diffracted waves are now possible. The transmission grating boundary conditions are 4.2. Surface relief gratings A m (x0)=0 (m*0), A 0 (x=0)°1. (5) Here the rigorous theory (equations (3)) must be used, Analytic solutions of (4) are possible for the thin due to the large values of the ratio e! IEO'. The value of (Q 10) diffraction regimes. the Q parameter still governs the diffraction regime Defining efficiency as the ratio of power in the first that the grating operates in, but there are no simple diffracted order to that in the incident wave, a maximum expressions predicting the performance in the limiting of 40.5% is found for a square wave grating in the thin cases of thin or volume behaviour. Figure 3 shows the regime (e.g. Magnusson and Gaylord 1978) while 100% theoretical variation in efficiency for a typical pho- is possible in the volume regime, irrespective of profile todoped grating. (Slinger 1988). For the multiwave regime, numerical solution of (4) is necessary. E.11flAanr1LLflI;L raa. .

4. Numerical results I.' 4.1. Bulk gratings

Equations (4) were solved numerically to determine the •.0 efficiency of first-order diffraction, q,, as a function of Q and . The results for a square wave grating are

shown in figure 2. Fresnel reflection losses have been I. •

O;ffroct.d intensity of on-80 ode 1I1I!1 I ' •.2 MMAIW

d/Et Figure 3. Theoretical calculations of q 1 for typical photodoped square wave surface relief grating. Replay is such that the grating is on-Bragg for the first diffraction order, with grating period equal to the replay wavelength.

For the grating analysed above, a maximum efficiency of 73% at d = 1.36 A is seen. Here (, Q) = (3.7, 0.35). The grating is in the multiwave regime. This efficiency is lower than the bulk grating case but should not be regarded as a maximum. It may be increased by Lai the use of suitable coatings or higher values of Q. L •1 Perhaps more importantly, the required grating depth is significantly less than for the bulk grating, being about Figure 2. Theoretical calculation of r 1 for a square wave 14 pm for operation at 10.6 pm. This could be of value bulk grating. Contour 1 = 10% efficiency. 2 = 20°!.. etc. in minimising grating exposure times. Note the log scale for Q.

neglected. The thin. multiwave and volume diffraction S. Summary regimes can all be seen. Perhaps surprisingly, high diffraction efficiencies of over 90% are possible in the In conclusion, for rectangular profiles, high efficiencies multiwave region. For the majority of grating appli- are achievable.in the itt for both bulk and surface relief

S80 439

Chalcogenide gratings formed by metal photodissolution gratings, provided the thicknesses are sufficiently large, Close D H 1975 Opt. Eng. 14 408-19 the required thickness for the surface relief device being Hulme K F. Jones 0, Davies P H and Hobden M V 1967 approximately a third of that for the bulk device. App!. Phys. Let:. 10 133-5 Janai M 1981 Phys. Rev. Lett. 47 726-9 Magnusson R and Gaylord T K 1978 J. Opt. Soc. Am. 68 Acknowledgments 806-9 Moharam M G and Gaylord T K 1981 J. Opt. Soc. Am. 71 811-8 This work was carried Out with the support of the - 1982 J. Opt. Soc. Am. 72 1385-92 Procurement Executive, Ministry of Defence. CWS Owen A E. Firth A P and Ewen P J S 1985 Phil. Mae. B would like to thank F S M Clube at GEC Marconi 52347-62 Research Centre for assistance with some of the surface Savage J A 1985 IR Optical Materials and their Annreflection Coatings (Bristol: Adam Huger) p 79 relief software. Slinger C W 1988 J. Opt. Soc. Am. submitted Solymar L and Cooke D J 1981 Volume Holography and © 1988 Controller HMSO. Volume Gratings (London: Academic) Syms R R A and Solymar L 1981 Opt. Quantum Electron. 13415-9 References Tai K L, Ong E and Vadimsky R 0 1982 Proc. Electrochem. Soc. 82-89 9-35 Bordogna J and Keneman S A 1977 Holographic Recording Wehmeier F H. Laudise R A and Shiever J W 1968 Mater. Media ed. H M Smith (Berlin: Springer) p 229 Res. Bull. 3 767-78

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Journal of Non-Crystalline Solids 115 (1989) 129-131 129 North-Holland

DRY ETCHED HIGH RESOLUTION POSITIVE AND NEGATIVE INORGANIC PHOTORESISI'

E. HAJTO, R.E. BELFORD, P.J.S. EWEN and AE. OWEN Department of Electrical Engineering, University of Edinburgh, Edinburgh EH9 3JL, Scotland

The etching properties of spin coated As-S and Ag-As-S films are reported. Novel results obtained on both the light induced effects and the dry etching of these films opens real opportunities for their use as high resolution photoresists.

1. INTRODUCTION 2. EXPERIMENTAL METHODS Light induced changes in the properties of 2.1. Sample preparation evaporated chalcogenide thin films have been studied Solutions of various strengths were made by dissolving for their potential use as inorganic photoresists 1-2• the powdered As2S3 glass in n-propylamine, the The advantages of their use in fabricating integrated resulting film thickness (200A - 2p.m) was dependent circuits (chemical and physical durability, broad on the concentration of the solution. Thin films were spectral sensitivity, high resolution, edge sharpening obtained by spinning these solutions onto substrates of and step-like function capabilities) have been described either Si wafers or microscope slides. The spun films in recent pubications 3-6 were annealed at 120°C for lhr in order to eliminate the organic solvent from the film. For the silver The technique of spin-coating offers the additional photodissolution experiments, evaporated Ag films were advantage that the layers can be deposited in the same used beneath the spin coated chalcogenide film as a way as conventional organic photoresists. The films silver source. A commercial wafer stepper (Optimetrix deposited by this method have basically the same 8010 X=430-600nm, power density=OmW.cm b -, -) properties as those obtained by vacuum evaporation or was used to illuminate the samples. sputtering techniques. Amorphous As-S films can be deposited in a wide range of thicknesses by spin 2.2. Plasma etching coating, such films films are uniform and free of Plasma etching experiments were carried out in an microstructure7. Their composition closely follows that International Plasma Corporation 2000 series system, of the bulk material. The annealed spin coated As-S which is a basic barrel-type plasma etcher. The film structure and optical properties have been chamber pressure (atmospheric - 10_I Torr) was the investigated 89. sole means of controlling the rate of flow of gas within the chamber. Etching was carried out with different Patterns can be generated by two different methods: plasma strengths (input powers) and barrel pressure photo-darkening of the As-S film and photo-dissolution combinations for each gas. The gases used were CF 4(g) of Ag into the As-S film, the former being a single step for As-S material and S(g) for Ag-As-S material. process and the latter a bi-layer resist process involving Sulphur vapour was generated in vacuo from a solid the deposition of a Ag layer. as well as the source and sublimed before entry into the chamber. chalcogenide film. Both negative and positive type resists are available using the above methods. The An "in situ" optical technique was used to evaluate properties of these resists were investigated using the etching rates using a He-Ne laser. The light is standard IC processing techniques. reflected from both the front (receding) and the back

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130 E. Hajto et al. /Positive and negative inorganic photoresist

(stationary) interfaces of the film, and the reflected of the film. The refractive indices were found to be light intensity is detected by a photodiode and plotted 2.24 for the unilluminated and 2.36 for the illuminated on a chart recorder. spin coated As2S3 (as determined by optical measurements). The etching time required to remove 3. RESULTS AND DISCUSSION the illuminated film is significantly shorter than that 3.1. The single layer system: Photodarkening required for the unilluminated areas. The calculated It was observed that illumination caused etch rate for illuminated spin coated As2S3 film is photodarkemng of the annealed spin coated films —230 Aisec, and for the unilluminated film is (associated with structural changes). This results in —92 A/sec. The results obtained for the etching differences in the chemical etch rate, therefore the kinetics are shown in Fig. 2. effect can be used for pattern generation in the film. A single As2S3 layer (thickness —0.5p.m) was deposited TIME (seconds) by spin coating and annealed at 120°C for 1 hour. 10 20 30 40 50 60 Half of the layer was then illuminated for 10 mm. The exposure time used here was for comparative work and too is not a measure of sensitivity. A plasma of CF4(g) 1000j Io was used to etch differentially the illuminated and z unilluminated areas of the film. The optimum etching 2000 0 conditions were: CF4 gas pressure 0.7 Torr, r.f. power 0 100W. The progress of the etching was monitored 3000 continuously by the in-situ technique described above. The recorded interference patterns are shown in Fig. 1. 4000

F-' U) FIGURE 2 z Etching kinetics for (a) unilluminated (b) illuminated F- As2S3 film F- C-, The etch rate is constant ie the plots are linear for both exposed and unexposed areas. The etch rate of the illuminated film is 2.5 times larger than that of the F- C.) unillurninated areas. A differential etch rate of this magnitude makes spin coated As2S3 films suitable for pattern delineation, and thus use as a positive photo- 30 60 15 45 resist with all the advantages this material offers. TIME (seconds) 3.2 The bilayer system: Photo-dissolution of Ag into FIGURE 1 As-S Reflected laser intensity curves as a function of etching As demonstrated in the previous section As 2S3 film time. (a) unilluminated (b) illuminated As 2S3 film can be etched in CF4 plasma but no suitable etchant The etching rates were calculated by the positions has been reported for the removal of the Ag doped relative to time, of succesive maxima and minima. The material, this has restricted the use of Ag-As-S system amount of film removed at any given time is; as negative photoresist. A plasma of S(g) was found to d°°MK/2n where M is an integer, X is the wavelength be very effective and selective to the doped material, of laser light used (6328A) and n is the refractive index yielding an amphoteric resist system ie either negative 442

E. Hajro et al. /Positive and negative inorganic photoresist 131 or positive depending on the etchant used. The etching stepper used. The samples used were Si wafers with a was monitored in the same way as described above. vacuum evaporated Ag layer of 1000A and a spun As-S The optimum etching conditions were: S(g) pressure = film of 300A. These very thin films 3 have the same 0.65 Torr, r.f. power = lOW, and an etch rate of etching characteristics as those shown in Figs. 3-4, but —160 Alsec was observed. Fig. 3 shows the they are too thin to give infomation as regards etch experimentally obtained interference pattern and Fig. 4 rates and thus vacuum evaporated samples were used to shows the the etching kinetics for the Ag-As-S bilayer obtain this data. system. 4. CONCLUSIONS Both photodarkening and silver-photodiffusion effects in spin-coated films of As2S3 can be used for U, pattern generation. The photodarkening effect offers a

C factor of 2.5 in the etching rate difference for illuminated and unilluminated films. This affords a positive type photoresist involving only a single layer —J deposition. The Ag-As-S double layer system has the drawback of having two deposition steps, but can act as 0 U either a negative or positive photoresist with all the 0 advantages described. a, 0 10 20 30 40 50 60 70 80 Time (seconds) REFERENCES Y.Mizushima and A.Yoshikawa, Jap. Ann. Rev. FIGURE 3 Comp. & Telecomm. Ed. Y.Hamakawa, (1982) Reflected laser intensity curve as a function of etching 277-290 time for Ag-As-S film. B.Singh, G.C.Chern and I.Lauks, J. Vac. Sci. Technol. 3(1) (1985) 327

10 R.E.Belford, E.Hajto and A.E.Owen, Thin Solid Films 173 (1989)

B.Singh, S.P.Beaumont, P.G.Bower and a, C.D.W.Wilkinson Appl. Phys. Lett. 41(9) (1982) 0 E 889 C, C.T.Kernmerer, CA K.L.Tai, R.G.Vadimsky, (A 0 • . J.S.Wagner, V.E.Lambertii and A.G.Timko, J. C, 1 Vac. Sci. Technol. 17(5) (1980) 1169 U H.Kokado, I.Shimizu and E.Inoue, J. Non-Cryst. 0 Sol. 20 (1976) 131 in n 0 40 51 Time (seconds) G.C.Chern and I.Lauks, J. AppI. Phys. 53 (1982) 6979 FIGURE 4 E.Hajto, P.J.S.Ewen, P.G.Hill and A.E.Owen, The information obtained from Fig. 3 in the form of Phys. Stat. Sol. (in print) thickness etched (%) vs. time. E.Hajto, P.J.S.Ewen. R.E.Belford, J.Hajto and In this case the samples were usee to obtain optical A.E.Owen, J. Non-Cryst. Sol. 96-97 (1987) 1191 resolutions of 0.5iim. which is the limit of the wafer 443

Thin Solid Films. 200 (1991) 229-237 ELECTRONICS AND OPTICS 229

INTERFERENCE GRATING FABRICATION IN SPIN-COATED As 2S3 FILMS

E. HAJTO AND P. J. S. EWEN Department of Electrical Engineering, Unirersitv of Ediithug/i. Edinburgh EH9 3JL (U.K.) R. E. BELFORD Department of Physics. Napier College, Colinton Road. Ediithurgh EHJO 5DT (U.K.) A. E. OWEN Department of Electrical Engineering, University of Edinburgh. Edinburgh EH9 3JL (U.K.) (Received August 29,1990; revised November 16. 1990; accepted December 5. 1990)

The optical properties and solubility of spin-coated As 2S3 films significantly change after silver is introduced by photodiffusion. The different types of holographic grating that can be fabricated in spin-coated Ag/As 2S3 double film structures by photodiffusion and subsequent selective etching are described in this paper. The silver diffusion occurs under the influence of visible monochromatic light and the grating pattern is produced by a holographic arrangement.

1. INTRODUCTION

Light-induced changes in the physical and chemical properties of evaporated As—S and other chalcogenide thin films have been widely studied ' 3. As—S films can also be deposited by the spin coating technique and such films have basically the same properties as those obtained by vacuum evaporation' - '. Amorphous As—S films can be deposited in a wide range of thicknesses by this technique and the resulting films are uniform'. Their composition closely follows that of the bulk material dissolved for spin coating and the structure and optical properties of the annealed, spin-coated As—S films are found to be very similar to those of the evaporated material'. Patterns can be generated in As—S films either by photodarkening, which produces a difference in etch rate between the illuminated and unilluminated part of the film', or by light-enhanced silver diffusion into the As—S film', which results in an even larger difference in the etch rate between the illuminated (i.e. silver-doped) and unilluminated film. Since the chalcogenide glasses are also well-known JR transmitting materials, patterning based on these photo-induced effects can be used to produce diffractive optical elements for use in the JR region' °. Such elements have potential applications in beam combining, filtering and spectral analysis and have advantages over conventional refractive components as regards weight, cost and ease of manufacture. Spin coating is a common technique for depositing materials, such as dichromated gelatin, which are used to produce diffractive optical elements for visible operation, and hence it would be useful to determine whether gratings can be created in chalcogenide films deposited by this technique.

0040-6090/91/$3.50 © Elsevier Sequoia/Printed in The Netherlands 444

230 E. HAJTO et al.

In this paper the light-induced diffusion of silver into spin-coated As,S 3 films is studied. It is shown that, using a holographic illumination method and subsequent selective etching, a surface relief grating of submicron resolution can be obtained. It is also demonstrated that the optical constants (absorption coefficient Ot and refractive index n) of the silver-doped As 2 S3 films differ considerably from those of the undoped films, so that the changes can be used to record either amplitude or phase holograms in the Ag/A5 2S 3 double film structures.

2. EXPERIMENTAL DETAILS

The Ag/A52S3 double film structures are prepared in two steps. First a thin (about 0.3 jim) silver layer is deposited by vacuum evaporation onto a glass substrate; then an amorphous As 2S3 layer (about 1 pm) is deposited using the spin coating technique''. For spin coating, solutions of As 2S3 were made by dissolving the powdered bulk As2S3 glass in propylamine. Thin films were obtained by spinning these solutions onto glass substrates previously covered with a silver film. Typically 2 g of As2S3 dissolved in 10 ml n-propylamine and spin deposited at 3000 rev mm for 20 s, produced an As 2S3 film of about 1 jim thickness. The samples were annealed (in the dark) at 120°C for 30 min in order to eliminate the remains of the organic solvent from the As2S3 film. The optical properties of the films were measured using a UV—visible—near-IR spectrophotometer (Perkin Elmer Lambda 9). The optical constants n and a were calculated from the transmittance and the reflectance data using the method suggested by Swanepoel' 2 and Abeles' 3. For these measurements the samples were deposited onto Corning glass 7059 substrates with refractive index

'substrate = 1.5049. The effect of silver doping was also studied. For this purpose the Ag/As2S3 structures were illuminated from the As 2S3 side (i.e. from the top of the double structure) with UV light using a Carl Suss mask aligner system. As a result of illumination, the silver layer completely diffused into the As 2S3 film. The optical constants of the silver-doped As 2S3 films were then measured. The structure of the thin films and the principal steps in the formation of the surface relief grating are shown in Fig. 1. The Ag/As 2S3 double structure is illuminated with the required pattern and the silver layer diffuses into the As 2S3 layer in the illuminated areas. After illumination, methanol saturated with ammonia is used for developing the surface relief grating. The optical arrangement for pattern generation is shown in Fig. 2. In this experiment we used the simplest form of interference grating in which the

Fig. I. Sample configuration used for holographic imaging. 445

As,S 3 INTERFERENCE GRATINGS 231

Fig. 2. Optical arrangement used for illumination. photosensitive films are exposed to a sinusoidal fringe pattern generated by two interfering beams of light. A continuous-wave argon ion laser is used for illumination, operating at a single wavelength of 5145 A. The laser beam is focused first through a spatial filter, and then expanded and collimated. The beam is divided into two parts of the same intensity using a prism, which is a more stable arrangement than the usual holographic method using a beam splitter and mirrors, because the optical path length is shorter and the number of optical elements is fewer. A stable optical arrangement is important in this case because of the relatively long exposure time required for the silver diffusion. When the two coherent beams of light intersect at an angle 20, they will generate interference fringes within the volume common to both beams with a spacing (also termed grating period) A given by '4 A=/2 sin 0 (1) where ). is the wavelength of the light and B is the angle of incidence. Therefore the grating period in the Ag/As 2 S 3 structure is determined by the wavelength . of the illuminating light and the angle of incidence 0 which is in turn determined by the angle of the prism. The finest spacing that can be obtained is )./2 which corresponds to 0 = 90°. This cannot be achieved in practice since it requires both beams to be incident along the surface of the sample, but a value of 0 = 60' yields a spacing of 0.6k. In our experiments two different prisms were used: prism (I) provided an angle of incidence 0 = 20° and prism (2) provided an angle of incidence 0 = 0.7°. The performance of the gratings produced (resolution, spectral purity, efficiency) was assessed using a low intensity He—Ne laser.

3. RESULTS AND DISCUSSION

Curve A of Fig. 3 shows the absorption coefficient of the undoped, spin-coated As-'S3 films (i.e. without silver) as a function of a wavelength in the visible range of 446

232 E. HAJTO et aL the optical spectrum. Curve B of Fig. 3 shows the absorption coefficient for the silver-doped As 2 S3 films. The effect of silver doping on the refractive index is shown in Fig. 4 (curve A for the undoped As 2S3 film, and curve B for the silver doped film). It can be seen that the absorption coefficient and the refractive index increase as a result of the silver incorporation into the As 2S3 network. These results correspond

5.0

4.0

3.0

2.0

0.6 0.8 1.0 1.2 wavelength (pm) Fig. 3. Wavelength dependence of the absorption coefficient of spin-coated As,S 3 films (curve A) and of silver-photodoped As 2S3 films (curve B).

3.0

2.8

2.6

2.4

2.2

0.6 0.8 1.0 1.2 wavelength (m) Fig. 4. Wavelength dependence of the refractive index of spin-coated As,S 3 films (curve A) and of silver- photodoped As,S 1 films (curve B). 447

As,S 3 INTERFERENCE GRATINGS 233 to a silver concentration of about 22 at.° in the As2S 3 films as calculated from the initial thickness of the silver and As 2 S 3 films. The large changes observed in the optical constants of the silver-rich (i.e. the illuminated) parts of the films are accompanied by a drastic change in other physicochemical properties, for example their solubility in a suitable solvent such as methanol saturated with ammonia. This agent dissolves the non-illuminated part of the As,S 3 film but the silver-doped part becomes practically insoluble. Figure 5 is a scanning electron micrograph of the surface relief grating produced in a spin-coated As2S3 film by light-enhanced silver diffusion and subsequent selective etching using the above solvent. In this experiment the sample was illuminated with the holographic arrangement of Fig. 2 using prism (1), i.e. the intersecting beams reach the surface of the sample with an angle of incidence 0 = 200. According to eqn.(l) this will produce a standing wav with the distance A between the neighbouring interference maxima (the grating period) of about 0.75 gm. The electron micrograph in Fig. 5 shows that the standing wave pattern is reproduced as a surface relief grating after selective etching. The applied laser power was 13.5 mW and the illuminated area was 1.84 cm', resulting in a laser power density of 7.34 x iO W cm 2 . The duration of the illumination was 10 min, i.e. an illumination energy of 4.4 J cm was obtained. This illumination energy caused the silver to diffuse to a depth of about 0.6 p.m, as can be estimated from the surface profile shown in Fig. 6 which is an electron micrograph of the grating taken in a tilted position with an angle close to 90°. Since the groove profile is basically a sine wave, it can be concluded that the silver diffusion depth (which determines the profile of the surface relief grating) is governed primarily by the light intensity profile. The period of the recorded fringes can be increased significantly if a small angle of incidence is used for illumination with the same wavelength (see eqn. (1)). This is achieved by using prism (2) which produces an angle of incidence 0 = 0.7° for the two intersecting beams. This resulted in a grating period A 22 p.m as seen in Fig. 7.

S.tnnng electron rniroraph ol the interference grating produced in spin-coated AsS h iker photod I Ilusto it. 448

234 E. HAJTO et al.

IuPM 1IUI4Y 47

Fig. 6. Scanning electron micrograph of the grating profile.

Fig. 7. Scanning electron micrograph of the interference grating with a grating period of about 22 jm.

The surface relief gratings prepared in this way are considered as thin holographic gratings. The distinction between thick and thin holographic gratings is usually made with the aid of the Q parameter" defined as Q = 2it,.d/nA 2 (2) where). is the illuminating wavelength. n is the refractive index, d is the thickness and .1 is the spacing of the recorded fringes. The holographic grating is considered thick when Q ~: 10 and thin otherwise. In our case a value Q 1.4 can be obtained using the experimentally determined parameters of i. = 0.6 pm. n = 3.0 (see Fig. 4), d = 0.56 pm (see Fig. 6) and A = 0.7 m (see Fig. 6). Holographic gratings are also classified by the mechanism by which the illuminating light is diffracted. In the amplitude grating. the interference pattern is

449

As-,S3 INTERFERENCE GRATINGS 235

recorded as a density variation of the recording medium and the amplitude of the illuminating wave is modulated. in the phase grating a phase modulation occurs when the illuminating wave passes through the film. In our case it is primarily the phase of the illuminating wave which is modulated and diffracted from the surface relief grating. The change i\4 in the phase is given by

A4 = l) d} (3)

where An is the difference in the refractive index, Ad is the difference in the thickness and i. is the illuminating wavelength. if the hologram is physically thin, d is very small and the contribution to A4 from the term d An is negligible so that

A=(n—l)M (4)

suggesting that the surface relief modulates the light by the differences in the

thickness of the film' 5 . For an ideal sinusoidal grating one would expect the light to be diffracted only

into the angles determined by the grating equation' 4 A(sinx+ sin fl) = in). (5) where A is the grating period, ot and flare the angles of incidence and diffraction, m is the order number and . is the wavelength of the light used. Therefore the grating period A can be determined from the measured angle of the diffracted beam. It was found that if the measuring light = 0.6328 pm) is used at normal incidence relative to the surface relief grating, the diffracted beams exit the surface at angles p = ± 56.4c . This gives A = 0.76 p.m for the period of the grating, in good agreement with the micrographs of the surface relief grating (seen in Figs. 5 and 6). The efficiency of a grating is defined as the fraction of the incident radiation that is diffracted into the required order. Figure 8 shows the measured efficiency of the surface relief grating using an He—Ne laser (i = 0.6328 pm) at normal incidence. These measurements were obtained on the same surface relief grating but used in reflection mode. (For these measurements the sample was covered with a thin aluminium layer in the order to increase the reflectivity.) The efficiency is determined

Angle of diffraction -60° +600

C 15 E t 10 C 0 U

in -1 0 +1 Order of diffraction Fig. S. Diffraction efficiency of the grating with 0.7 pm period as measured with 6328 A light at normal incidence. 450

236 E. HAJTO et al. primarily by the groove profile and if the grating is illuminated at normal incidence it cannot be higher than 50°,, in any order because of the symmetry. In practice the efficiency seldom exceeds 33 in this configuration' 4. Figure 8 shows that approximately 18 efficiency is measured at the diffracted orders of ± I (at angles fl = ± 56.4), suggesting that the profile of the surface relief grating is close to the "ideal" sinusoidal profile. Similar diffraction efficiencies were obtained in a vacuum- evaporated chalcogenide—silver double-layer system' . The roughness of the surface (see Fig. 5) might introduce a random variation in phase and amplitude of the diffracted light which might generate diffuse scattering. However, according to the theoretical analysis' 7 the spectral image of the diffracted light does not change significantly; the efficiency may decrease slightly but the distribution of light among the diffracted orders is much the same. It should also be emphasized that we did not observe spurious diffracted orders or diffuse scatter which might arise from periodic errors or imperfections across the surface of the grating. We suggest that the large variation observed in the optical constants of the As 2S3 films as a result of light-enhanced silver diffusion could also be used for recording thin amplitude or phase holograms. In this process no selective etching is necessary because the hologram can be recorded as changes in the absorption coefficient or the refractive index, which are in turn determined by the amplitude and the phase of the illuminating beam. It can be seen from Fig. 3 that silver doping CM - I increases the absorption coefficient (measured at 0.6 gm) from 2 = 1.35 x 10' to a = 4.67 x 10' cm ', which can be used for recording an amplitude hologram. In addition, the refractive index (measured at 0.6 gm) increases from n = 2.22 to n = 3 (see Fig. 4) as a result of light-induced silver diffusion. This effect can be used to produce a phase shift of the illuminating wave, i.e. to record a phase hologram. In our case these two effects occur simultaneously, indicating that the amplitude transmittance of such a hologram will be a complex function that describes the change in the amplitude and the phase of an illuminating wave on transmission through the material.

4. CONCLUSIONS

The optical constants and solubility of spin-coated As 2S3 films significantly change after silver is incorporated by photodiffusion and these changes can be used to record grating patterns in the films. The silver diffusion depth, which determines the profile of the surface relief grating, is governed primarily by the light intensity profile. The recorded pattern can be developed using selective etching or, alternatively, the observed changes in the optical constants as a result of silver photodiffusion can enable the material to modulate the amplitude and the phase of the transmitted light, thereby facilitating the recording of amplitude or phase holograms.

ACKNOWLEDGMENT

E. H. is grateful to the U.K. Science and Engineering Research Council and Pilkington Brothers plc. for the provision of a CASE studentship. 451

As,S 3 INTERFERENCE GRATINGS 237

REFERENCES

I A. Matsuda and M. Kikuchi. J. ipn. Soc. App!. P/irs.. 42(1973) 239-248. 2 A. E. Owen. A. P. Firth and P. J. S. Ewen. P/il/os. Meg. B. 52(1985)347-362. 3 A. V. Kolobov. S. R. Elliott and M. A. Tastuirdzhanov. P/il/os. 'dug. B. 6/(1990) 859--86S. 4 G. C. Chern and 1. Lauks. J. App!. Phi-s..54 (1983)4596-4601. 5 E. Hajto, P. J. S. Ewen. R. Belford. J. Hajto and A. E. Owen. J. Non-Cryst. Solids. 97-98 (1987) 1191-1194. 6 G. C. Chern and I. Lauks, J. Appi. P/irs.. 54(1983)2701-2705. 7 E. Hajto. P. J. S. Ewen. P. G. Hill and A. E. Owen. P/ivs. Stunts Solidi A. 1/4 (1989)587-597. 8 E. Hajto, R. E. Belford. P. J. S. Ewen and A. E. Owen. J. Von-Crrst. Solids. //5(1989)129-131. 9 K. Kase, G. C. Chern and I. Lauks. Thin So/id Film. //6(1984) L53-L54. tO A. Zakery. C. W. Slinger. P. J. S. Ewen. A. P. Firth and A. E. Owen. J. P/irs. D. 2/(1988) S78--S81. II G. C. Chern and I. Lauks. J. App!. P/irs.. 53 (1982) 6979-6982. 12 R. Swanepoel. J. P/irs. E. /6(1983) 1214. 13 F. Abeles. Optical Properties of So/utv. North-Holland, Amsterdam. 14 M. C. Hutley. D/fractioii Gratings. Academic Press. London. 1982. 15 H. M. Smith (ed.). Holographic Recording Materials. Topics in Applied Physics, Vol. 20. Springer. Berlin. 1977. 16 T. Fukaya. S. Matsumura.J. Tsujiuchi. E. Inoue and H. Kokado, Opt. Coniniu,i.. 7(1973)98-102. 17 M. C. Hutley, J. Physics E. 9(1976) 513-520. 452

PHILOSOPHICAL MAGAZINE B, 1991, VOL. 63, No. I, 349-369

Analogue memory and ballistic electron effects in metal—amorphous silicon structures

By J. HAJTO, A. E. OwEN and A. J. SNELL Department of Electrical Engineering. University of Edinburgh. Edinburgh EH9 3JL Scotland

and P. G. LE COMBER and M. J. Rose Department of Applied Physics and Electronic and Manufacturing Engineering. University of Dundee, Dundee, DDI 4HN, Scotland

[Received 11 June 1990 and accepted 15 June 1990)

We present experimental results showing that p 1 amorphous silicon memory structures exhibit polarity-dependent analogue memory switching The effect is non-volatile and we propose that it is associated with changes in a tunnelling barrier within the structure. It is also observed that conduction in the memory ON state is restricted to a narrow conducting channel through which the electrons can, under certain conditions, travel ballistically. As a consequence, quantized resistance levels associated with ballistic electron transport are observed under certain circumstances. In the presence of a magnetic field, additional steps in the quantized resistance levels occur. A particular feature of this quantized resistance is that the effect can be observed at relatively high temperatures (up to about 190K).

§ I. INTRODUCTION Results on metal—p'—n--i—metal amorphous silicon devices have provided experimental evidence that such structures exhibit extremely fast non-volatile polarity- dependent digital memory switching phenomena (Le Comber et al. 1985) after initial conditioning by means of a moderately high applied potential ('forming'). The most important result to emerge on formed devices is that in the ON state the currrent is carried by a highly conducting filament which is less than I pm in diameter. Filamentation has been demonstrated by experiments on the ON state resistance as a function of area (Rop, is independent of area), by thermal imaging techniques with liquid crystals and by direct observation with a scanning electron microscope combined with microanalysis. The latter indicated that the formation of the current filament may be associated with diffusion of the top metal contact into the amorphous silicon resulting in a region of intermingled metal and silicon. More recent experimental results (Rose et al. 1989) have demonstrated a new metal—p--metal amorphous silicon device which, rather than exhibiting a two-state digital operation, has a continuum of stable states which are non-volatile and fully programmable by single 10 as voltage pulses. It has also been suggested that the new analogue memory devices can be used as non-volatile and reprogrammable memory elements in analogue neural networks (Hajto, Rose, Snell, Le Comber and Owen 1990). In this paper we present a summary of the new results obtained on non-volatile analogue switching effects in amorphous silicon metal—p + —metal devices and discuss the possible physical mechanisms responsible for the phenomena.

350 J. Hajto a al.

§ 2. EXPERIMENTAL The samples used for this work were amorphous silicon Cr-p-V thin-film structures. The p layer was prepared by rf. glow-discharge decomposition of SiH 4 containing i0 vol. p.p.m. of B2H6. Films of 1000 A thickness were deposited on Corning glass substrates previously patterned with chromium bottom contacts. The p amorphous silicon was then patterned and an insulating layer was used to define an active device area of 10 6 em 2 The metal used for the top contact was normally vanadium. However, a number of different metals were also used and their influence on the memory operation will also be described. In accordance with our previous results (Le Comber a al. 1985), memory devices prepared in this way require an initial forming process. This means that the resistance of the as-deposited (unformed) device has to be lowered from R 109 Cl to R 10-IO'Cl (i.e. the typical value of an ON state). The forming can be achieved by biasing the sample with a single voltage pulse (duration 300 us; magnitude about 12 V) with positive polarity applied to the top vanadium contact. The formed samples were mounted in a 24-pin chip carrier and wire bonded. The carrier was then inserted into a standard socket in an Oxford Instruments cryostat in order to investigate the low-temperature characteristics. The current-voltage characteristics were measured using an HP 4145B semiconductor parameter analyser.

§3. RESULTS The metal-p * -metal memory structures exhibit a forming step which is different from the previously investigated metal-p-a-i-metal structures (Le Comber a aL 1985). The differences are demonstrated in figs. I and 2. In the case of metal.-p 4 -n-i-- metal structures, the resistance suddenly drops from about lO' ° fl (virgin state) to about 101 Cl, after the critical voltage (forming voltage I) has been applied. No change in the virgin resistance occurs when the sample is biased with voltages less than VF. This type of forming is termed hard forming. In contrast with this, the resistance of the unformed metal-p--metal structures can be lowered gradually by applying voltage levels with progressively increasing magnitudes. In this case no sudden change in the current or voltage signal can be detected when the sample is biased with a voltage pulse, as seen in fig. 2. This process is called soft fornung. Figure 3 shows the device resistance as a function of the soft-forming voltage (pulse duration, 300 ns). On reaching a critical voltage (about 14 V in fig. 3) the device resistance suddenly drops from about 106 to about l0-10 Cl. This is the memory ON state of the device. Once the memory device had reached its first (non-volatile) ON state all subsequent switching operations were performed with IG-lOOns pulses 1-5 V in magnitude. An example of the analogue switching effect is shown in fig. 4, where the sample resistance is plotted as a function of applied alternating WRITE and ERASE pulses (100 ns pulse duration each). It is important to emphasize the polarity dependence of the analogue memory behaviour. In the case of the WRITE pulses, positive polarity is applied to the chromium (bottom contact) while ERASE pulses have opposite polarity. The sample was first switched to an ON state (R = 2 x IW (1) and then a series of alternating WRITE and ERASE pulses were applied. The WRITE pulses were kept at a constant magnitude of 34 V but the ERASE pulses were incremented by 005 V steps from 12 to 34V after each WRITE pulse. It can be seen from fig. 4 that the sample resistance changes in an analogue manner as the magnitude of the ERASE pulse increases, that is the difference between Rcm and Rorr is a function of the magnitude of the ERASE pulses. A voltage range AV(ERASE) of 16V resulted in a change in 454

Analogue memory in metal-a-Si structures 351

Fig. 1

'yr

0 100 200 300 TIME (ns) Waveforms of a single forming pulse applied to an amorphous silicon Cr-p-n-i--AI structure showing 'hard forming'. V and I represent the voltage across the device and the corresponding device current respectively.

Fig 2 20 V(V) W-ew*q VP 10

f(mA) 0.31

0 200 400 •: TIME (ns) Waveforms of single pulses of increasing magnitude applied to an amorphous silicon Cr-p"-V structure showing 'soft forming'. V and 1 represent the voltage across the device and the corresponding device current respectively. 455

352 J. Hajto et al.

Fig. 3

50'

12 14 Fom*V VOUP (VI

Resistance of an amorphous silicon - 4 --Vmemory structure as a function of forming voltage.

Fig.4

R ( 0'

V .1.6 'e6

ri a Eras. Vos (V) Memory resistance as a Function of ERASE voltage in a Cr-p-V structum

1 03 resistance from R2 x a to R6 x Jos (I The shaded area in fig. 4 indicates the reproducibility of the analogue memory switching by showing the scattering in the resistance during repeated experiments (data from 100 cycles are included). Figure 5 shows another case where the ERASE pulses were maintained at a constant value of V= 34 V but the WRITE pulses were incremented from 12 V to 34 V in 005 V steps. The value of the OFF state resistance remained constant at about 6 x 10 3 0 (i.e. it changed back to this constant level from every ON state) whilst the ON state resistance decreased through a continuum of intermediate states over a similar A V to the ERASE operation. The shaded area in fig. 5 again represents the reproducibility of the analogue switching for 100 complete cycles. It is emphasized that the device will switch between any two resistance states within the range from about I kfl to I MCI by selecting the correct polarity and the magnitude of the WRITE and ERASE pulses. For all devices with a vandium top contact, the values of A V range from 1-5 to 2-0 V for both the WRITE and the ERASE operations. We have repeated the above experiments on metal-p'-metal devices with chromium as top metal and observed similar polarity-dependent changes in the memory state resistance. However, in the WRITE and ERASE experiments, intermediate states were found to exist only over a narrow zVof about 0-2 V as shown in fig. 6. Therefore these devices are considered as 'digital' devices. It is important to 5 VIM

Analogue memory in metal-a-Si structures 353

Fig. 5

10' R (Q) 10

1 2 3 V*VA Volts (V) Memory resistance as a function of WRITE voltage in a Cr-p -V structure.

Fig. 6

io$ Q) .-..... WRITE

f.02V 10' ERASE

10' 1 Z j Voltags (V) Memory resistance as a function of WRITE and ERASE voltages in a Cr-p-Cr structure.

emphasize that both the analogue (seen in figs. 4 and 5) and the 'digital' (seen in fig. 6) memory switching effects are non-volatile. Devices set to ON or OFF states have been monitored over 2 years without any significant change in their resistance. Also, operation at temperatures up to 160°C shows little change in the threshold voltages or in the device stability. However, it is also found that devices with certain top metal contacts such as molybdenum and palladium show a volatile memory switching effect. This is illustrated in fig. 7 where the signal through the device is continuously monitored at a low voltage level (at 05 V) that is below the voltage level of the programming pulses. The current decays rapidly alter the end of the programming pulse, that is the memory state is volatile. The role of the top metal contact has been investigated by fabricating devices with a range of different top metals but with otherwise identical physical parameters (i.e. about 1000A thickness of the p " layer, and a chromium bottom electrode). Using the analogue switching voltage window Van a guide, it is found that its value is significantly dependent on the top metallization contact This is illustrated in the table. It can also be seen that the definition of 'analogue' (z V? I V) or 'digital' (0-5 V or less) memory switching is somewhat arbitrary, because there is no sharp boundary between the two types of operation, and they are almost certainly associated with the same underlying physical phenomena. However, in the cases of molybdenum and palladium top contacts, a new type of volatile 457

354 J. Hajto et al.

Fig. 7

V lWRlTE.

I I

0.5 vol rssd 44 70.ERISE pi•

50 100 150 TIME (ps) Volatile memory effect in a Cr-pt-Mo structure.

Effect of top metallization on the switching behaviour. AV Switching Metal (V) characteristics Ag >01 Digital, non-volatile Al >01 Digital, non-volatile Cr 02 Digital. non-volatile Mn O5 Digital, non-volatile Fe O5 Digital, non-volatile

Ti - Unstable switching Au - No switching Cu - No switching W H) Analogue, non-volatile V 18 Analogue, non-volatile Ni 20 Analogue, non-volatile Co 20 Analogue, non-volatile MO 20 Analogue, volatile Pd 20 Analogue, volatile

switching effect is observed and in the cases of titanium, gold and copper no reproducible switching effects can be observed. These results suggest that the top metal contact plays a crucial role in determining the type of memory switching phenomena observed in these devices. In this paper we shall concentrate on the p + memory devices with vanadium top contact because these show typical non-volatile analogue memory switching. The current-voltage characteristics of analogue memory resistance states have been systematically investigated at both room temperature and lower temperatures and the following results obtained. It is found that all the room-temperature current- voltage characteristics show a 'linear-plus-power-law' behaviour in the various analogue memory resistance states (seen in fig. 4 The I-V curves can be described by a simple nonlinear relationship I=c1 v+c2 V•, (1) 458

Analogue memory in metal-a-Si structures 355

Fig. 8

1(A) - ON 1o•

10 isI kwPn 16

Vo1g(V) Current-voltage characteristics of analogue memory states plotted on a log-log scale. The characteristics are symmetric about the origin.

where C 1 and C2 are constants and the exponent n increases with increasing low-bias (linear region) resistance according to the relationship n = A + B log R. The observed power-law behaviour could indicate the possibility of space-charge-limited conduction at higher biases, although investigation of the thickness dependence has shown this to be unlikely. In accordance with the pulsed analogue memory switching results, a continuous transition of states can be found between the low-bias ON (about iO () and OFF (about 106 Q) states. The terms ON and OFF seem to be somewhat arbitrary therefore, and are only used in this paper for practical reasons. If the curves in fig. 8 are extrapolated above 1 V. they meet in the region of about 3-4 V, that is at typical programming levels (see figs. 4 and 4 It is also found that the exponent n does not depend on the thickness or the active device diameter but it is primarily determined by the low-bias (linear region) resistance of the analogue memory state. This indicates that the nature of the electrical conduction is quite similar in all memory states. However, repeatedly switching the device into the same resistance state need not always result in an identical value of the exponent. This appears to indicate that the 'same' resistance state can be achieved through different conduction paths within the same device. This is in accordance with the suggestion that the conduction path (filament) might have a structure similar to that of granular metals embedded in an insulating matrix and this structure may provide a variety of conduction paths (Giaver and Zeiler 1968). It is possible to extend the range of the I-Vcharacteristics (up to the critical field where switching occurs) using very short single voltage pulses of varying polarities and pulse heights. Figure 9 shows the room-temperature 'pulsed' current-voltage characteristics of the analogue Cr-p-V memory device. These characteristics are obtained using 400 ns pulses of progressively increasing magnitude and of both polarities. The 400 as pulse length is long enough to observe a plateau in the pulse signal, that is RC effects are avoided. Positive polarity refers to (positive) voltages applied to the chromium bottom contact. Starting from a 95 x iO fi OFF state (measured at 05 V (fig. 9(a))), the onset of the strong nonlinear rise in the current occurs at about + 1-1 V. Figure 9(a) is reproducible (i.e. it can be repeated many times without a change in the device resistance) up to a voltage level ofabout + 17 V. Further increase in the voltage height will result in a permanent decrease in the device resistance and consequently a change in fig. 9(a). The decrease in resistance is determined by the magnitude of the maximum voltage applied. 459

356 J. Hajto et al.

Fig. 9

14.

0.2k .• -2 - •'• i• 2 :MA)

••• -02 Cd

-0.4

Pulsed' current-voltage characteristics of a Cr-p-V analogue structure.

103 Figure 9(b) represents a memory ON state (R = 56 x fl at 0'S V) of the device and is reproducible up to a voltage level of about 5 V if positive polarity is applied. Further increase in voltage might destroy the device. However, if a negative voltage is applied to the same ON state (fig. 9(c). an ERASE process is observed at voltage levels of magnitude greater than about 17 V. On the other hand, if an OFF state (R = 9 x 10' l) is negatively biased fig. 9(d), no change in the OFF state resistance can be observed up to a voltage of about —5 V. The apparent polarity dependence suggests that the analogue memory switching is not determined simply by the magnitude of the applied power or energy. This is further supported by the comparison of individual switching transients with opposite polarity. Figure 10(a) shows the waveform of a single WRITE pulse from OFF (R=2x 10' (1) to ON (R=38x 101 (1) and of an ERASE pulse from OFF (R = 2 x IO (1) to a slightly higher OFF state (R = 24 x 10 - M In the second case a large change in the memory state resistance does not occur although similar voltage and current levels are measured. The calculated total charge (flowing through the sample) is also rather similar Qw = S'S x 10 I for the WRITE and QE = 7'2 x 10-11 C for the ERASE pulse. The rather high level of 'ringing' oscillations on these transients was caused by poorly matched cable impedances. These measurements suggest that the memory state resistance is determined by a combination of applied voltage level and the appropriate polarity. On the other hand, the memory resistance not being determined uniquely by the applied power or energy suggests that the memory switching does not depend significantly on the internal temperature of the device. Figure 11(a) shows a switching transient of an ERASE pulse at 300K where the device resistance is changed from R=3 x 103 Q to = 6 x 10' U. Figure 11(b) shows a similar ERASE transient at much lower temperature (42 K) where the device resistance has also changed from Rce = 3 X IW U to ROFF = 6 x 101 U. It can be seen that, despite the large temperature difference (more than two orders of magnitude), the current level and the threshold voltage (for achieving the same ERASE process as at room temperature) increase by a factor of less than two. It should also be emphasized that the device continues to operate even at liquid-helium temperature. The analogue switching effect is still observed at 4'2 K without large changes in either the threshold voltage or the current level. This suggests that the electrical conduction and the memory phenomena are possibly connected to a temperature-independent physical process which we propose may be associated with tunnelling. 460

Analogue memory in metal--a-Si structures 357

Fig. 10 4 VM 22

0 50 100 150 200 0 50 100 150 200 TIME (na) TIME (as) (a) (b) Waveforms or single-pulse memory switching (a) OFF-.ON transient; (b) OFF-OFF transient.

Fig. 11 8 Y(V) V

0 100 200 300 400 0 100 200 300 400 TIME (ns) TIME (ns) (a) (b) Waveforms of single pulse memory switching (a) ERASE transient at 300 K.(b) ERASE transient at 4-2K. The above suggestion is supported by recent results obtained during investigation of the low-temperature conductivity of the analogue memory states (Hajto et al. 1990). Typical current-voltage characteristics of a formed memory ON state at 4-2 K are shown in fig. 12. In the voltage region from 010036 V. the current around zero bias is of the order of 10 9 A but increases to about 10 - " A at voltages approaching 036 V. that is a strong nonlinear behaviour is found. It is important to emphasize that the room- temperature current-voltage characteristics of the memory ON state are linear. The observed large increase in the resistance around zero bias (fig. 13) is consistent with tunnelling conduction between metallic particles embedded in an insulating matrix (Giaver and Zeller 1968). Further experimental evidence for the tunnelling conduction comes from the temperature dependence of the device current Figure 14 shows the current at a fixed voltage (02 V) plotted against T2. The linear plot indicates a P dependence of the current at a constant V which is in accordance with tunnelling conduction. At 036 V, a current jump occurs and the resistance of the sample is lowered to the order of a few kilohms. After the first current jump at 036 V, further current steps can be observed at 047, 0-53 and &70V (fig. 12 (a)).At 0-36V (henceforth called the critical voltage Vi,) there is a dramatic change in the behaviour of the sample. At voltages lower than V, no discontinuities are observed but, at voltages higher than V. the resistance 461

358 J. Hajto et al.

Fig 12

I (mA) 0.4

0.3

0.2

0.1 1/

0.1 0.3 0.5 0.7 Vc(V)

Current-voltage characteristics at 42 K (B) with and (A) without magnetic field. (B) is displaced vertically for clarity.

Fig 13

id'

R(Q)

Vos(V) Resistance as a function of applied bias at 42K.

Fig. 14

0 500 1000 1500 2000 2000 0a) 1.n.riIa,' Current (at fixed voltage of 02 V) as a function of T2. 462

Analogue memory in metal-a-Si structures 359 is lowered and the current increases indiscrete steps. The current-voltage characteristics are symmetrical, that is the same behaviour is observed for the opposite polarity. Figure 12(B) depicts the current-voltage characteristics of the same sample under the influence of a (P2 T magnetic field. The curve has been displaced by 50 pA in the current scale for clarity. The direction of the magnetic field is 30° with respect to the conducting channel (Le. the filament). Additional steps can be observed at 034,042,0'S, 059 and 079 V together with the steps observed in the zero magnetic field case (fig. 12(B)). The effect of the magnetic field is reversible, that is, if the magnetic field is removed, the current-voltage characeristics revert to the zero-magnetic-field case. A number oil- Vmore characteristics have been obtained in which sharp and well defined steps can be observed at 42 K. The critical voltage V. at which the first current jump is observed and the magnitude of the first current jump are dependent on the resistance of the memory ON state investigated. Figure 11 shows the effect of changing the memory ON state resistance on the observed current steps. The curves have been displaced by 100 pA on the current scale for clarity. The critical voltage and the magnitude of the first current jump increase with increasing resistance (i.e. R, > R> RA but after the first jump the characteristics are rather similar, suggesting a similar conduction mechanism at higher voltages. The first current jump appears to be associated with the formation of a highly conducting path within the structure whose characteristics are independent of the low- bias behaviour (i.e. the different memory states). On the other hand, the position of the current steps is dependent on the direction of the voltage sweep, that is some hysteresis is observed as illustrated in fig. 16. Further information can be obtained if the resistance of a memory state (corresponding to fig 12(A)) is plotted as a function of applied voltage as Illustrated in fig 17. We propose that the first large current step (at (}36V) is associated with the formation of a narrow, highly conducting channel which significantly lowers the resistance of the sample. With further increase in the applied voltage, the resistance is lowered in steps, corresponding to quantized resistance values R = h/2ie2 where i is an integer. In the voltage range from 036 to 08 V. there are four steps (corresponding to the observed current rises in fig. 12(A))with i being 2, 3,4and 5. Higher voltages have not been applied to the sample because this could change the resistance of the particular memory state. If a magnetic field is now applied to the sample, further quantization of resistance is observed at values R=h/2(i++)e2. This is illustrated in fig 18 (the data correspond to fig 12(B)). It can be seen that extra steps inthe resistance occur at £ = 25, 35, 4.5 and 55. The effect of temperature is illustrated in 6g. 19. The curves have been shifted along the current axis for clarity. The observed current steps gradually decrease with increasing temperature until the effect is no longer observable at about 190 K. The results summarized in figs. 1549 are indicative of ballistic electron transport (Hajto et al 1990). §4. DiscussioN In analysing the main results of this work, two important facts should be emphasized. Firstly the observation of ballistic electron transport provides a vital due to the structure of the analogue memory element. Secondly, the programmability of the analogue memory provides information about the possible mechanism of the switching process itself. The starting point of our discussion is that the analogue memory effect in amorphous silicon metal-p--metal structures can only be observed if the sample is 463

360 J. Hajto et al.

Fig IS I (MA) 0.5

0.4

0.3

02 :TT.J. - 0 02 0.4 0.6 0.8 Vogs(V) Current-voltage characteristics of different memory states at +2K. R> R1 > R4 (curves displaced for clarity)

Fig. 16

0 - o me me me vcv Current-voltage characteristics at *2K showing the hysteresis observed on first increasing and then decreasing the voltage.

Fig. 17

oi 35 : Vcs() Resistance against voltage at 4-2K with no magnetic field: (---) indicates the idealized contribution from the ballistic transport channel. 464

Analogue memory in metal -a-Si structures 361

Fig, 18

H -

6 \\ S

587O.8 volig. (V) Resistance against voltage at 4•2 K with a 02 T magnetic field.

Fig. 19

I / uiA)

0 240 480 720 960 Voltage (mV) Current-voltage characteristics as a function of temperature. The curves are displaced vertically for clarity. subjected to an initial forming process. The forming process is characterized not only by the breakdown of the high-resistance state of the structure but, more importantly, also by presence of a positive feedback mechanism which provides a low-resistance ON state so that the breakdown is non-destructive and repetitive switching is possible. Furthermore, the experimental results suggest a strong influence of the choice of the top metal contacts (summarized in the table) on the type of memory switching observed (i.e. digital or analogue switching) and on the suss of obtaining stable and reproducible switching. These results are in good accordance with the previous observation (Gage et al. 1989) that the first switching event (i.e. the forming) causes a local structural modification of the p' amorphous silicon layer, producing a highly conducting filament which does not revert to the original amorphous material when the device is switched OFF. After the forming process the p * devices usually exhibit a lower OFF resistance than the unformed device in contrast with our original data for p-n-i devices (Le Comber et al. 1985). The temperature dependence of the conductivity is also greatly reduced by the forming process. The area independence of RON (Le Comber 465

362 J. Hajto et al. el al. 1985) suggests localized electrical conduction after forming. These results, together with the newly observed ballistic electron transport phenomena (see figs. 12-18), provide strong experimental evidence that the forming process creates a filamentary region consisting of a new material whose properties have changed significantly compared with those of the unformed original material. It is feasible that the high fields and current densities present during forming result in the development of high temperatures locally, which could lead to enhanced diffusion of metallic particles from the electrode into the thin amorphous film. Such a region would become the preferred current path carrying the electron current in the ON state. The current-voltage characteristics in the ON state at room temperature (see fig. 8) suggest that any material rearrangement within the filament occurs so as to destroy the rectifying properties of the original metal-p + Schottky contact, and it should also be noted that this is the case even in a typical OFF state. Lowering the temperature of the memory device reveals further information about the possible structure. The low-temperature current-voltage characteristics (see figs. 12 and 13) show that we have observed a zero-bias high-resistance anomaly and quantized resistance steps (associated with ballistic electron transport) in the ON state of amorphous silicon Cr-p + - v structures. The phenomenon of ballistic transport is observed in the case when the mean free path .t of the electrons is larger than the length of the conducting channel. The usual approach to the fabrication of devices based on ballistic conduction is to use a very high mobility material (usually high-purity GaAs), where the mobility can reach values of the order of 101 c& V 1 s, leading to values of the electron mean free path of the order of 1 pm. The value of A 1 pm is certainly longer than the device dimensions which can be achieved by modem submicrometre technology. The structure in which ballistic transport is most widely investigated is a GaAs-AlGa 1 As heterojunction with a split-gate field-effect transistor configuration, usually less than 05 pm in length, with a gap of about O-lJim (Wharam etal. 1988, Van Wees et al. 1988). As the voltage on the gate is made increasingly negative, the depletion region increases, narrowing the effective gap and hence the width of the conducting channel decreases. As a consequence, the (one-dimensional) channel width becomes comparable with the electron wavelength, that is becomes sufficiently small that quantization occurs. On the other hand, the channel length is sufficiently short that electrons pass through ballistically (i.e. without appreciable scattering). Our structure is quite different and a possible mechanism which explains the ballistic transport in amorphous silicon structures is outlined below. The current-voltage characteristics (see fig. 13) suggest that the filament has relatively large resistance around zero bias, that is a barrier for the current flow exists. The observed zero-bias anomaly is consistent with tunnelling conduction between metallic particles embedded in an insulating matrix (Giaver and Zeller 1968). The current-voltage characteristics from 0 to 036 V show a continuous, but strongly nonlinear behaviour owing to the the high electric field across the small tunnelling distance. In such a system the injection of free carriers is an activated process related to the increase in the electrostatic potential of a particle when a free electron is added to it. The activation energy can be provided entirely by thermal energy (hence the effect diminishes at higher temperatures) or, in the presence of an applied field, part or all of it can be provided by the field itself. The experimental observation of an approximate T2 dependence of the current at a constant voltage observation of Less than V., (see fig. 14) is also in agreement with the assumption that the electron transport is dominated by field-activated tunnelling processes in this region. 466

Analogue memory in metal-a-Si structures 363

These results can be partially explained by assuming that the conducting filament is composed of two parts: small-scale inclusions of permanently changed material connected by conducting channels which are formed, broken, dimensionally changed, re-formed, etc, during switching. Figure 20 illustrates an idealized model, having a single permanent inclusion extending from the top contact, with a narrow channel connecting it to the bottom contact. The evidence is (Le Comber et al. 1985) that the overall diameter of the filament at the top contact is less than 05 sun. The length of the channel must be consistent with tunnelling. With increasing applied voltage, the tunnelling current increases exponentially. This implies that at the critical voltage 1, the tunnelling barrier effectively breaks down' and a very large current flow occurs. This is not a destructive effect as the process is completely reversible and no material changes ensue. Because of the curved shape of the metallic inclusion illustrated in fig. 20, the current flow is restricted to a very localized area. Thus the channel can be considered to be an electron waveguide with confinement being brought about by the combination of the applied field and the geometry of the metallic inclusion. The observation of ballistic transport shown in figs. 15-19 can only be explained by assuming that the carrier transit time across the channel must be less than a scattering time, that is t

eu 2 (2) moy where m' is the effective mass, p is the mobility and d is the channel length. With d50A and Vit this leads to p100cm2 Vs 1. The question remains, however, of whether it is feasible to have such a mobility in the material of the conducting channel. Assuming that the channel length is about 50 A (typical tunnelling length), the potential gradient along the channel is about &36V/50A, that is E7 x 10' Vcsn '. The low-field value of mobility in amorphous silicon is about 10cm2 V s , but this might be increased by the presence of high local fields, when the more energetic carriers might have a larger mean free path because of the reduced scattering cross-section. There is also a possibility that, because of the presence of the metal, alloys could be formed, yielding still higher mobility values. In this connection it is worth noting that we have previously estimated a carrier mobility of l00cm2 V 1 s in similar devices from magneto-resistance measurements (Le Comber et al. 1984 Therefore, although there is considerable uncertainty regarding the structure of the channel and the mobility value there, it looks quite certain that it is

Fig 20

411

Schematic description of the filament showing the proposed metallic inclusion and the one- dimensional conducting channel. 467

364 J. Hajto et al. the small channel length which makes ballistic conduction possible at much lower values of mobilities than in the previous conveñtional' cases (Wharam et al. 1988). It is important to emphasize, however, that in our case the electron mobility does not need to have the same very high value as in the case o1GaAs—AJGa As heterostructures (Wharam et al. 1988). In our case the one-dimensional conducting channel is established by the dramatic increase in the tunnelling current at voltage Va,,. A further increase in voltage in the ballistic regime results in an increase in the cross-section of the conducting channel. This effect can be explained by assuming that the edge of the permanent inclusion has a curved shape as illustrated in fig. 20, that is the tunnelling barrier will break down at larger areas under the influence of higher voltages. The larger voltage applied will produce the same critical electric field at larger distances between the inclusion and the metal electrode therefore the area of the one- dimensional channel increases. It should be noted that the voltage has an opposite effect on the cross-section of the conduction channel in the case of split-gate structures where the increased voltage level 'squeezes' the channel width. In our case the electrons are confined by the applied potential in the conducting channel whose cross-section increases with increased voltage levels. This cross-section can be approximated by a rectangle (in a manner analogous to the 'electron-in-a-box' problem), whose dimensions L, and L, are comparable with an electron wavelength (fig. 20). The wavefunction V and the energy of the electrons in the lowest quantum level of the one-dimensional channel are given by P=L; 112 exp(ikic)g,(y)g3(z), (3)

A2 h [/\ 21 E = - kk 2 + (4) 2m' rj',) where g, and g,, are the normalized standing-wave functions of the forms 2"2 cos(iy/L,) and 2" 2 cos(z/L) respectively, L is the length of the conducting channel; L. and L, are the cross-section dimensions of the channel (fig 20). The first term in eqn. (4) describes the kinetic energy of a ballistic electron in motion along the channel. The second term represents the quantum energy level which is uniquely determined by the dimensions L, and Lr It is important to emphasize that the electron can accelerate freely along the channel, owing to the collision-free nature of the motion, that is its momentum can increase continuously as the applied voltage increases. On the other hand the electron momentum is quantized in the direction across the channel (defined by L. and Li). Therefore, as the voltage increases, two opposite effects can be observed; one is characterized by the increase in the electron kinetic energy along the channel and the other is associated with the increase in the cross-section of the conducting channel (i.e. with increase in L, and L1). According to eqn. (4) this latter effect will decrease the quantized energy levels until the nearest higher energy level (termed sub-band (Landauer 1985)) becomes lower in energy than the Fermi level and will be occupied by the 'more energetic' electrons. Consequently a new sub-band is created at higher voltage (i.e. at larger values of L, and L,) which in turn will lower the resistance in a quantized manner. This physical scenario was first suggested by Sharvin (1965) as a possible method for studying the Fermi surfaces of metals. It is thus a necessary consequence of the observation of quantized resistance values that the quasi- Fermi level must be within the sub-bands. This we believe to be a direct result of the extremely high current density in the channel, estimated to be of the order of 10 1 A cm'.

468

Analogue memory in metal—a-Si structures 365

According to the theoretical predictions (Landauer 1985, lmry 1986, Wharain et al. 1988), the resistance of a one-dimensional channel (in which the electrons punch through ballistically) should have a quantized value. This is because, in the absence of scattering, the change of electron kinetic energy is determined directly by the applied field across the one-dimensional channel. The current flow is given by 1=ne8v, (5) where n is half the number of carriers per unit length within one particular electron energy level (sub-band) and 8v is the increase in the electron velocity acquired on transit through the one-dimensional channel. The number of carriers can be obtained by integrating the density of states in one dimension: a.. (m*EF\1 /2 g,Jnv n= 'N(E)dE=---I (6) Jo 21th2 J 2h' where Eis the Fermi energy and vF is the Fermi velocity. it is assumed that the applied voltage is sufficiently small that the velocity acquired by the electrons during the transit in the channel is small compared with the Fermi velocity. In this case the increase in velocity is given by 6o=eV/mv. (7) Substituting eqns. (6) and (7) into eqn. (5) yields

I =!?V (8) ' 2xh for the current and h h (9)

where R is the sub-band resistance and g, is the spin degeneracy (g=2) in the non- magnetic field case. However, the value of R = h/2e2 applies to only one sub-band. If a new sub-band is created at a higher voltage (i.e. by changing the cross-section of the channel), the total resistance becomes h/2ie2 (i is an integer defining the number of sub- bands below E) in the same mannerasin the case Of parallel resistors. ThlsiSpossible because there is no scattering in the conducting sub-bands, that is they do not interact with each other. Consequently the r.sistance drops in a quantized manner with I = 2,3, 4and 5(as seen in fig 14 In the case of our results the plateaux of the individual quantum steps are not flat but they have a finite slope, that is the resistance decreases at higher voltages between the individual plateaux. This could be caused by the presence of additional non-ballistic conduction mechanisms. The broken line in fig. 17 indicates the resistance—voltage characteristics expected from the idealised ballistic component We emphasize that the data presented in figs. 15-19 are raw data, and no attempt has been made in the present work to distinguish between the various contributions to the conductivity. in the above model we have considered the effect of moving the sub- bands through the quasi-Fermi level by changing the dimensions of the channel. In principle, we believe it would be equally possible to explain the data with a model in which the sub-bands are fixed in energy and the quasi-Fermi level is varied by the changing concentration of injected charge in the channel as a function of applied voltage. 469

366 J. Hajto et al. When a magnetic field is applied, additional steps occur. This is probably due to the Zeeman splitting of electron energy levels, that is a lifting of the spin degeneracy splits each level into two. Therefore extra steps will be observed at R = h/20 + )e2 resistance levels as seen in fig. 18. The magnitude of the effect seen is rather surprising for a temperature of 42 K and a magnetic field of 02 T. However, this could be explained by assuming an anomalously high value for g,. The most important feature of the observed ballistic behaviour is that it can be observed up to about 190K namely, much higher temperatures than previously observed. We suggest that this might be due to the very small size of the ballistic channel. The model suggested above describes the possible mechanism for the ballistic transport and provides important information about the structure of the filament but, as it stands, it does not explain the memory switching. The most important difference between the two phenomena lies in the fact that ballistic transport is observed at applied voltage levels lower than about I V. that is at voltage levels where no memory switching occurs. The observed quantized jumps in the device resistance are of the type associated with threshold switching. if the voltage is lowered, the sample resistance reverts to the original low-bias case, that is there is no permanent change in the current-voltage characteristics of the device. On the other hand, memory switching occurs at voltage levels from about 15 V to about 4 V, resulting in a permanent change in the current-voltage characteristics of the device, that is a different resistance state of the memory. In order to explain the different and permanent resistance states a model for memory switching should embody one or more of the following features: changes in the conductivity of the whole filament; local changes in conductivity at a certain point(s) in the filament; changes in the filament geometry. Model (a) would presumably require an average energy for switching of a similar order to that established for forming, but it has been found that ON-.OFF-.ON switching can be achieved using single pulse energies of about 10 -10 .1 (see fig. 10). In addition, if it is assumed that the filament is homogeneous, there is no obvious way of introducing a polarity dependence. However, if there are inhomogeneitieS in the filament, it may be possible to increase substantially the local energy density and field strength. Consider for example, the simple one-dimensional arrangement shown in fig. 21. The regions A consist of a high-conductivity material separated by regions of lower-conductivity material B. The observed resistance of the filament will depend Fig. 21

(a) (b) (c) Possible one.dimensioflal arrangement of the conducting filament; (b). (c). schematic energy (a) diagrams for the tunnelling region.

470

Analogue memory in metal-a-Si structures 367

upon the proportions of A and B and hence the relative contributions to the filament resistance of regions A and B. If R 0 < R A, the properties of the type A material will be observed and, if RA < RB, the properties of the type B material will be observed. If the 'gaps' between the highly conducting regions are small, then it is also necessary to consider the possibility of inter-island tunnelling. In order to establish such a model, a more detailed description of regions A and B must be included, for example, as follows. (I) A is metallic or a degenerately doped material whose contribution to the filament is fixed and OA A 1(1') where V is the applied voltage and a is the conductivity. (2) B is a 'gap' between type A material which is sufficiently small to allow significant tunnelling to occur, consequently a n is some function of the voltage. One possible explanation is that in the ON state, R

' 3x10 9d2T2\ J(V, T)—J(V, O)(I + ( 10) • ), with din Angstrom units, 4 in electronvolts and Tin kelvins (Giaver and Zeller 1968). The T1 dependence of J at a constant Vhas been observed (see fig. 14) and the slope correlated with d=50A and 0=1eV. How might switching be achieved in terms of the tunnelling model? The low-bias isothermal resistance of 'region-B-like' parts of the filament can be expressed as R=cexp(d0 112), (11) 471

368 J. Hajto es al. where the constant c is determined by geometric factors and the transmission probability. This expression gives R directly in ohms if d is in Angstrom units and 'P is in electronvolts (with c as a small correction factor). Taking d = 50 A and 'P =1 eV, then R 1 =cexp(50) and increasing d by 5 A gives R 2 =cxp(55), that is R has increased by about a factor of ISO. An equivalent increase in resistance is obtained by keeping d constant and increasing 'P from I to 12eV. Therefore dimensional changes in the tunnel barrier on an atomic scale, or small (20%) changes in barrier height, could account for the presence of switching. A change in the particle size and/or their spatial distribution as a result of localized heating or high field effects could explain the differences between high- and low-resistance states. The high switching speeds (about tOns) and the insensitivity of switching to temperature suggest that large-scale structural changes are unlikely. The observation of the quantized jumps in resistance and especially the magnetic field dependence of the quantized resistance levels at values of R = h/2(i + )e2 provide an important experimental proof that the electron transport is ballistic in the formed Cr-p 4-V memory structures. The most important feature of the observed ballistic behaviour is that it can be observed up to about 190 K, namely much higher temperatures than previously observed (Wharam et al. 1988). This temperature dependence (see fig. 19) suggests that the states responsible for the quantized resistance levels are separated in energy by AE16x10 2 eV which is about kT, T being the temperature at which the differences between the quantized electron levels are smeared out. This energy indicates a change of about 40 A in the L, and L dimensions (as follows from eqn. (4)) between the subsequent steps which correspond to the energy separation of the observed quantum steps. Finally we emphasize that the effect has been observed in what is initially a metal-amorphous semiconductor structure. The 'classical mobility' of the amorphous silicon is many orders of magnitude lower than is expected for ballistic behaviour. It is not known with any certainty what effect the forming pros has on the structure of the conducting channel but the observed behaviour suggests the importance of the very small dimensions of the conducting channel rather than other physical parameters. The significance of using amorphous silicon sandwich structures lies in the forming process which allows the fabrication of such small structures. The strong influence of the metal contact on the observed memory switching behaviour indicates the importance of 'alloying' during the forming process. It has been reported for example that solid-phase amorphization or glass formation occurs in V-Si reactions induced by rapid thermal annealing (i.e. at conditions similar to forming) but do not occur in Co-Si and Cr-Si (Nathan 1988). Therefore the presence of a very small tunnelling conduction path in amorphous silicon Cr-p 4 -V structures might be due to a new type of solid-phase reaction (induced by forming) which creates a homogeneous (possibly amorphous) V- Si silicide.

§ 5. CoricuisioNs The experimental results presented in this paper show that Cr-p-V amorphous silicon structures exhibit polarity-dependent analogue memory switching. The analogue resistance values are stable after removing the programming voltages and up to temperatures of about 160°C, that is the effect is non-volatile. The temperature dependence of the current-voltage characteristics and of the memory switching transients suggest that the effect can be explained by either dimensional changes in a tunnelling barrier present in the structure on an atomic scale or small changes in barrier height. 472

Analogue memory in metal-a-Si structures 369

Perhaps the most important observation is that the conduction in the memory ON state is restricted to a narrow conducting channel through which the electrons can, under certain conditions, travel ballistically. As a consequence, quantized resistance levels at R = h/2ie1 values are observed where i is the number of occupied one- dimensional conducting channels (sub-bands) and the spin degeneracy is two (in the case when no magnetic field is applied). As the applied voltage is increased, the cross- section of the conducting channel in increased. This results in additional conducting channels (sub-bands) passing through the Fermi energy and consequently the resistance drops by quantized values. In the presence of a magnetic field, additional steps occur corresponding to the split levels at values of R = h/2(i + )e2 . The quantized resistance effect can be observed at relatively high temperatures (up to about 190 K), suggesting an energy separation of kT 16 x 10 2 eV. This indicates a change of about 40A in the dimensions of the cross-section of the conducting channel between the subsequent steps of the quantized resistance.

Acxowt&xmiam's It is a great pleasure to contribute to this Festschrift in honour of Professor Walter Spear on the occasion of his seventieth birthday. His contribution to the subject of amorphous materials has been outstanding and we are delighted to participate in this acknowledgment of his achievements. We have also had the privilege of collaborating with Professor Spear in the early stages of our work on amorphous silicon memory devices and we are grateful for his valuable contributions and for his continued interest.

Raiaacas GAGE S. M., HArm, J, REYNOLDs, S., CH0I, W. K., RosE, M. I., La CoMlan, P. G. SNEU, A. J., and Owa, A. E., 1989, J. non-crystalline Solids, 115, 171. GIAVER, I., Za.taa, H. K, 1968, Phys. Rev. Len., 20, 1504. Hrro .1., Owaw, A. E. GAGE, S. M. SNELI., A. .1., La CoIanER. P. G. and ROM. M. 1., 1990, Phys. Rev. Len. (submitted). HArm, J., Rosa, M. I., SNEU., A. I., La COMBER, P. G. and OWEN, A. E., 1990, Mater. Res Soc. Symp. Proc., 192, 347. hiay, Y., 1986, Directions in Condensed Matter Physics, edited by G. Grinstein and G. Mazcnko (Singapore: World Scientific). p. 101. LANDAUER, R, 1985, Localisatioi, Interactions and Transport Phenomena, edited by B Kramer, G. Bergmann and Y. Bruyserade (Berlin: Springer). p. 38. LE CoMBER, P. G. OwEN, A. E., SpaR, W. E., H4uro6 J. SNEU., A. I, Cuot, W. K. Rosa, M. I., and RamoLDs, S. 1985, J. non-crystalline Solids, 77 £78 1373. NATHAN, M., 1988, J. appl. Phys., 63, 5534. OWEN, A. E., La CoMBER, P. G. SAuAasyRouBE, G., and SPEAR, W. E., 1982, hoc. Inst. elect. Engrs, Part I, 129, 51. Rosa, M. 3., HArro, 3., La CoMBER, P. G., GAGE, S. M., CHot, W. K., SNELL, A. 3., and OwEN, A. E., 1989, J. non-crystalline Solids, 115, 168. SHARvmI, Y. V., 1965, JETP Len, 1, 151 VAN WERs, B. 3, VAB HourEN, H. BEENAEKEP., C. W. J. WiuAussoN, J. G. KovwNDIovEN, L P., VAN DER Mam., D. and Foxopi, C. T, 1988, Phys. Rev. Len., 60.84& WHn, D. A., PEPPER, M, Auiaz,, H. FROST, J. E. F. HAno, D. G. Paacoca, D. C, Rrrcmr, D. A., and Johns, G. A. C, 1988, J. Phys C, 21, L887. 473

O'4 r Journal of Non-Crystalline Solids 137&138 (1991) 1341-1344 North-Holland NON CRYS1ILINtSOUBS

PASS - A CHALCOGENIDE-BASED LITHOGRAPHY SCHEME FOR I.C. FABRICATION

M.N. KOZICKI and S.W. HSIA

Center for Solid State Electronics Research, Arizona State University, Tempe, AZ 85267-6206, U.S.A.

A.E. OWEN and P.J.S. EWEN

Dept. of Electrical Engineering, University of Edinburgh, The King's Buildings, Edinburgh EH9 3JL, U.K.

This paper contains details of a multi-layer resist scheme which provides excellent resolution capability as well as reducing many of the problems associated with the use of conventional microlithographic resists. In the PASS (planarized arsenic/sulfur/silver) scheme, surface planarization is first achieved by spinning on a layer which is self levelling by spin-casting. A thin film of A533S67 (at.%) is then vacuum deposited on this layer and topped with silver. The planar nature of the resist scheme reduces the problems of focus variations at steps in the circuit topology for optical lithography. During exposure, the silver diffuses rapidly into the As-S with little lateral spread. The As-S compound is soluble in a CF4 plasma whereas the As-S-Ag ternary compound is extremely insoluble under the same conditions. We can therefore dry develop the active layer. The unremoved ternary is then used to selectively protect the planarizing layer during the subsequent dry etch of this material. In experimental studies, the resist system exhibited extremely high resolution; contrast is typically in excess of 13 for optical illumination and electron-beam direct writing has produced 35 nm lines spaced by 35 nm in the active layer.

1. INTRODUCTION resolution but also result in decreased depth of focus. The rapid movement toward higher levels of The reduced depth of focus is a serious problem for integration in monolithic circuits has been made modern integrated circuits with multiple levels of possible by increased component packing densities metallization as these tend to have relatively large and smaller geometries. The reduced feature size changes in surface elevation. In addition, the within circuits is largely a result of advances in topography creates a problem for conventional spin- on resists as the thickness of the resist will tend to lithographic techniques. The minimum feature size which may be defined is change over the surface, becoming thinner at the determined by the combination of the capabilities of edges of raised features. The thin resist areas then the system optics and the contrast value of the become over-exposed and a change in linewidth photoresist. This latter factor for conventional optical occurs at the steps. Also, reflections from the resists is actually quite poor. An additional resolution substrate can lead to standing waves which reduce limitation stems from the fact that organic resists use photospeed and create uneven exposure throughout wet developers and these will not easily dissolve the depth of the resist. If the underlying layer is highly material in small spaces. A high-contrast resist reflective, whenever the layer passes over steps, system which can be dry developed would therefore incident light will be reflected not upward but laterally be desirable. into the resist which leads to reflective notching". To Unfortunately, the problems of current lithographic reduce this effect, dyed resists are frequently used but techniques do not stop with resolution. Higher these are extremely insensitive. This suggests that a numerical aperture (NA) lenses may provide better resist scheme which has a light absorbing photo-

1342 M.N. Kozicid etal. IPASS - a chalcogenide-based lithography scheme for I.C. fabrication

active region at the surface would be most desirable dip, we can remove the As-S regions by plasma etching or reactive ion etching (RIE) with CF4 to 2. THE PASS SYSTEM expose the underlying plananzing layer. The In the planarized arsenic/sulfur/silver (PASS) unrernoved ternary is then used to selectively protect multi-layer resist system, plananzation is achieved by the plananzing layer during a plasma etch step with a spinning on a layer which is not photoactive. This reactant which will remove the exposed underlying layer could be virtually any material which is self- material without attacking the ternary, 8 .9. 02 in the levelling by spin-casting. For instance, organic case of organic materials. We may therefore dry materials such as novolac resin, polyimide, or PMMA develop our exposed resist system. (poly methyl methacrylate) or inorganic spin-on The above process yields a negative of the mask glasses may be diluted by solvent to the appropriate pattern. However, it is also possible to produce a viscosity so that they are able to effectively planarize positive image by following a different development a particular circuit topography. The planarizing film is sequence. The resist is exposed as before but in this baked to remove the solvent and leave a film with little case the first RIE development step uses a sulfur gas volatile component. Note that this layer is made thick plasma which dissolves the ternary but not the arsenic enough to reduce the possibility of pinholes, sulfide2. An 02 plasma is once again used to etch particularly if high enough bake temperatures are the underlying planarizing layer. used to cause flow. The resulting structure may be used in two ways; The active layer is a film of As-S chalcogenide (1) as a surface mask during a subsequent ion glass, topped with a very thin coating of silver. Both implantation step, etch step, lift-off step, etc. and then these materials may be deposited by a variety of stripped afterwards, (2) the surface ternary is methods but evaporation and sputtering are favored removed to leave the patterned under-layer as an as they are relatively clean vacuum coating inter-metal dielectric which will remain as part of the techniques in which control of thickness and finished Circuit. stoichiometry is good. When exposed to light or other radiation around or above the bandgap of the glass 4. CHOICE OF CHALCOGENIDE (about 2.5 eV), the silver diffuses into the arsenic The particular arsenic sulfide compound for this sulfide to create a ternary compound which has very new resist scheme was chosen due to its inherent different material properties compared to the arsenic high resolution capability. A533S67 (at.%) forms an sulfide alone1. extremely homogeneous non-phase separated The total thickness of the active layer, arsenic ternary glass when combined with the appropriate sulfide plus silver, is typically less than 200 nm. The amount of silver. During exposure, the silver diffuses Ag thickness is less than 40 nm to allow sufficient light into the As-S with little lateral spread. In fact, the to reach the glass so that the sensitivity of the system mechanics of silver dissolution are such that an edge- is comparable to normal thicknesses of conventional sharpening effect occurs which enhances resolution photoresists. beyond what is predicted by diffraction-limited optics3. This leads to a practical contrast value which is 3. DEVELOPMENT greater than 10 (the best conventional resists have a The As-S compound is soluble in a CF4 plasma, contrast of only 3 - 5). Since the active material has whereas the ternary compound is extremely insoluble no large macro-molecular elements, the theoretical under the same conditions. Therefore, after removing resolution is essentially a few nanometers4. Other any unreacted silver by sputtering or a wet chemical chalcogenides, such as germanium selenide, exhibit 475

M.N. Kozicki et aL/PASS - a chalcogenide-based lithography scheme for iC fabrication 1343 more lateral diffusion and other diffusion materials, less than 1 and poor sensitivity (greater than 40 such as copper, will tend to thermally diffuse with no mJ/cm2) due to high absorption in the As-S. applied radiation. Therefore, the As-S/Ag system is However, even with this relatively crude wet the most ideal for applications in lithography. development step, a 70 nm pitch (35 nm linewidth) The As-S is almost opaque for wavelengths pattern has been produced for this combination with shorter than 400 nm due to the high absorption electron-beam exposure 6. In this latter experiment, a coefficient of the material in this region. The net result 5.5 nm diameter, 40keV beam was used to provide a is that very little exposure radiation reaches the line dose of 2.5 x 10-8 C/cm. substrate, particularly for I-line and below, to be Dry development using RIE with CF4 is also highly reflected back to the active layers. A dye element promising. Figures 1 and 2 show the etching could also be added to the plananzing layer if desired characteristics of A533S67 and the photodoped (particularly for G-line radiation) to further reduce ternary respectively for a reactive ion etch develop substrate reflections. This has obvious benefits for step in CF4. The etch rate of the binary is relatively optical lithography as reflective notching will be eliminated. Alignment may still be performed through ETCH RATE (A/sec) the resist using illumination between 500 nm and 1 550i micron 1 . Yellow light is most ideal for this purpose as 500 longer wavelengths will tend to be reflected and shorter wavelengths will expose the resist. 450

As an added benefit, since the active layer is 400 sensitive to a wide range of wavelengths, we may combine exposure techniques on the same resist 350

layer, e.g. optical and electron-beam, to improve 300 throughput for ultra-small geometry circuits. 250 5. SUMMARY OF EXPERIMENTAL RESULTS 200 In general, the A533S67 material is extremely 10 20 30 40 50 easy to deposit by evaporation with good control of ETCH POWER (W) layer stoichiometry as revealed by EDXA. To attain Fig. 1. Etching characteristics of As33S67 for RIE in the optimum amount of silver in the arsenic sulfide CF4. The pressure is 93 mTorr. film after photodoping, the As-S layer thickness has to be between 3 and 5 times that of the Ag layer and the high, approaching 500 Nsec. In contrast, the etch process should be driven to completion. This puts the rate of the fully exposed (fully photodoped) ternary is three component compound in the central glass negligible. Early work using 35 nm Ag on 200 nm forming region of the ternary phase diagramS. A533S67 gave a contrast of 5 with a sensitivity of 20 Although the Ag layer may be placed below the As-S, mJ/cm2. The layer thicknesses were subsequently which is necessary for thicker Ag layers so that the optimized to attain considerably higher contrast. radiation may readily reach the interface, the results in Using 30 nm Ag on 100 nm As33S67, a contrast of 13 the optical exposure case tend to be disappointing. is readily achieved with similar sensitivity. This is For instance, for 85 nm As33S67 on 21 nm Ag, I-line illustrated in Figure 3 which is a response curve for (365 nm) illumination followed by a wet development this resist. This particular scheme is also highly step (NaOH solution with IPA) produced a contrast of sensitive to a-beam exposure and features less than 476

1344 M.N. Kozicid et aL I PASS - a chalcogenide-based lithography scheme for !.C. fabrication

NORMALIZED REMAINING THICKNESS mask of 0.5 micron were readily transferred for I-line 1.1 illumination. 1.0 0.9 6. CONCLUSIONS 0.8 A chalcogenide-based resist scheme for high- 0.7 resolution microlithography has been reported. The 0.6 -0-- 4sec. scheme exhibits extremely high contrast for optical 0.5 * Ssec. and e-beam exposure and may be dry developed to 0.4 - 8s. allow the formation of very small geometry patterns. It 0.3 ----- 12sec. may significantly reduce many of the drawbacks of 0.2 • l4sec. conventional resists including depth of focus problems 0.1 and reflective notching while retaining good 0.0 0 50 100 150 sensitivity. ETCH TIME (sec) ACKNOWLEDGEMENTS Fig. 2. Etching characteristics of Ag doped As33S67 This work was supported by the National Science for RIE in CF4. The r.f. power is 50 W and the pressure is 93 mTorr. Foundation under grant number ECS-87-07089. Travel relating to the cooperative program with the NORMALIZED POST-DEVELOP THICKNESS Edinburgh researchers was also supported by the 1.0 NSF under grant number INT-87-22490.

0.8 REFERENCES M.N. Kozicki, V. Khawaja, A.E. Owen, P.J.S. Ewen 0.6 and A. Zakery, Proc. Sixth VLSI Multilevel Interconnection Conference (1989) 251. R.E. Belford, E. Hajto and A.E. Owen. To be 0.4 published in Thin Solid Films. K.L. Tai, R.G. Vadirnsky, C.T. Kemmerer, J.S. 0.2 Wagner, V.E. Lamberti, and A.G. Timko, J. Vac. Sd. Technol., 17 (1980) 1169. o.c Y. Mizushima and A. Yoshikawa, in: Amorphous 0.4 0.6 0.8 1.0 1.2 1.4 1.6 Semiconductor Technologies and Devices, ed. LOG(DOSE, mJ/cm2) Hamakawa (Ohmsha, Tokyo, 1981) p. 277. A.E. Owen, A.P. Firth and P.J.S. Ewen, Phil. Mag. Fig. 3. Response curve for an AsS/Ag image layer for B, (1985) 347. RIE in CF4. The r.f. power is 50W and the pressure 93 miorr. G.H. Bernstein, W.P. Liu, Y.N. Khawaja, M.N. Kozicki, D.K. Ferry and L Blum, J. Vac. Sci. Technol. 100 nm wide may be created by line doses around 5 x B, 6 (1988) 2298. 10-10 C/cm. Having the Ag on top has the added benefit of reducing charging during e-beam exposure. Using a 1 micron thick layer of novolac based material, topped with 120 nm As33S67 and 25 nm Ag, followed by a CF4 develop and 02 RIE step to etch the.novolac, the smallest features on an optical test 477

Photodoped chalcogenicies as potential infrared holographic media

C. W. Slinger, A. Zakery, P. J. S. Ewen, and A. E. Owen

The extension of holographic techniques from the visible to the infrared is important. Potentially, holographic diffractive elements have a large range of uses in this wave band. Examples include mirrors, lenses, filters, and beam combiners. All these elements would have similar advantages to those enjoyed by their visible band diffractive analogs. The metal photodJssolution effect in chalcogenides shows promise as one of the few techniques for producing low-loss holographic materials for use at any given wavelength from 0.6 to beyond 16 im. To date, the work has concentrated on the photodissolution of silver into arsenic sulfide glasses. Both bulk and surface relief gratings can be fabricated simply by holographic or mask exposure. In principle, kinoforms (e.g., blazed zone plates, and Fresnel lenses can also be made. The results of material studies show that phase gratings with high modulation and low absorption can be produced. A coupled-wave analysis is used to calculate the likely grating performance, and some initial grating characterization results are presented. The limitations of the medium are discussed and possible solutions are considered.

Introduction ments. Bulk modulation here is taken to mean that The extension of optical techniques from the visible phase modulation occurs, by variation of refractive into other regions of the electromagnetic spectrum is index, throughout the volume of the material, as an important goal. In particular, there are civil and opposed to phase modulation caused by thickness military requirements that use the thermal band variation (a surface relief structure). They differ atmospheric windows of 3-5 and 8-14 p.m. Here, in significantly in their angular and wavelength selectiv- addition to conventional reflective and refractive ele- ities—bulk structures are generally more selective ments. infrared diffractive optics have a role to play. because of their lower index modulation. Note that a They offer similar advantages to their visible band limitation of surface relief structures is that it is analogs. These include' being lightweight and having the ability to perform novel beam-forming operations difficult to determine how reflection grating behavior that are difficult to accomplish by using conventional can be obtained from them, although there has elements. Their dispersive characteristics may well recently been some progress in this direction. 3 limit their broadband imaging applications, but con- Whereas a relatively large range of materials is versely, enable diffractive elements to be used as available for diffractive elements in the visible wave band.4 filters and other, more complex, spectral domain the same cannot be said of thermal bands. devices. Hybrid refractive-diffractive elements may Some surface relief type structures have been fabri- also be of use .2 In addition, the high cost of bulk cated (notably by reactive-ion etching of germanium), materials currently used in thermal band refractive but no high-efficiency, bulk-modulated structures elements (e.g., germanium for the 8-14-p.m band) have been made for use in these bands. Some workers may lead to significant price advantages for diffrac- have used photoinduced phenomena in chalcogerudes tive elements. to demonstrate gratings, 3-8 but these were restricted Both surface relief and bulk modulation structures to visible wavelengths. The emphasis here, however, are important classes of transmissive-diffractive ele- is on research toward structures for use in the infrared, in particular, the thermal bands. Preliminary work is presented on the use of C. W. Slinger is with the EM1 Division, Royal Signals & Radar photodoped chalcogenides as an infrared medium Establishment, Great Malvern, Worcestershire WR14 3PS. UK that are capable of being used to form both bulk and The other authors are with the Department of Electrical Engineer- surface relief diffractive gratings. Blazed zone-plate- ing, University of Edinburgh, Edinburgh EH9 3JL, UK. type structures can also be fabricated. The process of Received 19 April 1990. grating formation is discussed first, followed by re-

2490 APPLIED OPTICS i Vol. 31, No. 14 I 10 May 1992 478

.s,.rnNlc 9\DIATiO' suits of optical characterization of the materials. I I Coupled-wave theory is then used to predict the MASKI - = = = diffraction performance of the sorts of grating likely A to be produced. This is followed by some preliminary SUBSTRATE experimental measurements made on gratings. Fi- CIt.iCOGE.'tDE -- - nally. the current limitations of the photodoping METAL process are discussed. and some of the approaches by which they may be overcome are listed.

Grating Formation The chalcogenide glasses are so named because they DOPED contain one or more of the chalcogenide elements S. REGIONS -. -- - Se. and Te. which may be combined with one or more of the elements Ge, Si, As. and Sb, among others, to form a wide range of binary or multicomponerit ,- - glasses. These glasses are primarily known for their DOPED good infrared transmission characteristics . 9 In addi- REGIONS tion, they exhibit a remarkably wide range of photo- induced phenomena'° including photodarkening, D photobleaching, photopolymerization, and photocrys- ta11ization. Many chalcogenides (possibly all. in BULK PHASE principle) exhibit a metal photodissolution effect— GRATDG also known as photodoping—in which illumination causes metal atoms to dissolve into the glass. The E mechanism is not fully understood, but it is thought

to be a radiation-enhanced solid-state process. involv- SURFACE RELIEF ing a three-dimensional intercalation reaction. 12 Ini- PHASE R tial interest in photodissolution centered on its use as Fig. I. Schematic illustrating the grating formation obtained by a high-resolution, inorganic photoresist in the semi- using the metal photodissolution effect: A. start of exposure; B, conductor industry, 13 and indeed, photoinduced ef- exposure continues while doped regions are formed: C. end of fects on chalcogenides are still under study for appli- exposure; D. unused metal removed to form a bulk-modulated cation in this area. 1415 This paper considers infrared transmission grating'. E. removal of undoped chalcogenide to form grating formation that uses the photodissolution a surface relief transmission grating. effect. The principle is shown schematically in Fig. 1. An amorphous chalcogenide layer is deposited by. for example. evaporation or spin coating's onto a boundary, the width of the transition region being as substrate. A thin metal film is deposited on top of the much as 20% of the photodoped layer thickness. If the chalcogenide. Light of a suitable wavelength is ar- silver source is exhausted, it has been found that the ranged to form an intensity pattern in the chalco- photodissolution still continues under the influence genide, corresponding to the profile of the desired of the illumination, silver atoms being supplied from grating. This pattern can be generated by exposure the doped regions themselves. It is important to note through a mask. as shown here either binary or that the migration of the metal is only along the grey scale or by conventional holographic tech- direction of the incoming radiation. No doping occurs niques. Grey scale or diffracting masks would be in the uniluminated areas. There is an abrupt bound- particularly useful for blazed structures (although ary between the doped and undoped regions. After a other techniques could also be used). These would suitable amount of time has elapsed. the exposure is allow generation of such structures in a single expo- stopped and any unused metal is chemically removed. sure, rather than the multiple-step procedure that is The optical properties of the doped and undoped required when conventional photoresist staircase ap- material differ to give a periodic structure that is capable of diffracting light—a bulk grating has been are used. proaches2 formed. This bulk structure can be transformed into Initially , the actinic radiation is absorbed at the l ass–metal interface. This causes a surface relief profile, if desired. by either simple chalcogenide g 4 ) growth of metal-doped regions into the chalcogenide. alkali (ammonia in methanol) or reactive-ion )CF etching. The doped and undoped regions differ very Subsequently, incoming radiation, probably absorbed by a factor of 10 or at the undoped–doped glass boundary, causes further markedly in their etch resistances t growth of the doped region with a corresponding more for ion etching). Thus structures of at least depletion of the metal reservoir. The silver concentra- - 10-20 im in depth can be fabricated. The chalcogenides used in this paper were arsenic tion is uniform throughout the photodoped region. where 0

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exhibit the same effect. 18.12 The actinic radiation can be anywhere from x rays through 'to beyond visible red wavelengths. The choice depends on the chaico- IIIHRIIIHII genide-metal system used and the thicknesses of the 1 IUIIIIIii

chalcogenide film. Thus, the reaction can be stimu- U: - lated by photons with energies above or below the optical band-gap energy, although the process is more IIIIIi..ij, ix4 efficient when above band-gap illumination is used. U. U Theor-v suggests that, for silver and As 2S3 films of !111111111- several micrometers thickness, visible green wavelengths are optimum.

Bulk Material Properties WAVEINOTH For efficient diffraction, the main requirement of the 1n Fig. 3. Refractive index of undoped and doped material as a grating material is that it should be of low loss in the function of wavelength. The materials are as in Fig. 2. wave band of use. For bulk gratings, a refractive- index variation (again in the wave band of use) is also necessary. sample is more absorbing in the visible, probably The excellent infrared transmission characteristics because of a combination of higher intrinsic absorp- of chalcogenides are well known and documented tion and the photodarkenirig effect. This latter contri- (see, e.g., Ref. 9). For amorphous As 2S3 in particular, bution can be removed by annealing. The overall Young19 has reported absorption coefficients of < 1 transmission of the doped glass is lower, mainly cm' for 0.8 to beyond 10 i.Lm. He also reported a because of the higher reflection losses caused by its refractive index of from 2.4 to 2.3 in the 1-20-p.m larger refractive index, as shown in Fig. 3. There is a region. refractive-index difference between undoped and Proustite (crystalline Ag3AsS3) has been found to doped samples of 0.4 across most of the transmission be weakly absorbing between 0.65 and 12 jm and has region, showing the high modulation that can be indices of 'iE = 2.52 and no = 2.73 in the infrared. 20 achieved by photodoping. No data, however, have been published on the optical The refractive index of the doped regions has been properties of As-S-Ag glasses. Measurements were found to be related directly to the amount of silver therefore made by using a combination of mechanical present (Fig. 4). Several groups, using a variety of stylus techniques and transmission as a function of different techniques, have shown independently that wavelength. The interference-free transmission T. the metal concentration in a sample undergoing and the refractive index of undoped and silver-doped photodissolution has a steplike profile and not the As1S3 were determined by using the method devel- Fickian distribution characteristic of normal dopant oped by Swanepoel, which does not require measure- diffusion in semiconductors. 11 Hence the silver concen- ment of the reflection spectra. 2' The samples were 1.7 trations in the samples that yielded the data in Fig. 4 and 1.77 l.Lm thick, respectively, the doped sample are expected to be essentially constant with depth. By being formed by photodissolution of 0.17 m of Ag suitable choice of the silver thickness, relative to that into 1.6-irn As2S3. The results are shown in Figs. 2 and 3. 3.2 Both doped and undoped samples show good trans-

mission from 0.7 lLm to beyond 13 j.tm. The doped 0 ,2eopto • ' 500 9 SILVER 3.0 0 • 750 R SILVER 0 1000 0 5tLVER 0 500 8 SILVER 0 0 CM 135 - 0

z 0 Cr U, 2.6 • • - U, Cc 0 0 I U, z 4 cc 2.9 1

2.21 . 500 900 1700 10 VVELENGTH WAVtiNOTH/p.m Fig. 4. Refractive index versus wavelength for 500-nm thick Fig. 2. Interference-free transmission T. of undoped and doped As3eS70 films photodoped with various amounts of Ag. The As evaporated As 2S1 glass material as a function of wavelength: concentration in the flints was controlled by varying the thickness undoped thickness 1.7 doped thickness 1.77 t u,zn from 1.6-m of the initial Ag layer, and these thicknesses are indicated in the As-_,S, doped with 1:0 rIm of Agi. figure.

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other. bulk holographic recording media. For exam- of the chalcogenide. the index modulation of the ple. a two-plane-wave. holographic exposure produces gratings can be varied as desired. The ability to a sinusoidal intensity distribution in the chalco- fabricate gratings with specific index modulations genide. The square root time dependence of the may be an advantage in some circumstances. photodis solution process (see later), combined with the requirement of growth from a source of metal Theoretical Analysis atoms, results in a parabolic–sinusoidal structure. The performance of the gratings produced by the This can be analyzed by slicing the grating perpendic- metal photodissolution effect can be predicted. to ular to the x direction. In this way, the profile good accuracy. by using a coupled-wave analysis. The generated by the sinusoidal intensity distribution can underlying theory has been covered elsewhere tsee. be represented as a rectangular grating of variable e.g.. Ref. 22). and only the main results are presented here. The replay situation described below is used for mark–space ratio: simplicity. Other geometries and grating configura- 2a€ t1=-51fliLiLr). tions can be analyzed by relatively straightforward O€+L. extensions of the theory. The grating is assumed to be lossless. phase modulated. unslanted (i.e., the grating fringes are perpen- with dicular to the substrate surface), and have a profile !cØ-i that can be described by an even function. The slab of - thickness d containing the grating is taken to be y–z plane with a normal in the x infinite in the where .€ = e. - € mjfl, Emax and €mm are the maximum direction and bounded by homogeneous regions. The K = and minimum relative dielectric constants, corre- grating vector K lies in the y direction. where sponding to the doped and undoped regions. respec- 2 1 .\ andA is the grating period. The grating param- z, but tively, and f is a saturation factor. eters may generally vary as a function of y and Further simplifications to Eq. (1) can be made in it is assumed to be locally plane. 23 An infinite, mono- 1 (generally a value A0 and free-space some cases. If the ratio €j /€O ' chromatic plane wave of amplitude of 0.2 is the limit), the second derivatives in Eq. (1) z direction, is incident wavelength X, polarized in the can be neglected with little loss in accuracy. 22 For in the x–y plane. bulk and surface relief gratings in photodoped chalco- The coupled-wave equations that describe the prop- genides, typical values for this ratio are –0.1 and 1.0, agation of diffraction orders inside the grating can be written in terms of the dimension- respectively. Thus, for bulk gratings the first-order (0

j_ —jmfl)m + ThA n, Cos 2 80 2J3 d dAl —jmt))m + P)A., .'-jA,,j +A,,) = 0. 131 d. (1) with transmission grating boundary conditions of

where A, is the amplitude of the mth diffraction A0(x=O1l. A,,,(x0)0 (m;'O). (4) is the propaga- order. i<. = I3€ '/4o', = 2V€0 1'X tion constant in the grating, € is the ith Fourier Analytic solutions of Eq. (3) are possible for the cases harmonic of the phase modulation profile at depth of fi << 1 (Ref. 25) and fl 1 (Ref. 26), correspond- x. C = x x!cos 0 0 is a modulation parameter. 1) = ing to thin (Raman–Nath) and volume diffraction '4 is a K2 !213Ki is a volume parameter P = (sin 8)23. K regimes. respectively. Generally speaking, thin re- Bragg parameter. and 80 is the propagation angle of gime behavior occurs to good accuracy for Q < 0.1. wave 0 inside the grating. and volume diffraction for fi> 10. Between these two These equations, subject to the grating boundary types of behavior, the grating occupies the multiwave conditions t matching of the tangential electric and diffraction regime and numerical solution is neces- can be solved numeri- magnetic fields at x = 0, d), sary. cally—only in the limiting case of fI 1 are some .24 Usually, one of the main requirements for a diffrac- relatively simple analytic solutions possible The tive element is that the first-order diffraction effi- amplitudes of the diffracted waves can thus be deter- ciency m is high (>80%, say). , can be defined as the mined as a function of the grating parameters. ratio of power in the ith diffraction order at x = d to It is important to realize that the true form of the that in the incident order at x = 0. In the volume grating produced by the photodissolution process is regime. for mth order on-Bragg replay rn + P = 0), essentially that of a surface relief structure, im- near 100% efficiency is possible, regardless of grating mersed in a dielectric medium. Thus holographic or In the other regimes, the achievable effi- grey scale mask exposures result in forms of grating profile.27 ciency does depend on the grating profile. that are generally different from those produced in 2493 10 May 1992 I Vol. 31. No. 14 1 APPLIED OPTICS 481

Figure 5 shows the first-order diffraction efficiency depths to achieve them. A typical result is shown in as a function of 4 and fI for a square-wave, bulk Fig. 6. in which first-order diffraction efficiency is grating. These results were obtained by numerical plotted as a function of the normalized grating thick- solution of Eq. (2) for P = –1 (on-Bragg for the first ness for a square-wave profile grating. Such a grating diffraction order). Reflection losses have been ne- could be obtained by reactive ion etching of the above glected. The square-wave grating is equivalent to bulk structure. The equivalent result for a sinusoidal exposure of the chalcogenide through a binary mask. profile surface relief grating is also shown. The efficiencies are seen to agree with the anal tic For the cases shown in Fig. 6, peak efficiencies of solutions in the thin and volume regimes having 63% and 77% occur at d = 0.76X and 1.1X for the maxima of 40.5% and 100%. respectively. In the square and sinusoidal profiles, respectively. Note that multiwave regime, it is interesting to note the pres- these figures should not be regarded as the maximum ence of several high-efficiency regions. For example. achievable. They can be increased by suitable coat- m is >96% at (,fl) = ('iT/2,2.3) and >90% at ings or changes in the grating parameters (choice of (3.0. 0.5). Such operating points may be useful in lower index or decrease of the grating period). A 0.15k some applications. For a bulk square-wave grating thick conformal coating of 1.58 index increased the with refractive indices of 2.3 (undoped) and 2.65 calculated peak efficiencies to 83% and 87% for the (doped region) at (, fl) = (ii/2, 2.3), a grating thick- square and sinusoidal profiles, respectively. ness of 2.19x is required—around 23 j.i.m at X = 10.6 For structures other than the relatively high fre- p.m. quency diffractive gratings (A = X) analyzed above. It is of interest to compare the performance of the for example, kinoforms (e.g., zone plates) and Fresnel bulk grating with that of the equivalent surface relief lenses, a coupled-wave analysis is not necessary. structure. For the latter, the rigorous coupled-wave Fresnel lenses rely on refractive principles. so are Eq. (i) must be solved numerically. The lack of capable of 100% efficiency, although the thicknesses analytic solutions for the second-order theory does required are comparatively much higher than in not mean that trends cannot be identified. It was diffractive devices. Blazed zone plates can also achieve found that the volume parameter 11 still influenced 100% efficiency, the depth required being given by the number of significant diffraction orders that were d = X1(n - 1). The so-called binary optics 28' 2 are also present. Thus, generally speaking, the larger the capable of high efficiencies (over 90% for six-level refractive index of the surface relief structure (and phase structures, for example). therefore the larger the index modulation), the smaller Thus, for both bulk and surface relief structures. the grating thickness d needed for peak efficiencies, typical of those capable of being produced by although these peak efficiencies were lower than photodoped chalcogenides, high efficiencies should be surface relief gratings of smaller refractive index. achievable. Low-index gratings gave higher peak efficiencies (because of their higher 0 values) but required greater Experimental Work As discussed in the following section, these material systems currently require long exposure times, hence to date, fabrication of gratings has been restricted to 11tU • '\! chalcogenide thicknesses of up to 2 p.m or so. The

d/A Fig. 6. Theoretical calculation of efficiency ill as a function of the Fig. 5. Theoretical calculation ofefficiencyn, as a function of the normalized grating thickness for square xi and sinusoidal volume parameter fI and the modulation parameter Z for a surface relief transmission gratings. The grating period .t is equal square-wave transmission grating obtained by using the first- to the replay wavelength >.: the refractive index of the structure is order, coupled-wave Eq. (3). Contour 1 = 10% efficiency. 2 = 2017c. 2.65. Calculations were made from the second-order coupled-wave etc. Note the log scale for Q. Reflection losses have been neglected. Eq. (1). Reflection losses are implicitly included.

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modulation parameter in the infrared is thus small. Measurements have therefore been at visible and near-infrared wavelengths. Surface relief gratings have been made by using holographic and mask exposures followed by alkali etch. The grating can be of good quality, exhibiting few surface defects. Several bulk gratings (both thin and volume types) have also been made by using holographic exposures. Recordings were made by using the interference of two plane waves from a single-frequency, argon-ion laser operating at 514.5 nm (theory indicates that wavelengths corresponding to the optical gap energy of the chalcogenide. -2.4 eV for As 2S3, are the most effective for inducing photodissolution in the thick Fig. 8. Scanning electron microscope micrograph of a cross samples: shorter wavelengths are highly absorbed section through a bulk. holographic grating recorded by interfering in the chalcogenide, whereas longer wavelengths are two plane waves to produce a sinusoidal intensity pattern. The less efficient at stimulating the effect). Typical power sample prior to exposure was As3sS7s (2.5 1jin)/Ag (450 rim), with densities were 0.25 W cm 2lwave. The samples were the unused ..g removed after an exposure of 120 sand illumination made by evaporating As 2S3 (supplied by Servofrax) to intensity of 150 mWjcm 2 ,recording wave. The profile of the a thickness of 1.6 p.m on microscope slides and embedded structure is clearly visible and appears to be close to the subsequently coated (without a break in vacuum) parabolic—sinusoidal profile discussed in the text. with 275 nra of Ag. After exposure, the samples were washed with a solution of iron (ill) nitrate, to remove image of a cleaved bulk grating. The doped regions the unused Ag. By varying the interbeam angles of can be distinguished from the undoped arsenic sul- the recording waves, we made both thin, multiwave fide, and the shape of the doped regions appears to and volume gratings. For example, an interbeam follow the parabolic-sinusoidal profile predicted from ± 20 to the substrate normal angle of approximately the kinetics results. produced bulk gratings having a significantly thin Interbeam angles of ± 25° results in gratings display- character. On replay, using a He-Ne laser at 623.8 ing significant volume behavior at visible wave- rum, we could observe many diffraction orders that lengths. Figure 9 shows the measured 0 and +1 were typical of Raman-Nath behavior. Figure 7 is an diffraction orders as a function of replay angle for an optical phase contrast micrograph of such a grating of early version of one such grating with an exposure period 22 p.m, formed by photodoping 3-p.m As 30S70 time of 120 s. Replay was in air, using a He-Ne laser with 1 I.Lrn of Ag. The definition of the grating is at 632.8 nm. For the, case shown, just under 9% reasonable. although the small-scale structure that is efficiency is achieved, all but 1% of the energy being apparent may be due to some kind of inhomogeneity removed from the incident wave. Reflection losses in the photodoped material, for example, phase- (nearly 30% at the Bragg angle), absorption. and separated clusters. A change in the arsenic sulfide scatter account for the missing energy. The absorp- composition may well remove these clusters, and in any case, scatter from them will decrease at longer wavelengths. - 0.1 Experimeni Figure 8 shows a scanning electron microscope Theor , ,

/ 0 2 - 0.05 /_I \

0 Replay angle /degrees Fig. 9. Response of an early, near-volume grating recorded by sinusoidal interference pattern A = 0.609 m. The diffraction order intensity as a function of replay angle was measured in air at a 632.8-nm replay wavelength. The theoretical curves were generated from first-order coupled-wave theory incorporating Eq. 12). Diffraction orders of —3. —2. —1.0, +1. +2, and rS are permitted. Fig 7. Phase contrast photomicrograph of a plane grating with a The key parameters are = 1.875 m. e,,,,, = 8.41. c,,,,,, = 6.2. and period of 22 im. produced by holographic exposure of an A530S7e bulk losses of = 0.44 m* kUiiiUUU.3. 1j.mhAg l-imy sample. 2495 10 May 1992 I Vol. 31. No. 14 / APPUED OPTICS 483

tion may be partly due to the photodarkening of the z,0.7 chalcogenide (a shift in the visible absorption edge). Photodarkening has little effect in the infrared band. ' In addition, it can be removed by annealing or AO' ' H reduced by using a different chalcogenide composition. Additionally, this early sample suffered from poor film adhesion. Equations (2) were modified to include bulk losses and absorption modulation (see, e.g., Ref. 29). A theoretical analysis of the grating was made, based on Eq. (4) and numerical solution of the modified equa- I Replay angle /degrees tions. The results are shown as solid curves in Fig. 9. A Talystep profilometer was used to determine the Fig. 11. iAs in Fig. 10 but measured at 1.5 j.Lm. First-order thickness d of the sample. The parameters derived diffraction efficiency is > 35% in this case. The anomalies in thE from the fit are within the range expected from the response are more pronounced than in the previous figure. bulk material characteristics. The agreement between theory and experiment is generally good, the and 10 show anomalous behavior in their angulai main features of the curves being well matched. responses (examples of which are marked a in tht Figures 10 and 11 show the measured diffraction figures); their cause is as yet unknown, but they ma) performance of more recent gratings at 632.8 nm and be caused by noise grating phenomena 31 or othei 1.5 pm, respectively. The angular response shown in interference effects at recording. Fig. 10 is from a grating recorded as described above, but using 2.5 urn of As 30S70 with 490 nm of Ag and exposed at angles of 30°. A peak diffraction effi- Possible Limitations of the Photodissolution Process ciency of 34% can be seen. This increase in efficiency Possible problems that may limit the applicability 0: over the previous result (Fig. 9) can be attributed to the photodissolution process include its speed and thi increased film quality and both the lower intrinsic classes of grating that can be fabricated. The rate al absorption and a reduction in photodarkening of the which the process occurs determines the exposur( lower arsenic content film. In fact, photodarkening is time required for a given grating thickness d. Coupled negligible near the composition of As 30S70.30 wave theory can be used to predict the values of d tha The grating whose angular response is shown in are required for a particular type of grating an Fig. 11 was optimized for performance at near- wavelength of operation. The results for a square infrared wavelengths, and recording angles of the wave grating, assuming undoped and doped refractivi plane waves were therefore reduced to ± 12.5°. 2.6 p.m indices of 2.3 and 2.65, respectively, are given it of As33S67 coated with 1.0 p.m of Ag were exposed for Table 1. Volume operation at (, Cl) = (r/2, 10) wa 240 s. Despite the reduced values of at the longer assumed for the built grating. For the surface relie replay wavelengths of 1.5 p.rn, the lower absorption profile, the grating period A was taken to be equal ti and reduced scatter result in a first-order diffraction the replay wavelength X. The thickness required for efficiency that exceeds 35%. These results are encour- blazed zone plate is also shown. aging and confirm that, for greater grating thick- It can be seen that, for far-infrared operation nesses (such that -. ,r/2), high efficiencies (>90% thicknesses of several micrometers are required. Not neglecting reflection losses) should certainly be achiev- that the surface relief structures are approximately able at infrared wavelengths. However, both Fig. 9 factor of 3 or 4 thinner than the equivalent bull gratings—an advantage in their favor. Silver photodis solves to a depth of >20 p.m in some chalcogenides. 3 Plh but can diffractive structures be produced to th required depths for high efficiency?

0 To answer this question it is necessary to study th factors that govern the speed of the process. Thi 0 depth to which the chalcogenide has been doped cal be written as a function of the wavelength of thi 0 Z Table 1. Required Thicknesses for Square-Wave Gratings

Replay Wavelength/m

0.632 4.0 10.6 Replay angle /degrees Fig. 10. Angular response of a more recent grating, again mea- d.',.m sured in air at 632.8 nm. The details of the grating are in the text. A Bulk 1.39 8.77 23.2 first-order diffraction efficiency of 34% can be seen. Note the Surface relief 0.48 3.0 8.06 which are marked a in the anomalies in the response, examples of Blazed zone plate 0.38 2.4 6.42 figure.

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actinic radiation , A0 its intensity I, and exposure time bulk and surface relief gratings having low-ti medium slant angles (<45°, say) in addition to kini d - f(.f,r). forms can certainly be made. But it is difficult I (5) envisage the writing of gratings having larger slant (i.e., such that the grating vector no longer lies cbs An increase in photodissolution rate with photon to the plane of the substrate), since a Ag source rnu energy is counterbalanced by the increasing absorp- be associated with each fringe, and fringe growt tion of the material (both undoped and doped) with starts from the source. Thus fabrication of reflectio decreasing wavelength. Theory suggests that visible (conformal) gratings would require some effort. green wavelengths are optimum for thicknesses greater than a micrometer or so. The dependence of the rate on intensity is probably linear for low powers Conclusion but seems to saturate at higher levels (typically 300 A preliminary experimental and theoretical investig MW cm -2 at 514.5 am for 4 gm of As 30S70;Ag). The tion of the metal photodissolution effect in chalc effect of the exposure time on the photodissolved genide glasses has been presented. In particular, th depth is not straightforward. Initially, the doped silver doping of arsenic sulfide glasses has bee thickness increases with time I as k 1 'Jt until the Ag examined. The bulk optical properties of this materi reservoir is exhausted. After this occurs, the doped system show low losses from the visible red to beyon region acts as the source of Ag and d varies as k2Vt. 15 p.m. Thus surface relief structures obtained b The photodissolution rate also depends on the using this process should be of use at any give chalcogenide composition. Experiments have been wavelength in this wide band. In addition, refractiv performed investigating this by measuring the trans- index measurements show that phase modulatio mission and reflection of a sample while exposure should also be possible over this wave band. enablin is occurring. 33 The optimum composition for the bulk diffractive structures to be made. As0Sj../Ag system was around As 30S70. Fortuitously, Theory has shown that high efficiencies should b this is close to the composition for which minimum possible. Experiments on visible band gratings showe photodarkening occurs. However, the rate does not reasonable agreement with the theory, when thi vary significantly for small variations about this regime and near-volume transmission gratings wer optimum. fabricated. Measurements of grating efficiency a To give some idea of the rates found in practice, near-infrared wavelengths (1.5 jim) have confirme typically a 700-s exposure with a power density of 300 that high efficiencies should be possible after suitabl mW cm -2 at 514.5 nm was necessary for complex grating thicknesses are achieved. Although the large photodissolution of a 1-jim thick As 30S70 sample, with material thicknesses required for 8-14-p.m band struc an excess of Ag, at 21°C. Therefore, unless great care tures may be difficult to fabricate, several approache is taken le.g., by using active fringe stabilization could overcome this limitation. Currently, the pro techniques holographic exposures for far-infrared cess is restricted to transmission-type structures grating fabrication would be difficult. Mask expo- although work is under way to try to fabricat reflection gratings. sures. while overcoming the stability problems, may still be awkward. Although the emphasis in this paper has been fo However, several avenues of study may yield an use within the 3-5- and 8-14-p.m thermal bands, th increase in the photodissolution rate. These include large bandwidth optical properties of the materia seeding of the chalcogenide with a small percentage of system may enable it to be applied usefully to othe spectral regions, for example, in the near infrared fo, Ag before doping. A 1% addition of silver to AS40S60 optical communication applications. increases the conductivity by a factor of 2 x 103.34 The ionic and electronic conductivities are thought to The authors thank Abdelkarim Zekak for the scan influence photodissolution, so the rate should in- ning electron microscope micrograph used in Fig. 8 crease with seeding, although our results so far have This work was carried out with the support of thi not supported this. It is nevertheless possible that Procurement Executive, Ministry of Defence. other additives may enhance the diffusion or reaction rates. Application of an electric field during exposure References may also increase the doping rate. Other possibilities D. H. Close, 'Holographic optical elements. Opt. Eng. 14 include the use of a photosensitive silver layer to 408-419 (1975). minimize the exposure requirements, 35 the use of G. J. Swanson and W. B. Veldkamp, 'Advances in computer double metal layers (e.g., Ag and Cu) and other generated binary optical elements. in Annual Meeting Techni chalcogenide—metal systems. Finally, rates at higher cal Digest, Vol. 11 of OSA 1988 Technical Digest Serie temperatures are greater, 6 although the advantages (Optical Society of America. Washington. D.C.. 1988). pape may be limited by lateral doping that is due to the W1J2: "Diffractive optical elements for use in infrare, closely related thermal dissolution effect.' applications," Opt. Eng. 28, 605-608 1989). J. J. Cowan. "Embossed volume holograms: the Aztec Struc Other current limitations of the photodissolution ture,' in process include the restricted class of structures that Proceedings of the Second International Con ferenc on Holographic Systems. Components, and Applications. (El the process is capable of fabricating. Transmission Conf. Pubi. London 311.38-441989,.

10 May 1992 1 Vol. 31. No. 14 / APPLIED OPTICS 249 485

L. Solvmar and D. J. Cooke. Volume Holography and Volume K. F. Hulme. 0. Jones, P. H. Davies. and M. V. Hobder Gratings Academic. London. 1981). "Synthetic proustite (Ag,iAsSs ,: a new crystal for opticz S. A. Keneman. "Hologram storage in arsenic trisulphide thin mixing," AppI. Phys. Lett. 10. 133-135 (1967). fllms."Appl. Phys. Lett. 19.205-207(1971). R. Swaneooel. "Determination of the thickness and optico M. 1. Kostyshin. E. P. Krasnojonov. V. A. Makeev. and G. A. constants of amorphous silicon.' J. Ph'.-s. £ 16. 1214-122: Sobolev. "The use of light sensitive semiconductor metal (1983). systems for holographic investigations. in Proceedings of the M. G. Moharazn and T. K. Gaylord. "Rigorous coupled.wav International Symposium on Holography. J. Vienot. J. Bula analysis of planar-grating diffraction." J. Opt. Soc. Am. 71 bois, and J. Pasteur, eds. (U. Besancon Press. Besancon, 811-818 (1981): "Diffraction analysis of dielectric surface France. 1970). paper 11.7. relief gratings," J. Opt. Soc. Am. 72.1385-1392 (1982). C. H. Kwak, J. T. Kim. and S. S. Lee, "Scalar and vector R. R. A. Svms and L. Solvrnar, "Localised on'dimensiona holographic gratings recorded in a photoanisotropic amor- theory for volume holograms." Opt. Quantum Electron. 13 phousAsS3 thin film.' Opt. Lett. 13.437-439(1988). 415-41911981). A. G. Poleshchuk, E. G. Churin. and Yu. I. Yurlov. Applica- J. A. Kong, 'Second order coupled mode equations for spatiaii tion of an evaporated photoresist )As 2S3) in the production of periodic media," J. Opt. Soc. Am. 67,825-829(1977). kinoform optical elements." J. Imaging Sci. 30, 132-135 R. Magnusson and T. K. Gaylord, "Diffraction efficiencies (1986). thin phase gratings with arbitrary grating shape," J. Opt. Soc J. A. Savage. Infrared Optical Materials and Their Antireflec. Am. 68,806-809(1978). tion Coatings (Hilger. London, 1985). H. Kogelnik, "Coupled wave theory for thick holograir A. M. Andriesh. V. V. Ponomar, V. L. Smirnov, and A. V. gratings." Bell Syst, Tech J. 48, 2909-2947 (1969). Mironos. "Applications of chalcogenide glasses in integrated C. W. Slinger. "Multiwave diffraction analysis of transmissior and fiber optics (review)," Soy. J. Quantum Electron. 16, phase gratings," RSRE Memorandum 4325 (Royal Signals I 721-736(1986). Radar Establishment, Great Malvern, Worcestershire W'111 A. E. Owen. A. P. Firth. and P. J. S. Ewen. "Photo-induced 3PS, UK, 1989. structural and physico-chemical changes in amorphous chalco- H. Dammann. 'Blazed synthetic phase'oniv holograms," Op genideserniconductors," Phios. Mag. 852.347-362(1985). Uk 31, 95--104 (1970). G. Kluge. "A new interpretation of the photodoping effect in R. R. A. Syms and L. Solvinar, "Planar volume phase hoio amorphous As- and Ge-chalcogerudes." Phys. Status Solidi A grams formed in bleached photographic emulsions,' Appi 101,105-114 (1987. Opt. 22. 1479-1496(1983). A. P. Firth, P. J. S. Ewen, A. E. Owen, and C. M. Huntley, A. Zakerv, P. J. S. Ewen, C. W. Slinger, A. Zekak, and A. E "Inorganic resists based on photo-doped As-S films," in Ad- Owen. 'Apphcation of chalcogenide glasses in integrated anc vances in Resist Technology and Processing II, L. F. Thomp- diffractive optics," in Proceedings of the 14th Interna,rionco son, ad., Proc. Soc. Photo-Opt. Instrum, Eng. 539. 160-165 Conference on Amorphous Semiconductors. G. H. Bauer, W (1985). Fuha. and L. Ley, eds. (Elsevier. Amsterdam. 1991). E. Haito and G. Zentai. "Submicron resolution amorphous G. D. G. Riddy and L. Solvmar. "Theoretical model of recon- chaicogenide optical grid," Non-Cryst. Solids 90, 581-584 structed scatter in volume holograms," Electron. Lett. 22, (1987). 872-873(1986). A. Buroff and D. Rush. "Photolithographic performance of E. Inoue, H. Kokado, and I. Shimizu, "Photo-doping of metal a-As2S3 resists under synchrotron radiation," Non-Cryst. Sol- into chalcogenide glasses." in Proceedings of the 5th Interna- ids 90.555-588) 1987). tional Conference on Solid State Devices. J. Jpn. Soc. App. G. C. Chern and 1. Lauks. "Spin-coated amorphous chalco- Phvs. 43. 101-105 (1974). genide films," J. Appi. Phys. 53.6979-6982(1982). P. J. S. Ewen. A. Zakerv, A. P. Firth. and A. E. Owen. "Optical K. Kase. G. C. Chem. and!. Lauks. "Dry etching of gratings on monitoring of photodissolution kinetics in amorphous .A. spin-coated As2S3 films," Thin Solid Films 116, L53-L54 films. Phos. Mag. B57.1-12 11988). (19841. N. F. Mott and E. A. Davis, Electronic Processes in Von- H. Mizuno, K. Tanaka. and M. Kikuchi. "Photo- and thermal- Crystalline Materials, 1st ed. (Clarendon. Oxford, 1971. pp. diffusions of metals into As 253 glass,' Solid State Commun. 330-340. 12.999-1001(1973). J. I. Masters. G.M. Goldberg, and J. M. Lavine. "Photographic P. A. Young, "Optical Properties of vitreous arsenic tn- Ag photodoping of amorphous A5253 films.' IEEE Trans. sulphide.' J. Phys. C 4.93-106(1971). Electron Devices Lett, ED1_ 1. 61-62(1980).

2498 APPLIED OPTICS / Vol. 31. No. 14 1 10 May 1992 486

14 Photo-induced changes in chalcogenide glasses and their applications

P. J. S. EWEN and A. E. OWEN

14.1 Introduction

The chalcogenide glasses are one of the most widely known families of amorphous materials and have been extensively studied over the past three decades, partly because of their interesting fundamental properties and partly because of their applications. Indeed, it was the discovery of electronic switching in chalcogenide systems such as Si—Te—As—Ge (Ovshinksy, 1968) that stimulated the upsurge of interest in amorphous materials in general that occurred during the 1960s. The chalcogenides are so called because they contain one or more of the chalcogen elements 5, Se or Te. These elements can be combined with many others, for example As, Sb, P, Si and Ge, to form a wide variety of glass-forming systems, ranging from simple binary combinations to more complex multi-component systems such as As—Te-.Ge—S—P. The As—S, As—Se and Ge—Se systems are among the most widely studied of the binary combinations and include the well-known compound glasses As2S3 , As25e3 and GeSe2 . In general, the chalcogenide glasses are relatively easy to prepare in either bulk or thin-film form and their properties are often comparatively insensitive to the details of preparation, which is an advantage both in experimental studies and in applications. Further information on the preparation and general properties of these glasses can be found in various texts and review articles (Elliott, 1988; Mott and Davis, 1979; Madan and Shaw, 1988; Savage, 1985; Zallen, 1983). The most important applications of chalcogenide glasses are now in the field of optics and arise chiefly from either their infrared-transmitting properties or the many photo-induced effects they exhibit. They have potential uses in integrated optics, optical imaging (e.g. holography and very large scale integration (VLSI) lithography), optical data storage and infrared optics. In this chapter the various types of photo-induced effects that occur in amorphous chalcogenides are classified and described, with particular emphasis on the As—S system, and their applications are reviewed. Further information on the fabrication of chalcogenide glasses and their use as infrared waveguides is given in chapter 13. 487

288 HIGH-PERFORMANCE GLASSES

14.2 The classification of photo-induced effects in chalcogenide glasses

Chalcogenide glasses exhibit a wide variety of light-induced changes, ranging from relatively subtle effects involving minor atomic rearrangements and manifested mainly by shifts in the optical absorption edge, to more substantial atomic and molecular reconfigurations which cause a variety of physical and chemical changes. These effects are of fundamental interest because of the information they yield on defects and metastable structural states in amorphous solids, and because they are mostly unique to the amorphous state. There are several earlier reviews of this field (Elliott, 1985; 1986; Owen et al., 1985; Tanaka, 1981a,b). At least seven distinct photo-induced structural or physicochemical changes have been observed in amorphous chalcogenides when samples in a suitable form are exposed to light or other irradiation, viz. photocrystallization, photopolymerization, photodecomposition (in compounds), photo-induced morphological changes, photovaporization, photodissolution (of certain metals), and light-induced changes in local atomic configuration. In general these changes are accompanied by changes in the optical constants of the material and particularly shifts in the absorption edge, i.e. photodarkening or photobleaching. Other characteristics of the material, such as the elastic constants (Kolomiets and Lyubin, 1978; Tanaka et al., 1981), may also be affected. There are two reasons why the chalcogenide glasses are so susceptible to light-induced changes (Elliott, 1985; 1986). Firstly, they have a great deal of structural flexibility, partly because the chalcogen atoms are normally only twofold co-ordinated, and partly because their structures tend to be chain-like or layer-like rather than three-dimensional continuous random networks. The second reason is that the chalcogen atoms possess lone-pair electrons which are normally non-bonding but which can be involved in light-induced reactions to produce structural defects consisting of threefold or singly co-ordinated chalcogen atoms. The highest-ly- ing levels in the valence band of these materials consist of the states associated with these non-bonding electrons and are therefore the states that are preferentially excited by illumination. The photo-induced phenomena that occur in the chalcogenides may be classified according to whether they are primarily structural or physicochemical in nature and whether they are reversible or irreversible, in the sense that the system may revert partly to its initial state after some annealing treatment at a temperature higher than that at which illumination took place, or will recover completely on annealing at the glass transition temperature, Tg. The reversible effects are generally observed most readily in well- annealed vapour-deposited films, or in melt-quenched glasses, which are also comparatively well annealed as a natural consequence of their

488

PHOTO-INDUCED CHANGES IN CHALCOGENIDE GLASSES 289

08SEVO Is POCRLY.AMIsA1.EO VAPOJR OEPO9TEO FLJ

I OAIGESIII.00AL I I MORPHOLOGICAL STR*JCflJPL

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7iOMSERVE0 RI POORLY.AIIEAI.EO VAPOJR.OEPOIsED FU40A IN MO AIRRIALRD SAMOLES

Figure 14.1 Classification of photo-induced structural and physicochemical phenomena in amorphous chalcogenide semiconductors.

method of preparation. By contrast, the irreversible changes that are primarily structural are found mainly in poorly-annealed vapour-deposited films, while the irreversible physicochemical phenomena occur whatever the state of annealing of the amorphous sample. The inter - relationships between the reversible or irreversible, and structural or physicochemical effects are illustrated schematically in Figure 14.1.

14.3 Reversible photo-induced effects

14.3.1 Photo-induced changes in local atomic structure

Reversible photo-induced changes in local atomic structure are subtle effects which are induced by light of photon energy greater than or close to that of the optical band-gap, Eg , of the chalcogenide glass. They are manifested mainly by small shifts (<-0.1 eV) in the optical absorption edge to lower energies, that is photodarkening. These shifts are accompanied by small changes in the radial distribution function determined by diffraction experiments, or in the vibrational spectra, and also, normally, by barely discernible changes in volume (contraction or expansion). Amorphous As 2S3 is the best-known example of a chalcogenide glass exhibiting these structural changes (Tanaka, 1981a,b) but they have also been observed in other As—S and As—Se glasses (particularly a—As2Se3 and As5oSe50), and in a—GeS2, amorphous Ge—Se compounds (particularly a—GeSe 2) and a—As4Se5Ge (Kolomiets and Lyubin, 1978; Tanaka, 1980). Photodarkening does not occur in Group IV or Group V amorphous semiconductors and appears to be a characteristic of most, though not all, chalcogenides (amorphous Te does not exhibit this 489

290 HIGH-PERFORMANCE GLASSES effect, and in the case of As 2S3 and As2Se3 it can be destroyed by the addition of Cu (Liu and Taylor, 1987)). Since photodarkening is sensitive to pressure (Pfeiffer and Paesler, 1989; Tanaka, 1984; Tsutsu et al., 1984), van der Waals bonding may also have a role in the process (Tanaka, 1983). It should be noted that photodarkening does not occur in crystalline As2S3 , indicating that this particular photo-induced effect is probably unique to the amorphous state (Tanaka, 1981a,b) and must involve structural features only present in the disordered material. These effects are reversible in that annealing restores the initial structure and- properties, and in some cases the initial state can also be recovered by exposure to light whose photon energy is less than that used to photodarken the sample (Hamanaka et al., 1981). It has been suggested that the structural change induced by light consists of minor bond rearrangements involving the chalcogen atom relaxing between the minima of double-well potentials (Averianov et al., 1980; Kolomiets and Lyubin, 1978; Tanaka, 1980). Figure 14.2 shows a schematic model of conjectured bistable local bonding geometries in a—As2S3 and the associated double-well potential: illumination induces a transition from arrangement A to arrangement A', possibly as a result of the strong electron—phonon coupling involved in the recombination

ra 1' U

R--+ Figure 14.2 A schematic model of bistable bonding geometries in a—As2S3 and the corresponding double-well potential. A and A' represent atomic configurations before and after illumination, R is some configurational coordinate and U the potential. 490

PHOTO-INDUCED CHANGES IN CHALCOGENIDE GLASSES 291 of a photo-excited valence electron (Murayama et al., 1980). X-ray absorption studies suggest that the actual structural change occurring during photodarkening in As2S3 is more complicated than that shown in Figure 14.2 and may involve a twisting of adjacent AsS 3 pyramids about their shared S atoms together with an increase of the As—S--As bond angle (Lee et al., 1989; Yang et al., 1987). A photo-induced increase in interlayer separation has also been suggested (Lowe et al., 1986). Another type of photo-induced change in local atomic structure which may also occur in chalcogenides is that produced by the 'self-trapped exciton' (Begelson and Street, 1980; Elliott, 1985; 1986). This involves the breaking of bonds—for example, in the case of a—As 2Se3 , an optically excited electron-hole pair would cause an As—Se bond to break and a Se—Se bond to form, resulting in a pair of metastable defects Se and Asj, possibly accompanied by deformation of the local structure. Further work is necessary to confirm the existence of these photo- induced changes. A third type of local structural change has been proposed to account for the appearance of optical anistropy in certain chalcogenides when illuminated with plane-polarized light (Grigorovici et al., 1983; Hajto and Ewen, 1979; Hajto et al., 1982; Hajto and Janossy, 1987; Zhdanov and Malinovskii, 1977; Zhdanov et al., 1979). This phenomenon, which has been termed the 'vectorial effect', is believed to involve the re-orientation of small anisotropic structural units already present in the amorphous material. The principal axes of these units are initially randomly oriented but under illumination with plane-polarized light the units are forced to re-orient themselves in such a way that the axes become aligned with some preferred direction defined by the plane of polarization. In the case of GeSe 2 , it has been suggested that the anisotropic units are the 'outrigger rafts' identified by Phillips (1981). The induced anisotropy can be erased by unpolarized or circularly polarized light (Hajto etal., 1982).

14.3.2 Photodecomposition

As the name implies, photodecomposition is the light-induced dissociation of a compound into its constituents. It has been studied most frequently in amorphous As—S and As—Se compositions and illumination of photon energy greater than Eg is necessary (Asahara and Izumitani, 1975; Berkes et al., 1971; Hamanaka et al., 1976; Keneman et at., 1978; Tanaka, 1981a,b). Absorption of a photon of energy greater than Eg causes a dissociation of the arsenic—chalcogen bond, and this is followed by thermal diffusion of the arsenic to form arsenic clusters, 491

292 HIGH-PERFORMANCE GLASSES which are highly absorbing. Photodecomposition causes small but detectable changes in X-ray diffraction and vibrational spectra, but is manifested mainly by a red-shift in the optical absorption edge, i.e. photodarkening. Provided the energy and intensity of illumination are not too great the photodecomposition is reversible on annealing at Tg. Clearly the proportion of dissociated bonds must be small. Small clusters of arsenic atoms can, in fact, be formed without significant diffusion of the arsenic: a local bond re-organization such as that illustrated for a—As 2S3 in Figure 14.3 will give rise to As2 units (i.e. As—As bonds). and several such re-organizations occurring in the same locality will produce clusters of three or four arsenic atoms. In well-annealed films or bulk samples of a—As2S3 , heteropolar As—S bonds are favoured, although a small percentage (about 1%) of homopolar As—As and S—S bonds may be present (Ewen and Owen, 1980). It is possible that above-band-gap illumination alters the bond statistics from a distribution close to that for the chemically-ordered-network model towards one characteristic of a random-network model, that is the randomness of the bond distribution is increased. Isolated As—As bonds are energetically unfavourable and when produced by the process illustrated in Figure 14.3 will soon revert to the initial state (Figure 14.3(a)). Larger clusters, in contrast, are expected to be relatively stable and hence can give rise to the photodarkening. At Tg , however, the atoms are more mobile, and annealing at this temperature causes even larger clusters to 'dissolve', so that the effect is reversible. Exposure to below-band-gap illumination also reverses the photodarkening. Direct evidence for the formation of As—As bonds in

OAS

(a) (b)

Figure 14.3 Schematic model of local bond reorganizations in a—As2S3 leading to the formation of an As—As bond. The transition between the initial structure (a) and the reorganized structure (b) is reversible. 492

PHOTO-INDUCED CHANGES IN CHALCOGENIDE GLASSES 293

well-annealed a—As2S3 after exposure to above-band-gap light has been obtained in Raman studies (Frumar et al., 1984). The increase in film thickness that accompanies this photodarkening in a—As2S3 is accounted for by the increase in the average bond length resulting from the replacement of As—S bonds by the longer As—As bonds. Measurements of the fractional change in film thickness can thus be used to estimate the percentage of As—S bonds converted into As—As bonds under illumination, and a value of approximately 6% has been derived (Frumar et al., 1984). It is interesting to note that if isolated As—As bonds were stable, the two structures depicted in Figure 14.3 could be described as bistable bonding arrangements, by analogy with the two bistable geometries shown in Figure 14.2. It is thus possible that stable homopolar bond formation in chalcogenides is one origin of metastable states.

14.4 Irreversible photo-induced effects

14.4.1 Photocrystallization

Chalcogenide glasses, by definition, have a non-crystalline structure, lacking long-range order. For some chalcogenides illumination can break interatomic bonds directly, and/or indirectly via local heating, thereby allowing the atoms to re-arrange themselves into an ordered crystalline structure. The photocrystallization of amorphous solids is well known and was first observed over two decades ago in a-Se (Dresner and Stringfellow, 1968; De Neufvilie, 1974). Photo-induced crystallization and 're-amorphization' has been studied in thin films of As—Te—Ge compositions, using Kr-ion and dye lasers (Von Gutfield and Chaudhari, 1972; Weiser et al., 1973). Thus, in a sense the process is reversible, but 're-amorphization' requires that the temperature of the film first be raised to the melting point of the crystals, and this can be achieved with a laser pulse. Strictly, therefore, this is an irreversible process.

14.4.2 Photopolymerization

When chalcogenide compounds are deposited as thin films by evaporation the as-deposited material often contains some of the molecular species of which the vapour is composed; for example, As 4S4 molecules have been identified in As2S3 vapour (Solin and Paptheodorou, 1977) and in as-deposited a—As2S3 films (Nemanich et al., 1978). In the case of a—As2S3 these molecular species are embedded in an amorphous As—S network and illumination causes them to polymerize and combine 493

294 HIGH-PERFORMANCE GLASSES with the network (Treacy et al., 1980). Polymerization causes densifica- tion and in general, on prolonged exposure, the density and structure of the thin films become virtually identical with those of melt-quenched glasses and well-annealed films (Chang and Chen, 1978; Chang and Hou, 1978). Photopolymerization has been proposed as an explanation of reversible photodarkening in well-annealed amorphous As—Se films (Grigoro- vici and Vancu, 1981). For such a model to be applicable, molecular species must exist in the film prior to exposure and annealing must cause depolymerization. It is not clear how photopolymerization can result in the film expansion that often accompanies reversible photodarkening. Photopolymerization has also been observed in the pseudo- binary system (As2S3) 1 _(As2Se3) ( Onari etal., 1985). Photopolymerization of As 454 molecules has been reported in crystals of a- and /3-As4S4 (Porter and Sheldrick, 1972), so this particular photo-induced effect is not unique to the amorphous state.

14.4.3 Photo-induced morphological changes

'Photo-induced morphological changes' is a term proposed for use to denote effects usually described in the literature as 'photovolumetric changes' or 'photocontraction', since the phenomenon has its origin in changes in the macroscopic structure of chalcogenide thin films, that is in their morphology. Changes in morphology have been observed mainly in thin films of Ge—Se compositions which, during vapour-phase deposition, grow on the supporting substrate in a columnar structure (Singh et al., 1979; 1980a,b; Spence and Elliott, 1987). On illumination, particularly with ultraviolet light, the films contract in volume. The magnitude of the change depends to some extent on composition but mainly on the angle of incidence at which the vapour-deposited film is grown. Contrac- tions in volume of up to 12% can occur. The effect is thought to be due to a collapse of the columnar morphology of the films, precipitated by a photo-electronic excitation of the electronic defect states that are characteristic of amorphous chalcogenides (Singh et al., 1980b).

14.4.4 Photovaporization

Photovaporization has been observed mainly in amorphous As 2S3 thin films and occurs at elevated temperatures (> —150 °C). It is strictly a photo-oxidation reaction followed by thermal evaporation of the volatile oxidation products (Janai, 1981a; Janai and Rudman, 1974): light causes a surface reaction between the oxygen in the air above the film and the 494

PHOTO-INDUCED CHANGES IN CHALCOGENIDE GLASSES 295

As2S3 to form arsenic oxide and sulphur, both of which evaporate off, resulting in a thinning of the film.

14.4.5 Photodissolution of metals

Of the various photo-induced phenomena that occur in chalcogenides, the metal-photodissolution effect is probably one of the most useful as far as applications are concerned and is therefore worth describing in more detail. When a chalcogenide layer in contact with a metal layer is illuminated, metal ions dissolve into the glass and migrate through it in the direction of illumination, thereby altering the composition and structure of the glass and changing its physical properties, particularly its solubility and optical constants. This effect is termed 'photodiffusion', 'photodoping' or 'photodissolution', and the structure of the system during the process is shown in Figure 14.4. The most widely accepted explanation of the effect is that it is essentially a light-induced chemical reaction between the metal and the chalcogenide to form a new material, which may be a single compound or a phase-separated mixture. This reaction product separates the metal and chalcogenide layers, so that for the reaction to proceed the new material must be both an ionic and an electronic conductor, i.e. it must allow both the metal ions and the electrons to diffuse through it to reach the product- chalcogenide interface (X—Y in Figure 14.4), where they can continue to

ILLUMINATION REFLECTED I It LIGHT

MOVING 1 1! —As-S GLASS INTERFACE X[ •.• • V + : ••. .. . : :: • •: _ PHOTODORED 1.. - • • . ••• . • • •• • LAYER

—METAL

-----i—SUBSTRATE

Figure 14.4 A schematic cross-section through a sample during the photodissolution process. Under illumination, metal ions dissolve into the As-S glass to form a new photodoped' material. The photodoped layer continues to grow as long as the sample is illuminated. The light stimulating the process is believed to be absorbed in the photodoped layer close to the interface X-Y. 495

296 HIGH-PERFORMANCE GLASSES react with the chalcogenide. The process is analogous to the tarnishing of metal films. The purpose of the light is to lower the kinetic barrier preventing the reaction at room temperature, so that the light stimulating the effect is expected to be absorbed near the doped- undoped interface X—Y, which has been confirmed experimentally (Owen et al., 1985; Kolobov et al., 1990). The effect has been observed in many chalcogenide systems, e.g. As—S, As—Se, Ge—S, Ge—Se, As—S—Te and As—Se—Te glasses, and probably occurs in most amorphous chalcogenides, whether in the form of unannealed vapour-deposited films, well-annealed films or melt- quenched glasses. A variety of metals, metal compounds and alloys have been used as the metal-ion source, e.g. Ag, Cu, Zn, A9 2S, A92Se, AgCl, AgNO3 and alloys in the Ag—Cu system. The metallic source is not necessarily an evaporated film, but can be a bulk substrate (Ewen et al., 1983) or even a thin coating produced by dipping the chalcogenide into a solution of a metal compound (Petrova et al., 1984). The amount of metal that can be introduced into a chalcogenide glass by photodissolution can be considerable, e.g. 30-40 atom% in some cases (Janai, 1981b; Yamaguchi et al., 1982). (For this reason 'photodoping' is possibly not an appropriate term since doping generally suggests the introduction of small amounts of extraneous atoms. However, the term 'photodoping' is widely used in the literature, the reaction product often being referred to as the 'photodoped' material and the initial chalcogenide as the 'undoped' glass.) In addition, the metal ions can be driven quite deep into the chalcogenide by photodissolution, for example in the Ag—As 16STe4 combination, Ag was found to penetrate at least 20 p.m into the glass (Inoue et al., 1974). One of the characteristics of the photodissolution effect is that the interface between the reaction product and the chalcogenide (X—Y in Figure 14.4) is usually relatively sharp, much sharper than that resulting from the Fickian distribution expected if the metal ions simply diffused into the chalcogenide in the same way as, for example, dopant diffuses into a semiconductor. Also, the metal concentration throughout the reaction product layer is essentially constant (Inoue et al., 1974; Janai, 1981b; Matsuda et al., 1973), so that the metal concentration profile during the process is often termed 'step-like'. Figures 14.5(a) and 14.5(b) show measured profiles (obtained by electron microprobe X-ray analysis) for Ag in As2S3 glass (a) for a photodoped sample and (b) for a thermally doped sample. The step-like nature of the profile in the photodoped case compared with that for the thermally doped material is apparent, but it should be noted that 'step-like' does not necessarily imply a perfectly abrupt interface between the photodoped and undoped material, only that the interface is sharper than that resulting from simple diffusion. 496

PHOTO-INDUCED CHANGES IN CHALCOGENIDE GLASSES 297

Ll ~I As

PHOTO-DOPED

(b)

THERMALLY

DOPED Ag

It 8im

Figure 14.5 Ag and S concentration profiles (relative to As) in Ag-doped a-As2S3 obtained by electron microprobe X-ray analysis (Matsuda and Kikuchi, 1973); (a) is for photodoped material and shows the 'step-like' Ag profile; (b) is for thermally doped material and shows a more gradual variation in Ag concentration in the interface region between the doped and undoped materials.

Photodissolution can be stimulated by photons with energies anywhere between the deep ultraviolet (250 nm) (Polasko et al., 1985) and the infrared (800 nm) (Lis and Lavine, 1983) spectral regions, the exact range of energies depending on the sample geometry and material compositions. (It also occurs under X-ray (Buroff and Rush, 1987; Kolwicz and Chang, 1980; Saito et al., 1988), electron-beam (Bernstein et al., 1988; Singh et al., 1982) or ion-beam irradiation (Wagner et al., 1981).) The chalcogenide —metal combination may be illuminated from the chalcogenide side or, for very thin metal films, the metal side, although in the latter case the maximum thickness of photodoped layer achievable will be correspondingly small. The spectral sensitivity of the process depends on absorption in the chalcogenide/metal for illumination from the chalcogenide/metal side, as well as on the spectral dependence of the basic process. It should be noted that the process can still be stimulated with photons of energy below the band-gap energy for the undoped chalcogenide. The usual sample configuration used in 497

298 HIGH-PERFORMANCE GLASSES

observing the photodissolution effect consists of a 0.1-1-p.m thick amorphous chalcogenide film on (or beneath) which has been deposited a thin metallic layer typically 0.01-0.05 m in thickness. Illumination from either the chalcogenide or the metallic side with light above or below Eg for the chalcogenide causes the metal to dissolve rapidly into the amorphous film and migrate through it. Evidence for the precise structural changes occurring during photodis solution has been obtained by Raman (Firth et al., 1983) and extended X-ray absorption fine-structure spectroscopy (EXAFS) (Greaves, 1990) measurements. It should be noted that photodissolution may also occur in crystals, though less effectively (Imura et al., 1983; Owen et al., 1985).

14.5 Applications of light-induced effects in chalcogenide glasses

14.5.1 Imaging properties of the metal photodissolution effect

Of the various photo-induced phenomena that occur in chalcogenides, one of the most important with regard to applications is the metal photodissolution effect and it is therefore useful to note briefly some of its general imaging properties. The metal photodissolution effect has two important features as far as imaging applications are concerned. Firstly, because the dissolving metal ions do not migrate outside the illuminated area an exact image of the intensity distribution at the surface of the chalcogenide film is created in the interior. Secondly, because such large amounts of metal (e.g. 30-40 atom%) can be driven into the glass, its structure and hence its properties are significantly altered: for example, the difference in refractive index between the doped and the undoped material may be as high as 0.5 and the doped material may be impervious to etchants that remove the undoped chalcogenide. Figure 14.6 shows schematically how images are formed using the effect, and how either embedded or surface-relief structures may be obtained in the film. For most metal —chalcogenide combinations at room temperature the metal ions will dissolve into the glass only under illumination, so that if illumination ceases there is no further movement of the metal ions and no degradation of the stored image. However, at higher temperatures (>175 'C for the Ag—As253 system) thermally stimulated isotropic diffusion of the metal ions may occur.

14.5.2 VLSI lithography

Resist technology is a key issue in the continuing trend to reduce semiconductor-device dimensions, and the development of new resist PHOTO-INDUCED CHANGES IN CHALCOGENIDE GLASSES 299

ILLUMINATION

MAS

As2S: to Ag son MENEM

IIlIIIIIIIIIl. JI AFTER ETCH L

Figure 14.6 Schematic illustration of pattern formation by metal photodissolution using the Ag/As—S system. Ag dissolves into the chalcogenide in the illuminated region but not in the masked areas. Because there is negligible sideways migration of the Ag, a faithful image of the mask is produced in the chalcogenide and the photodoped region has an approximately rectangular cross-section. If illumination is terminated (Io = 0) there is no further movement of the Ag and an embedded structure is formed. A surface-relief structure can be produced by removing the undoped material with an alkali etchant. materials is seen as one of the main routes to advances in this area (Brunsvold et al., 1987). The technology and fundamental science of inorganic resists based on photo-induced effects in chalcogenides (particularly the metal photodissolution effect) have been extensively studied, much of the earlier work being carried out at AT&T Bell Laboratories and at NTT. Reviews of this field have been published by Tai et al. (1982) and Mizushima and Yoshikawa (1982). Interest in these chalcogenide resists as a possible alternative to standard organic-polymer resists was originally stimulated by the fact that they could be used to achieve submicrometre resolution using conventional optical exposure sources (Sze, 1983). Because of their high contrast and high absorption coefficient in the ultraviolet, chalcogenide resists do not suffer problems with standing-wave formation or with matching the modulation-transfer function of the exposure system, unlike earlier organic resists, which 499

300 HIGH-PERFORMANCE GLASSES

required expensive e-beam or X-ray exposure systems to produce integrated circuits with submicrometre features. Research in this area has concentrated on resists employing the metal photodissolution effect as the imaging mechanism. Resists based on the many other photo-induced effects that occur in these, glasses have also been investigated (Janai, 1981a; Mednikarov, 1984) but seem less promising, in some cases because of poor sensitivity. The most extensively studied chalcogenide resist system based on metal photodissolution is Ag2Se/Ge—Se (Tai et al., 1982), although much work has also been carried out on Ag/As—S resists (Chang et al., 1979; Firth et al., 1985; Kolwicz and Chang, 1980). In terms of characteristics such as resolution, contrast and line-width control, the performance of chalcogenide resists generally matches or surpasses that of organic resists and they are compatible with standard industrial techniques of device fabrication such as plasma-etching and spin-coating. (Working devices have been fabricated using these resists (Mizushima and Yoshikawa, 1982; Utsugi et al., 1984).) Evaporation or sputtering techniques can be used to deposit them as thin, uniform films, which is an important requirement for high-resolution lithography. Contamination problems, with e.g. Ag, can be avoided by using a multilayer resist system in which the chalcogenide resist is used as a thinimaging layer above a thick organic layer which planarizes the wafer topography and acts as a barrier between the wafer and possible contamination sources in the chalcogenide. Such multilayer resist systems also tend to yield the best resolution because they eliminate the effects of substrate topography and reflectivity. Undoped Ge—Se films are resistant to most of the acid solutions used in silicon processing (e.g. HF, HF—NH4F, H3PO4 , HCl and H2SO4) but can be dissolved by alkalis. In contrast, Ge—Se films photodoped with Ag hardly dissolve at all in alkalis, so that an alkaline developer such as an aqueous solution of NH3 can be used to achieve a negative process (unexposed resist removed). The swelling deformation that occurs during the development of polymer resists is not present in these Ge—Se resists. Positive processes (in which the exposed resist is removed) can also be achieved using an additional heat treatment (Mizushima and Yoshikawa, 1982) or other light-induced effects (Mednikarov, 1984). 'Dry' development of chalcogenide resists by plasma etching is also possible. In the case of the Ge—Se system, CF 4 , CHF3 or SF6 can be used as the etchant gas and differential etch rates as high as 500:1 have been achieved (Mizushima and Yoshikawa, 1982). The theoretical limit to the feature size that can be produced in chalcogenide resists using a standard deep ultraviolet exposure system is —0.3 gm (Tai et al., 1982), and 0.4 jum lines and spaces have already been produced in these resists over typical wafer topography (Ong and LIME

PHOTO-INDUCED CHANGES IN CHALCOGENIDE GLASSES 301

Hu, 1984). These resists have also been used with excimer laser sources and have demonstrated a resolution capability of 0.2 m (Polasko et al., 1985) for this type of exposure. Chalcogenide resists are also sensitive to X-rays (Buroff and Rush, 1987; Kolwicz and Chang, 1980; Saito et al., 1988), ion beams (Wagner et al., 1981) and electron beams (Bernstein et al., 1988; Chan et al., 1986; Mizushima and Yoshikawa, 1982; Singh et al., 1982), which are the sources required for very high resolution: feature sizes down to 0.05 j.m have been achieved in these resists using X-rays, and down to 0.03 gm using e-beam exposure. For X-ray exposure, chalcogenide resists are expected to achieve better resolution than organic materials because they have a higher density than typical polymer resists. One reason for the high resolving power of the chalcogenide resists is the so-called 'edge-sharpening' effect they exhibit (Tai et al., 1980). This tends to compensate for diffraction and interference effects in the aerial image and is illustrated schematically in Figure 14.7, which shows the edge of an opaque region of a mask and the corresponding intensity distribution over a Ag 2Se/Ge—Se resist film. Because of diffraction effects some light leaks sideways under the opaque region and so the aerial image is not perfectly step-like. Below the transparent region of the mask the illumination intensity is relatively high and Ag from the Ag2Se sensitizing layer rapidly photodopes the Ge—Se film. As a result,

Ge-Se

two Cb t i

* 3

Figure 14.7 Schematic diagram of the light intensity distribution at the edge of an opaque region of a mask over a A92Se/Ge—Se resist layer. Also shown are the Ag concentration (CAg) profiles at various times, r, in the A92Se layer during its photodissolution into the Ge—Se layer. Ag ions in the A92Se layer beneath the edge of the opaque region diffuse sideways into the illuminated part because of the concentration gradient in the region of the edge. 501

302 HIGH-PERFORMANCE GLASSES

in this region the sensitizing layer is depleted of Ag, which causes Ag ions in the weakly illuminated area below the opaque region to diffuse sideways along the Ag2Se layer to compensate for this deficit. This means in turn that less Ag is available at the edge of the masked region so that photodoping is less efficient there. The effect of the light leakage will therefore be less pronounced than for an organic resist and the edge of the photodoped region correspondingly sharper. The ultimate resolution capability of a resist is governed by its 'grain' size, which for an organic resist will be of the order of —lOOs of A since these grains are composed of large polymer molecules. The 'grain' size for chalcogenide resists will be smaller than this because these materials are essentially covalent solids, with the largest structural units being only a few lOs of A in extent. Hence the ultimate resolution capability of the chalcogenide resists will be extremely high, and this has been confirmed experimentally (Mizushima and Yoshikawa, 1982): analysis of transmission electron microscope images of fine metal particles deposited on a A92Se/Ge—Se resist by evaporation indicate that the resolution limit for this system is less than 100 A. Another characteristic feature of chalcogenide resists is their high contrast: values of contrast, y, between 5 and 10 have been reported for conventional ultraviolet illumination (Huggett et al., 1983; Kozicki, 1989; Tai et al., 1980) and greater than 10 for excimer laser exposure (Polasko et al., 1985). High contrast (5 < y < 9) is also obtained with electron beam sources (Balasubramanyam and Ruoff, 1981; Mizushima and Yoshikawa, 1982; Singh et al., 1982), which is an important advantage for this type of exposure as it enables discrimination against backscattered electrons. A y value of 3.5 has been reported for X-ray synchrotron radiation (Saito et al., 1988). The main shortcoming of chalcogenide resists at present is their sensitivity, which for conventional optical illumination is typically 3-5 times poorer than that of organic resists, although good sensitivity (5 mJ/cm2 ) is achievable with excimer laser exposure (Polasko et al., 1985). Their sensitivity can be improved, though at the expense of additional processing steps (Masters et al., 1980). It is possible, however, that this problem may be overcome in the future by using another of the many chalcogenide —metal combinations exhibiting the photodissolution effect. In the case of e-beam exposure, sensitivities comparable to that of poly methyl methacrylate (PMMA) have been observed (Chen et al., 1986), while for X-ray exposure sensitivities 2-3 times better than that of PMMA have been achieved (Saito et al., 1988). It should be noted that in a production environment, throughput depends not only on exposure time but also on the time spent in wafer-handling and alignment in the exposure system, so reduced sensitivity is not necessarily a serious disadvantage (Utsugi et al., 1984). 502

PHOTO-INDUCED CHANGES IN CHALCOGENIDE GLASSES 303

Over the period of this research into the lithographic applications of chalcogenide glasses there have been improvements in the performance of organic resists, with the result that some are now capable of submicrometre resolution using conventional ultraviolet exposure systems. Although this means that the chalcogenides may have lost their economic advantage, it is possible that they may yet find uses in either very high resolution lithography, using X-rays, electron-beam or ion- beam techniques, or in certain specialized areas such as photomask production (Mizushima and Yoshikawa, 1982). Research is continuing into both the theory and the practical aspects of chalcogenide resists (Belford et al., 1989; Bernstein et al., 1988; Das and Al-Jishi, 1989; 1990; Hajto et al., 1989), a particularly promising area being the development of an in vacuo lithographic process (Kozicki, 1989). Organic resists, which are solvent-based and therefore have volatile components, are not compatible with this concept. All-vacuum processing is seen as a key step in reducing particulate and gaseous contamination of substrates (Toy, 1989), which is becoming an increasingly important issue as the trend to smaller geometries continues and as new device structures, such as those based on superlattices, become widespread.

14.5.3 Infrared diffractive optics

Diffractive optical elements operate as a result of the diffraction of light rather than its bending or reflection, the best known example being the Fresnel lens or zone plate, which was originally developed in the nineteenth century as a lightweight alternative to planoconvex lenses for use in lighthouse optics but is also used nowadays in other large-aperture applications such as photocopiers and overhead projectors. Apart from reduced weight, these diffractive optical elements can, in many situations, have other advantages over conventional refractive or reflective components, and this is true not only for the visible but also for infrared operation. Because they are essentially planar elements they can be cheaper and easier to manufacture than reflective or refractive components such as Ge lenses. Diffractive elements can be produced to perform the functions of, for example, mirrors, lenses, filters and beam combiners, and they can also carry out beam-forming operations that are difficult to accomplish using conventional elements (Frolova, 1988). Hybrid refractive/diffractive elements may also have uses (Swanson and Veldkamp, 1989). Figure 14.8 shows the diffractive analogues of some optical components. The applications of infrared optics are many and varied and general areas include energy management, thermal fault detection, astronomy, electronic circuit-inspection and night vision (Johnson, 1988). 503

304 HIGH-PERFORMANCE GLASSES (a)JJ

(-)

Figure 14.8 Diffractive analoguesD of optical elements: (a) prism; (b) axicon; (c) lens (Frolova, 1988).

The range of materials that are suitably transmissive in the infrared is small, most optical elements being made from Ge, Si or ZnSe. Chal- cogenide glasses are well-known infrared transmitting materials, having pass-bands (depending on composition) from the visible to beyond 15 p.m, and they have been employed to produce infrared windows, filters and fibres (Cimpl and Kosek, 1987). Because the chalcogenides also exhibit many photo-induced effects it is possible to create diffractive elements by using one of these effects to produce surface relief or embedded structures in a chalcogenide film. Photo-induced effects in chalcogenides have been used to fabricate holographic gratings (both phase and surface-relief types) for use at visible wavelengths (Andreish et al., 1986; Bordogna and Keneman, 1977; Kase et al., 1984; Yaji and Kurita, 1983) and are currently being investigated as a technique for producing infrared-diffractive elements (Slinger et al., in press; Zakery et al., 1988). As in the case of the very large scale integration (VLSI) lithographic applications, of the various photo-induced phenomena that occur, the metal photodissolution effect is probably the most useful as far as grating applications are concerned, because it produces the largest 504

PHOTO-INDUCED CHANGES IN CHALCOGENIDE GLASSES 305 change in the properties of the chalcogenide, particularly the refractive index and etch resistance. In the case of the Ag/As—S system, both photodoped and undoped As—S films are transparent over the range 2-12 gm and the difference in refractive index between the Ag-doped and undoped material over this wavelength range can be as high as 0.5, so that phase gratings with high modulation are achievable. Phase gratings have an embedded structure and this is created automatically by photodoping. Surface-relief gratings can be produced by removing the undoped chalcogenide in an alkali etchant. Theory suggests that near-band-gap illumination is the most efficient for producing the deep structures required for infrared operation: if the photon energy is too high, the light is absorbed in the undoped chalcogenide before it can stimulate photodissolution; if the photon energy is too low, the photodissolution process proceeds very slowly. For the As—S glasses near As 2S3 this means that the exposing wavelength should be in the green, and the 514.5 nm line of the Ar laser is therefore an ideal source. In general, for efficient operation these infrared elements have to be relatively thick (of the order of several micrometres at least), which is in contrast to the VLSI applications, where much thinner films (-0.2 /.Lm) are used. Coupled-wave theory can be used to predict the required thicknesses for different types of grating element at different wavelengths of operation (Slinger et al., in press) and typical values are given in Table 14.1. Chalcogenide films can be deposited using a variety of techniques (including thermal evaporation, sputtering, spin-coating, plasma deposition and hot-pressing) and thicknesses of up to at least 100 j.tm are achievable. However, the depth to which metal ions can, in general, photodope chalcogenide films has not been established. In the case of the Ag/As—S system, photodoping to a depth of 4 gin has been observed (Owen et al., 1990) and it is likely that photodoping to a depth of at least 6 gm is possible, so that most of the structures in Table 14.1 are obtainable with this system. As mentioned earlier, Ag has been observed to photodissolve to a depth of at least 20 m in As 16STe4 , and if this is typical of most chalcogenides then even the 23-p.m structures required for bulk gratings at 10.6 p.m operation should be

Table 14.1

Grating thickness d (gm)

Replay wavelength (gm) 0.632 4.0 10.6

Bulk 1.39 8.77 23.2 Surface relief 0.48 3.0 8.06 Blazed zone plate 0.38 2.4 6.42 505

306 HIGH-PERFORMANCE GLASSES achievable. Because the photo-dissolution process is essentially diffusion-controlled the required exposure time increases as the square of the grating thickness, so that long exposures may be required to produce very thick structures. However, several possibilities exist for significantly increasing the speed of the photodissolution process, such as the simultaneous application of heat (so-called photothermal doping). Coupled-wave analysis of the likely performance of bulk and surface- relief structures typical of those capable of being produced by photodoped chalcogenides (Slinger et al., in press) suggests that high diffraction efficiencies (>80%) should be achievable in the infrared.

14.5.4 Optical mass memories

Because the density of information storage in optical media can be up to two orders of magnitude better than in magnetic discs, there has been extensive research into materials capable of optical recording. Chalco- genide films have potential in this area and have been investigated as optical recording media for mass-memory applications such as data discs for video recording or computer information storage (Madan and Shaw, 1988). In these applications the basic mechanism used to record infor - mation is generally either photo-induced crystallization of an amorphous film (alternatively, amorphization of a crystalline film) (Jung et al., 1989; Matsushita et al., 1987; Uchida et al., 1989; Watanabe et al., 1983; Yagi et al., 1987; Yamada et al., 1987) or light-induced morphological changes such as the generation of bubbles or holes in the film (Vriens and Jacobs, 1984). Both of these mechanisms are essentially thermally induced, the heating being provided by the optical excitation, which is usually a focused laser beam. In the case of the photo-induced phase-transition mechanism, the optical constants of the amorphous and crystalline material are sufficiently different that discrimination between the exposed and unexposed regions of the film can be achieved by monitoring reflectance. In a typical system (Watanabe et al., 1983), light from a diode laser is focused to a spot a few micrometres in diameter: to record, the laser power is increased above the recording threshold for the material (usually a few tens of milliwatts), while to read data, the power is reduced below this threshold and the reflected light from the beam spot is collected and measured. Of the various chalcogenides exhibiting the effect, Sb 2Se3 , Sb1Te1_ (particularly Sb2Te3) and Te(GaSe 1_) 1 _ glasses are among the most suitable found to date. Optical recording of frequency-modulated video signals has been demonstrated in Sb 25e3 films used in conjunction with a reflective layer of Bi 2Te3 , which increases energy coupling into the photo-sensitive layer (Watanabe et 506

PHOTO-INDUCED CHANGES IN CHALCOGENIDE GLASSES 307 al., 1983). In these experiments the recording material was deposited on a PMMA disc substrate, and the recording threshold was <5 mW at a rotation speed of 1800 r.p.m. This memory effect is reversible and so can be used in either write-once or erasable recording: erase times of 0.5 p.s and reversibility >iO write—erase cycles have been realized in doped with 5 wt% Ge (Matsushita et al., 1987); Te08(Ga0.05 Se0.95)0.2 5 GeSb2Te4 exhibits an erase time of 50 ns and reversibility of >10 cycles (Yamada et al., 1987). In the case of the pseudo-ternary alloy system GeTe—Sb2Te3—Sb, a reversibility of >10 6 cycles has recently been reported (Uchida et al., 1989). Of the morphological mechanisms, the most important is probably the creation of well-defined holes in thin films of chalcogenide material (Vriens and Jacobs, 1984). Uniformly shaped holes with clean edges and dimensions of the order of 1 p.m can be generated in a variety of chalcogenides (for example AsTe 80 , Ge 10Te90 , As15Se, Ge loSego and Sb20S) by using a focused laser beam. The beam is believed to evaporate the centre of the irradiated region and to soften the remainder so that material is pulled to the edges of the exposed area by surface tension.

14.5.5 Integrated optics

Optical fibres are now replacing copper wire as the transmission medium in telecommunications because of their significant advantages in terms of data rate, cost and information-carrying capacity. However, the performance of optical-fibre communication networks is limited, not by the properties of the fibres, but by the speed of the electronics used in switching, routing and signal processing. To exploit the full potential of these optical-fibre systems new optical devices and circuits must be developed to process light signals directly. Such devices require materials with non-linear optical properties, that is materials in which the optical constants can be changed by intense optical radiation. The interconnects in these optical circuits will generally be waveguides and fibres. To minimize attenuation in optical fibres, the wavelength of the light used in optical communications must be in the infrared: typical commercial fibres operate in the range 1.3-1.6 p.m at present. For ultra-long- distance communications, attenuation losses must be reduced still more, which implies shifting the wavelength of operation further into the infrared. The use of chalcogenide glasses as materials for infrared-transmitting fibres and waveguides has been extensively studied and has been reviewed by Andreish et al. (1986). As these applications are covered in detail elsewhere in this book (chapter 13) they will not be discussed 507

308 HIGH-PERFORMANCE GLASSES further here, except to note that as well as interconnecting chalcogenide devices, chalcogenide films can be used as coupling elements between active and/or passive optical components made in other materials such as silicate glasses and LiNb0 3 . A wide variey of chalcogenide integrated optical components has been studied, including filters, mixers, deflectors and demultiplexers. (As many of these components contain grating structures, there is some overlap between research in this area and the work being carried out on infrared diffractive optics in chalcogenides.) Figure 14.9 shows a schematic diagram of a demultiplexer fabricated using the metal photodissolution effect (Andreish et al., 1986). Single- and double-throw optical switches have been made in As—S waveguides (Tanaka et al., 1985), based on the reversible photo-induced optical-constant changes described in section 14.3.1. Although these devices have long switching times, it is likely that much faster switches can be produced based on the non-linear optical properties of these materials. GeS 2 and As2S3 are known to be highly non-linear, the latter having a non-resonant third- order electronic susceptibility of 2.2 x 10-12 esu and a response time of less than 10 Ps (Hall et al., 1989; Nasu et al., 1989). Finally, it should be noted that much of the technology required for the fabrication of optical integrated circuits in chalcogenides has already been developed: it is essentially identical with that used for the chalcogenide resists employed in VLSI lithography. Indeed, the processing requirements may be less stringent than those for VLSI circuits, since the dimensions of optical-device features will generally be of the order of the light wavelength, which for infrared operation will be greater than 1 gm. In this case, conventional optical exposure techniques can be used in patterning the chalcogenide film.

14.6 Conclusions

As a result of extensive research over the past three decades, considerable information now exists on the properties and applications of

ENTRY FIBRE DIFFRACTION GRATING

EXIT FIBRES FOCUSING ROD PRISM

Figure 14.9 Schematic diagram of a demultiplexer made using the metal photodissolution effect (Andreish et al., 1986). 508

PHOTO-INDUCED CHANGES IN CHALCOGENIDE GLASSES 309 chalcogenide glasses. They are generally 'well-behaved' materials and can be made in bulk form or deposited, by a variety of techniques, as films with thickness ranging from about 100 A to 100 pm. The chalcogenides exhibit 'a wide diversity of reversible and irreversible photo- induced effects which makes them unique amongst glass-forming families and leads to many potential applications in the area of optical imaging or recording. Because such a large number of chalcogenide systems exist there is considerable scope for optimizing the performance of the imaging and recording media based on these photo-induced effects. Research is actively continuing in this field, and with the growing interest in 'nanotechnology' and 'photonics' there is considerable scope for applications of the light-sensitive properties of these materials. 509

332 HIGH-PERFORMANCE GLASSES

Lines, M. E. (1984b) Scattering losses in optic fibre materials. II. Numerical estimates. J. App!. Phys., 55(11) 4058-4063. Lines, M. E. (1988a) Theoretical limits of low optical loss in multicomponent halide glass materials. I. Non-Cryst. Solids, 103(2-3) 265-278. Lines, M. E. (1988b) A possible nonhalide route to ultralow loss glasses. I. Non-Cryst. Solids, 103(2-3) 265-278. Lu, G., Aggarwal, I. and Bradley, J. P. (1988) Noble metals as a source of continuous scattering in fluoride glasses. I. Amer. Ceram. Soc., 71(3) C156-157. Lucas, J. (1989) Fluoride glasses. J. Mater. Sci. 24(1) 1-13. Midwinter, J. E. (1979) Optical Fibres for Transmission. Wiley, NY, USA. Mitachi, S. and Tick, P. (1988) Oxygen effects on fluoride glass stability. Mater. Sci. Forum. 32&33 197-202. Miya, T., Terunuma, Y., Hosaka, T. and Mujashita, T. (1979) Ultimate low-loss single-mode fibre at 1.55 jA m. Electron. Lett., 15(4) 106-108. Moynihan, C. T., Macedo, P. B., Maklad, M. S., Mohr, R. K. and Howard, R. E. (1975) Intrinsic and impurity infrared absorption in As 2Se3 glass. J. Non-Cryst. Solids, 17 369-385. Pitt, N. J., Sapsford, G. S., Clapp, T. V., Worthington, R. and Scott, M. G. (1986) Telluride fibres for transmission in the 8-12 micrometres waveband. In Proc. SPIE mt. Soc. Opt. Eng., Vol. 618, Infrared Optical Materials and Fibres IV, pp. 124-129. Pitt, N. J. (1987) Loss mechanisms in chalcogenide glass optical fibres. In Proc. SPIE mt. Soc. Opt. Eng., Vol. 799, New Materials for Optical Waveguides, pp. 25-32. Poulain, M., Poulain, M. and Lucas, J. (1975) Fluorine glasses containing zirconium tetrafluoride. Optical properties of a glass doped with Nd 3 . Mater. Res. Bull. 10(4) 243-246. Saito, M. and Takizawa, M. (1986) Teflon-clad As—S glass infrared fibre with low-absorption loss. I. App!. Phys., 59(5) 1450-1452. Saito, M., Takizawa, M. and Miyagi, M. (1989) Infrared optical fibres with vapour- deposited cladding layer. J. Lightwave Technol., 7(1) 158-160. Sanghera, J. S., McKenzie, J. D. and Hulderman, F. (1989) UV radiation damage of As2Se3 glass fibres. Mater. Lett., 8(10) 409-414. Savage, J. A. (1985) Infrared Optical Materials and their Anrireflection Coatings. Adam Hilger, Bristol. Shibata, S., Terunuma, Y. and Manabe, T. (1981) Sulphide glass fibres for infrared transmission. Mater. Res. Bull, 16(6) 703-714. Sun, K. H. (1979) Fluoride glasses. Glass Tech. 20(1) 36-40. Takahashi, H. and Sugimoto, I. (1983) Decreased losses in germanium oxide glass optical fibre prepared by VAD method. Jap. I. App!. Phys., 22(3) L139-140. van Heel, A. C. S. (1954) A new method of transporting images without aberrations. Nature, 173 39. Wallenberger, F. T., Weston, N. E. and Dunn, S. A. (1990) Melt spun calcium aluminate fibres: infrared transmission. I. Non-Cryst. Solids, 124(1), 116-119. Zhang, X. H., Fonteneau, 0., Ma, H. L. and Lucas, J. (1989) Potential for application of tellurium halide glasses for thin films and optical fibres. In Proc. SPIE mt. Soc. Opt. Eng., Vol. 1128, Glasses for Optoelecrronics, pp. 301-305.

Chapter 14

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References added at proof The following extensive reviews on photodarkening and photodissolution in chalcogenide glasses appeared whilst chapter 14 was in preparation: Pfeiffer, 0., Paesler, M. A. and Agarwal, S. C. (1991) Reversible photodarkening of amorphous arsenic chalcogens. I. Non-Cryst. Solids, 130 111-143. Kolobov, A. V. and Elliott, S. R. (1991) Photodarkening of amorphous chalcogenides by metals. Adv. in Physics (in press). 515

Amorphous and Microcrystalline Semiconductor Devices Volume II Materials and Device Physics

Jerzy Kamcki, Editor

Artech House Boston • London 516

Chapter 14 Electronic Switching in Amorphous- Semiconductor Thin Films I. Haj:o, A. E. Owen, and A. I. Snell Department of Electrical Engineering, University of Edinburgh, Scotland, U. K. P. G. LeComber and M. I. Rose Department of Applied Physics and Electronic & Manufacturing Engineering, University of Dundee, Scotland, U. K.

The aim of this chapter is to summarize experimental and theoretical work on electrical instabilities such as electronic switching and memory phenomena in amorphous thin films and particularly in amorphous silicon thin-film structures. Section 14.1 is a general survey of the field, comparing and contrasting experimental observations and theoretical models of switching in a variety of amorphous thin films, including a brief review of the early work on amorphous silicon. In Section 14.2 recent work in the authors' own laboratories on digital and analog amorphous silicon switching devices is described and discussed in more detail. For the present purposes, electronic switches are two-terminal solid-state devices that can be changed from a nonconducting OFF state to a conducting ON state by an appropriate electric signal. Electronic switching is associated, explicitly or implicitly, with negative differential resistance (NDR). Two types of NDR exist: current-controlled NDR (CCNDR) [1) shown in Figure 14.1(a) and voltage-controlled NDR (VCNDR) 121 shown in Figure 14.1(b). Conduction in CCNDR involves the creation of conducting filaments in which the current density differs from that of the surrounding material [3] or circuit-controlled oscillations (4). VCNDR is associated with stationary [2] or steady-travelling high-electric-field domains [5), or with sustained circuit-controlled oscillations [6]. Both CCNDR and VCNDR can give rise to a range of switching phenomena.

641 517

642

CCNDR VCNDR

0 (a) 0 (b)

Volts -' Volts -+

Flgire 14.1 Schematic forms of (a) CCNDR and (b) VCNDR.

14.1 GENERAL SWITCHING PHENOMENA IN AMORPHOUS SEMICONDUCTORS

14.1.1 Threshold and Memory Switching

Many examples of threshold and memory switching have been reported in initially homogeneous thin films of a variety of materials including simple oxides [7J, transition-metal-oxides [81, elemental boron 191, metal-semiconductor-metal structures [10], and Langmuir-Blodgett films [11J. Interest in amorphous semiconductor films was stimulated by the discovery of reversible switching in certain amorphous semiconductors [121. From 1968 onward, interest concentrated primarily on a class of covalently bonded alloys of group IV, V, and VI elements called the chalcogenide glasses. These materials, with appropriate electrodes, exhibit a transition from a low conductance (OFF) to a high conductance (ON) state under the influence of sufficiently high fields. To illustrate the switching characteristics that are often observed, we shall first describe some typical results for the chalcogenide glasses. A typical switching device consists of a thin film of chalcogenide glass (thickness approximately 0.1- 10 m) sandwiched between two electrodes of refractory metals. Conduction is ohmic up to fields of about I(Y Vcm '. At higher fields, nonohmic processes become evident and the current rises exponentially with applied voltage. Switching occurs at fields of about 10 Vcm and schematic current-voltage (I- V) characteristics are shown in Figure 14.2(a). Upon switching (at the threshold voltage VT,,), the voltage across the device drops sharply along the load line until a holding voltage V, of about 1V is reached. Experimental results show that conduction in the ON state is filamentary in character [131. The device may be maintained in the ON state as 518

643

I :N\

cItag. VH frz' •... V, VTW

VrM ON

'ON

OW IY

VTH

Flpre 14.2 Schematic representation of (a) threshold switching and (b) memory switching. long as the current does not drop below a critical holding current 'H and if that is not maintained the device switches back to the low-conducting OFF state. The switching process is highly reproducible, reversible, and essentially independent of polarity. Devices of this kind are called threshold switches; they are nonpermanent, or volatile, and always revert to the OFF state in the absence of an appropriate bias. The schematic I-V characteristics for a different type of switching are shown in Figure 14.2(b). There is again a critical switching voltage for the OFF to ON transition, but both ON- and OFF-state characteristics extrapolate through the I-V origin. In these devices, if the high ON-state current is maintained for about a millisecond after the OFF to ON switching, the material along the current filament is modified and a high density of crystallites forms, creating a permanent bridge of high-conductivity material between the two electrodes (14]. After this modifi- 519

rri

cation of the original structure has occurred, the device remains in the low-resistance ON state even after the electric field is removed. It is possible to return the device to its high-resistance OFF state by applying a sufficiently high, short, current pulse. This "erase" pulse melts the high conductivity material in the conducting channel. Subsequent rapid cooling of the melt restores the original amorphous phase. Devices of this kind are permanent (nonvolatile) and hence are known as memory switches. The amorphous-to-crystalline transition outlined above is, of course, only one possible mechanism by which memory switching can be implemented. In general, memory switching refers to any mechanism that results in the ON and OFF states persisting more or less indefinitely in the absence of any bias. To summarize, switching phenomena can be characterized into two main categories: Threshold switching, in which continuous electrical power is required to maintain the highly conducting ON state; Memory switching, in which both ON and OFF states can be maintained without electrical power. It is common to describe switching phenomena in terms of ON and OFF states, but as we will show in Section 14.2.2, switching devices are not always rigidly bistable in their operation, that is, they are not always digital devices. In some cases, a continuous range of intermediate states is observed between the ON and OFF states, giving an analog memory effect. The speed of the switching transients varies from device to device and covers a range from <10 ns to >1 ms. The actual switching is extremely fast (nanoseconds) but is preceded by a delay time of the order of microseconds near the threshold. The delay time decreases exponentially with the overvoltage, that is, with the voltage above the threshold. Figure 14.3(a) shows a simple circuit that can be used to measure the voltage and current waveforms during a single switching pulse to obtain the speed of the switching transient, and Figure 14.3(b) shows schematic waveforms of the measured voltage V5 and current Is across the device during an OFF to ON transition (associated with either threshold switching or a memory "write-in" operation). There is a sudden rise in the voltage across the device at time t but the current remains low (i.e., the sample is in its low-conducting OFF state) for a certain time termed the "delay time" (t0), equivalent to t - to. At : there is a sudden rise in the current and at the same time a similarly fast decrease in the voltage associated with the OFF to ON transition (i.e., a sudden increase in the device conductivity). The duration of the ON to OFF transition (:2 - t) is called the "switching time," and it is usually much faster than the delay time :,.,.. Figure 14.3(c) refers to the erase operation for a memory device. This situation obviously does not arise in a volatile threshold device. When the voltage pulse is applied at time :o, the current is high at first, as the sample is in its ON state. After 520

I VS is _FL RL Sample'*Si= A2

oftoottransItion (C) ONto OFF traneition A v IT[

...... ISI ...... t ow t 06

to II t time to 1, t'

Rgure 14.3 (a) Experimental arrangement of the switching circuitry. (b) waveforms of WRITE transient; and (c) waveforms of ERASE transient.

a delay time !DE, the sample conductance decreases rapidly during the erase switching time (12 - ti), and at the same time the voltage across the device increases. Most chalcogenide devices, whether threshold or memory, can sustain many hundreds of cycles of ON—,.OFF—.eON transitions. The best threshold devices can switch up to Hr before they fail. Good memory devices will operate for at least 106 WRITE-ERASE cycles before failure.

14.1.2 The Forming Process: A Precursor to Threshold and Memory Switching

Newly fabricated devices rarely show threshold or memory switching effects without an initial modification of their as-deposited structure, a process usually called forming. Forming is achieved by the application of suitable voltage pulses that are always higher in magnitude than the subsequent programming pulses. This invari- ably produces an irreversible change in the electrical characteristics of a device, often with a substantial decrease in the overall terminal resistance. The processes involved in forming depend on the type and quality of the thin film, the geometrical structure of the device and in some cases the electrode material. The changes can be either physicochemical, involving a redistribution of the 521

constituents, or they can be electronic, wherein a quasi-permanent change in the occupancy of some electronic states takes place. The changes can occur throughout the bulk of the film or in a localized region. The most common localized effect is the formation of a filament of highly conducting material that extends completely or a part of the way through the device. Such permanent filament formation is a consequence of temporary filamentary breakdown usually observed in metal-insulator-metal and metal-semiconductor-metal sandwich structures under double- injection conditions [31. Barnett and Milnes have considered current and voltage instabilities in semiconductors and have shown that with current-controlled negative resistances, such as those expected in double-injection structures, energy considerations imply the formation of a filament. The filament develops first at some device inhomogeneity, is stationary. and grows in size about the nucleation region as the current is increased. This filament should not be confused with the moving transverse wave of a voltage-controlled instability (Gunn effect), which was also considered by Barnett and Milnes. Rather earlier, Ridley [151 had also shown by general thermodynamic arguments that CCNDR implies the formation of current filaments, while field domains are involved in VCNDR. Filamentary conduction has been experimentally observed in a wide range of materials and sandwich structures. These include single crystal silicon [161, gallium arsenide [17], zinc telluride [181, compensated germanium (19], and polycrystalline silicon 1201, all of which show I-V characteristics associated with threshold switching of the kind shown in Figure 14.2(a). In these cases the filament collapses and disappears as the voltage is removed, and there is no permanent change in the device structure or in its electrical characteristics. In the case of amorphous thin-film structures, however, the development of the filament during the forming process is always accompanied by an irreversible modification of the original structure. This means that the filament—or part of the filament—is permanently 'written in" to the originally uniform amorphous structure (i.e., it will not disappear after the voltage is removed). Permanent filament formation can occur by crystallization of an amorphous film [14], stoichiometric changes [211, diffusion of the electrode material into the film [7], or ionization of deep traps (221. All of these changes usually refer to a localized modification of the structure in the area of the filament. Reports of bulk forming effects are much rarer, one example being bulk diffusion of electrode material into the film (23]. The modification of the initial structure during the forming process is the most important factor determining the subsequent switching operation. This is because in most cases forming creates a new device within the originally deposited structure, whose characteristics determine the ensuing threshold-switching or memory-switching operation. For example, a permanent filament may have a cross- sectional area many times less than the device area, but its effective conductance is often so much higher than that of the surrounding material that it becomes the dominant conducting path. 522

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It is important to emphasize that in both the threshold- and memory-type devices, the initial switching mechanism appears to be the same [24]. It is often initiated by field-dependent nonohmic conductivity and consequent instability. Whether what follows is threshold switching, memory switching, or, in some cases, destructive electrical breakdown depends on the properties of the material and the presence or absence of suitable feedback in the system.

14.1.3 The Mechanism of Threshold Switching

The detailed I-V characteristics of chalcogenide-based threshold-switching devices have been widely investigated since the original publication by Ovshinsky in 1968 [121, and a general survey of the most significant results has been provided by Adler, Henisch, and Mott [25]. The suggested physical models can be divided into two categories; the first assumes that threshold switching is due to thermal processes [261, and the second considers that it is associated purely with electronic phenomena [27, 28]. Thermal or, more generally, combined thermal and electrical mechanisms are sufficient to explain threshold switching. In the thermal models, the increase in conductivity under the influence of an electric field exceeding the switching threshold V is controlled only by Joule heating by a "thermal runaway." This is regarded as the principal driving force of the switching mechanism. The explanation of threshold switching in terms of thermal processes involves a feedback loop, illustrating how, in materials with a thermally activated conduction process, thermal runaway can occur and cause a sharp drop in the device resistance. The feedback 100p is:

high field -. increased current - increased power dissipation

greater conductivity .- internal temperature rise

The essential link is the sensitivity of the conductivity to temperature. Whether the final result is threshold switching or memory switching, the conditions leading to the electrical instability can usually be formally described by an energy-balance equation [26]:

A(T, E, a) = B(T, a) (14.1) where A is the rate at which energy is gained from the field E at temperature T, and B is the rate at which it is absorbed or dissipated. The parameter a is introduced to denote any other relevant property of the current carriers depending on the particular experimental conditions. The variation of the two sides of this energy- balance equation will be generally as shown in Figure 14.4, in which the abscissa 523

648

Rate of Change of Energy E3

E2 / E

INCREASING FIELD, E

T,E or 0 -

Fe 14.4 Schematic representation of solutions to energy-balance equations.

represents some appropriate parameter such as temperature, energy f, or injected charge Q. The rate of gain of energy from the field will normally be given by (14.2) A = o(T, E)2

or some variant of this. The conductivity a-(T, E) is generally a function of temperature and field. With a sufficiently large conductivity that is temperature-dependent, the material will heat up through Joule heating, and, if thermal processes are dominant, then

I C) O T (14.3) B - V2 = tI---K at

The characteristics of purely thermal threshold switching can be accurately predicted from the solution of (14.2) and (14.3) in a one-dimensional form with conductivity expressed only as a function of temperatures that is, or = a(T). The general solution is shown in Figure 14.4; the abscissa in this case represents temperature and the intersection of B and field E2 determines the point at which instability sets in. It should also be noted that even in the case when the isothermal conductivity is ohmic, the dynamic conductance will not be constant because of 524

649

Joule heating. The most detailed calculations of the thermal, one-dimensional model have been reported by Warren 1261. The general conclusion, summarized more fully at the end of this section, is that such a simple model is not sufficient to explain switching in relatively thin films (e.g., <8 gm). although the evidence shows it is adequate to account for the switching characteristics of thicker films. Kroll and Owen et al. have provided the most detailed mathematical description of threshold switching phenomenon in amorphous semiconductors based on a combined thermal and electrical model (i.e., a = a(T, E)) (29, 301. In Kroll's calculation, the I-V characteristics were obtained from very general mathematical considerations concerning stability and bifurcation of solutions of the nonlinear equations for temperature and field, and he showed that switching occurs by the nucleation and growth of a hot spot approximately in the center of the device interior. The primary results of Kroll's analysis are shown in Figure 14.5. The model predicts the formation of the S-shaped CCNDR associated with high-current filaments (Figure 14.1(a)). The existence of a region of negative differential resistance on the I-V characteristics was determined by radially uniform solutions of the thermal balance equation. It was also shown that the dashed portions of seg-

Voltage Vt

Fg.re 14.5 I-V characteristics: solid curve, stable solutions; dashed curve, unstable solutions; dotted line, load line. From: 1291. 525

'so

ments (b) and (c) are locally unstable against temperature and field perturbations which keep the total current through the device constant, and that branches (a) and (d) are stable with respect to such perturbations. Switching is initiated at some point not too far below turnaround (see point 1 in Figure 14.5) as the result of a macroscopic critical fluctuation. Once the switching is initiated, the device dis- charges through the embryonic channel and switches along the load line to the stable vertical portion of branch (d). Here all the current flow is carried in a hot channel. The possibility of thermally induced negative differential conductivity was suggested and described in other publications [31, 321. Experimental evidence of high filamentary temperatures during threshold switching has also been reported [33]. Several authors have put forward alternative nonthermal electrical models for creating conditions of instability leading to switching. One of the most frequently used electrical models is based on double injection with recombination in which, in the ON state, carriers are injected at both electrodes, giving rise to a high density of carriers in the valence and conduction bands. In this case the energy input is either stored in the dielectric as injected charge q,., or is lost through recombination. An energy-balance equation (14.1) could therefore be written as

j.06 o(T,E)2= _L!! (14.4) Moe, di

where €, is the relative permittivity, 0 is a suitable geometrical factor, and j, is the recombination current driven by some appropriate internal potential 4'a,. In this case the abscissa of Figure 14.4 would represent q.,. A qualitative explanation along the above lines was suggested first by Henisch 127J. Similar but more quantitative models have also been proposed by Lucas 1281 and Mott [34J. The conductive ON state is sustained by double injection, provided the applied voltage exceeds the mobility gap. It is assumed that the tails of localized states (typically present in amorphous semiconductors (351) extend from the conduction and valence bands and overlap somewhere near the center of the energy gap (or mobility gap). This provides a completely compensated set of positively and negatively charged states above and below the Fermi level pinned at or near the center of the gap. These features are inherent in the Cohen-Fritsche-Ovshinsky (CFO) model of the electronic band structure of chalcogenide glasses [361 and are further supported by more recent theoretical interpretations involving the concept of negative-U states 1351. The injected electrons and holes will recombine with and neutralize the positively and negatively charged states, setting up a negative and a positive space charge adjacent to the cathode and anode, respectively. The regions of space charge will limit the current flow in the vicinity of the electrodes, and the field will be redistributed in a way that would be decreasing near the electrodes and increasing 526

651

in the center. As the applied voltage is increased, more charge is injected and the space-charge regions grow until eventually they meet and overlap. The physical situation then rapidly becomes unstable. When the space-charge clouds overlap they neutralize each other, causing the field in the interior to collapse and allowing the same or larger current to flow with a lower applied voltage, causing a negative resistance characteristic. The conductivity in the center increases and the field decreases, while the field near the electrodes increases. Electrons and holes are accelerated rapidly across the neutral region and, because of the increased field, injection of carriers at the electrodes increases. Both effects increase the rate at which space-charge overlap occurs (i.e., there is a positive feedback and hence an unstable situation). The stable state, corresponding to the ON state, is achieved when the space charge has spread right across the structure and the bands are practically flat. Schottky-type barriers are established at the electrodes, but because of the high density of traps and because they are thin (about 1 nm), electrons can easily tunnel through from the Fermi level of the metal into the conduction band of the chalcogenide glass and, similarly, holes from the anode tunnel into the valence band. Because the space charge has been neutralized and the traps are filled, the conductivity in the device is now high (i.e., the drift mobility is no longer limited by trapping). Homma [371 published a systematic and detailed comparison between the threshold switching properties of a chalcogenide material (TeoAs,Ge7Si1g) and a nonchálcogenide alloy (Cd23Ge12As) of nearly equal forbidden gap but higher conductivity. The results showed that two alloys of rather different resistivities have the same threshold voltage and also that symmetrical threshold voltages may be associated with highly asymmetric threshold currents. Furthermore, in both cases. the threshold current is light-sensitive (both materials are photoconductive) but the threshold voltage is not but remains practically unchanged by illumination despite a power increase by a factor of three or more. This indicates that a critical field is involved in the circumstances and determines the onset of switching. These results provide evidence against thermal interpretations and thereby support electronic models. Microwave noise and transient ON-characteristics (TONC) measurements [38] also suggest that the Joule heat is not the main cause of the formation of the current filament. Also, relaxation processes and polarization effects [391 observed at low temperatures show an effect on threshold switching opposite to that expected from the thermal theory. It is found that if, during double-pulse experiments, the first pulse is of insufficient height to cause switching, the magnitude necessary for the second pulse to cause switching is lowered if the pulses are of the same polarity and increased if they are of opposite polarities. It appears that contact, injection, and trapping effects must play some role in explaining certain of the asymmetric effects described above. Therefore, although the electrothermal theory describes well the observed initial switching instabilities, it is also clear that contact, injection, and trapping effects must play some role in explaining certain 527

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of the asymmetric electrode effects and polarization and relaxation phenomena described above. Many other conditions of electrical instability leading to switching have also been investigated. For intrinsic electronic breakdown in the single-electron approximation, for example, the function A in (14.1) is of the form [24)

e2E2 ) A = ei()E2 = ( 14.5) M where g, the mobility, and i the relaxation time, are functions of energy. The electron loses energy to the lattice through phonon interactions, and the value of B (14.1) can be written as

ku B 1. (14.6) = r)(2N. + 1) 1 - ii where r is the mean time between collisions and is a function of energy, and N is the average number of phonons of frequency w. Thus, in Figure 14.4, the abscissa represents energy f, and the field E2 again corresponds to the onset of instability. Electron-eletron collisions may also lead to electrical instabilities 1241. With a high collision rate and a rate of energy exchange due to electron-phonon interactions, the electron energy-distribution function is Maxwellian with a mean temperature Td above the ambient temperature T. or the lattice temperature T. This means that the energy rate of change terms A and B (14.1) can be averaged to A and B, having a similar functional dependence so that the general picture of Figure 14.4 remains valid at least up to energies f. at the maximum of curve B. This collective description has a strong analogy with thermal runaway, and the abscissa corresponds to the electron temperature 7'd In this case, therefore, it is the steady- state electron temperature that rises until, at a critical value, no equilibrium is possible and instability sets in. Near C., however, the electron-phonon energy transfer increases, implying an increase in the internal temperature of the material, with consequential thermal effects. Impact ionization and avalanche breakdown can also produce instabilities [flJ. If the electron density is low, a high-energy electron may collide with an atom instead of another electron and thus ionize it, producing a hole and two low-energy electrons. The two electrons, in turn, will be accelerated to high energies and ionize more atoms; thus the process can cascade to cause an electron avalanche. Summarizing these results, there is no doubt that, besides thermal effects, there are significant electronic effects causing nonohmic conduction and CCNDR in the preswitching region of chalcogenide glass switches. A comparative study of their relative importance has been given by Owen and Robertson [24) and Adler 528

653

[251. The main point of discussion has concerned the extent to which switching can be attributed to thermal or electronic processes. In either case (thermal or electronic), heating may be the initial process, but the critical field E. at which switching occurs may behave differently with different thicknesses of the films. For thin films (< —1 pm) conduction at breakdown is nonohmic. Therefore it is expected in this case that Vs will be independent of the film thickness and will decrease linearly with temperature (34]. The nonohmic field- dependent conductivity implies electronic processes such as double injection or avalanche breakdown, as discussed above. Furthermore, the constant value of the threshold voltage in the presence of illumination [37], the microwave noise and TONC measurements [38], and the polarization effects [391 also provide evidence for electrical mechanisms. Based on these experimental results, there are strong reasons for believing that in chalcogenide thin-film (d 1 pm) threshold-switching devices ("Ovonic switches" (121), thermal considerations do not play a significant role. However, for thick films (8 pm) for which current at breakdown is ohmic, E, should be proportional to the inverse of the thickness and decrease exponentially with increasing temperature [34]. Kolomiets (40] found a dependence on the thickness showing that in thin films (-.8 pm), the breakdown field was independent of the thickness and depended only weakly on temperature, suggesting that the switching mechanism in thin films is not associated with simple thermal breakdown. On the other hand, in thick films (2-- 8 pm), the breakdown field is inversely proportional to the thickness and shows strong temperature dependence, confirming the essentially thermal nature of the breakdown in thicker films. Stocker [41] described a quantitative model for threshold switching in semiconducting films a8 pm thick based on thermal considerations only (i.e., no field dependence of conductivity) and has Shown that the switching behavior of these thicker films can be described adequately by a simple thermal model. The most useful approach seems therefore to be to compare, wherever possible, the functional dependence of parameters such as the threshold voltage and delay time on, for example, temperature, field, and geometry with the predictions of the related models. With several possible mechanisms competing, the dominant process can change drastically as a function of such factors as temperature or the geometry of the specimen.

14.1.4 Mechanisms for Memory Switching

As described in the previous paragraph, the precursor to memory phenomena in thin (-1 pm) film is the onset of nonlinear field-dependent conductivity leading to instabilities and threshold switching. For threshold switching, the material must be capable of carrying a much increased current, either uniformly or locally in a 529

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filament, but reversing spontaneously to the original nonconducting OFF state when the holding voltage is removed. For memory switching. the dielectric material must be capable of changing into a permanent conducting state in some way—an overall or localized change in the atomic or microscopic structure for example—but one that can be reversed to the OFF state by a suitable current (energy) pulse. Obviously the system must also be able to absorb the reversing pulse without destructive breakdown. Alternatively, the memory-switching process may be based on charge storage (i.e., without a modification of the structure). Models for memory switching can therefore be divided into two broad categories: electronic or structural. The first of these is based on the long-term storage of charge, with no major structural modification, to account for the nonvolatile nature of the switch. The most commonly proposed charge storage sites are traps, either in the bulk or at an interface between two dissimilar materials. The necessary characteristic of such traps is that they have a release time comparable to the retention time of the memory, which may range from a few hours to many years. If the charge is stored in the amorphous thin-film sandwich structure, it can affect the conductance in a number of different ways. For example, when stored in the bulk or at interfaces, it can cause band bending, which in turn modifies the conductance. Simmons and Verderber have used an electronic model to explain their observations of memory switching in thin silicon monoxide (SiO) films fitted with gold electrodes [2]. The I-V characteristics of their device (Figure 14.6) show a VCNDR, and the device could be switched between several memory states (A, B, C, D) that persisted for a few weeks. The forming process in these devices is due to gold ion injection into the bulk of the silicon monoxide film, the source of the ions being the positively biased gold electrode. The sample cannot be formed at liquid nitrogen temperatures but forms more readily at elevated temperatures, indicating that ion injection with subsequent ion migration is the origin of the forming process. The current-voltage characteristics and the weak temperature dependence of the conduction process suggest that the main factor determining the current flow in this system is tunneling. For a reasonable current flow in the system, adjacent ionic sites must be positioned within –30A of each other, which means that the injected ion density must be at least of the order of 1019 cm. The authors concluded that in these devices the conduction and memory processes are electronic in nature and that the memory effect is due to charge trapping in the bulk of the silicon monoxide, which in turn modifies the conductance of the metal- silicon monoxide barrier. Hovel and Urgell have also suggested an electrical model to explain their observations of memory switching in epitaxial ZnSe films grown on crystalline Ge [42]. In the OFF state the current flow through the device is limited mainly by the high-resistance ZnSe layer. In the ON state, the barrier at the ZnSe/Ge interface has been narrowed by the ionization of traps at the interface from neutral to positive, thereby permitting easy tunneling through it. The same general type 530

655

VT Voltage (V)

I1ge 144 Schematic diagram of dynamic I-V characteristics illustrating several memory states (A. B. C. D) and the threshold voltage yr. From: 121.

of phenomenon has been observed in ZnSe-GaAs [431. GaP-Ge 1431. AIN-Si and SbSi-Sn02 heterojunctions (451. Switching in chalcogenide glass memory devices is based on a rearrangement of the structure in the ON and OFF states, as seen in Figure 14.7. During initial forming pulses, a filament of material crystallizes in the amorphous structure [14]; this normally happens near the center of the device where the highest temperatures are attained. This modified material has a higher conductivity than the bulk and therefore becomes the preferred current path during subsequent voltage pulses. In the erase operation (i.e., switching from ON to OFF), large voltage pulses with steep trailing edges are applied. These melt the crystalline filament, causing it to solidify in an amorphous form. To write in again, switching from OFF to ON, pulses with more gradual trailing edges are used. These pulses also melt the filament but allow the material to solidify in a crystalline form. Manhart has also proposed a structural model to explain memory switching in silicon monoxide sandwich structures with silver electrodes 1211. In these structures, a forming pulse was again used to reduce the device resistance from its initial value of —1012 to —30011. According to Manhart's model, the forming process gives rise to a temperature increase sufficient to induce silver from the electrodes 531

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lll I - Filament

Write in Erase =I@

flg.n 14.7 Memory-switching process in metal-chalcogenide-metal devices. to migrate into the silicon monoxide film to produce a metallic filament. This explains the subsequent linear I-V characteristics and positive temperature coefficient in the ON state. Two different mechanisms were suggested for the switching. The first assumes that the erase pulse (by, trailing edge 1000 ns) melts the metal filament only at the narrowest point. The metal disperses into the silicon monoxide, breaking the link. To write in again and hence reconstruct the filament, a write pulse (6-8V, trailing edge >0.1 ms) encourages more metal from the electrodes to enter, that is, a long pulse is able to heat the boundaries sufficiently to cause new diffusion from the electrodes. The second model also suggests that the erase pulse melts the metal filament. After the removal of the pulse, rapid cooling leads to the formation of a high-resistance amorphous mixture of metal and silicon monoxide phases. To write in again, this material is melted again but then cooled more gradually. The difference in the freezing points of the metal and the silicon monoxide cause them to separate, thus restoring the filament. Memory switching in thermally grown microcrystalline NiO has been reported by Gibbons and Beadle E71. They too suggest that a metallic filament is formed in their structures, not by the diffusion of metal from the electrodes, but by the collection of nickel atoms at a structural defect in the oxide. This occurs during the forming stage of the device where a filament of material becomes sufficiently hot for this stoichiometric change to take place. The device can have an ON-state resistance of approximately 100(3 and an OFF state resistance of 25 MI). In the ON state a nickel filament is formed, which connects the ohmic contacts. Once the device is switched ON, the device can be very rapidly switched OFF by essentially burning out a small section of the conducting filament. Once the filament is 532

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formed, only the section removed or reoxidized during the OFF state need be replaced. The main body of the filament, which remains more or less intact during the OFF-state transient, provides a large nickel reserve for this replacement. This will obviously limit the lifetime of the device.

14.1.5 Electrical Switching in Amorphous-Silicon Structures

The earliest reports of switching in amorphous silicon (a-Si) were published in 1970 (13, 461 contemporaneously with some of the early literature on switching in chalcogenide glasses. More detailed experiments on the same structures were reported later (9, 471. These investigations studied vacuum-evaporated films of a-Si in the range 0.3-2.0 p.m thick, fitted with titanium electrodes. Similar observations were made on evaporated films of germanium, boron, and boron plus carbon. As threshold devices, these a-Si structures had threshold voltages V& of 5-10V, OFF-state resistances in the range of 1-30 W. and an ON resistance of about 10011. In common with the chalcogenide glasses, there was a delay time i, before switching of 20-50 As or more at room temperature, and the actual switching time was at least several microseconds. Feldman and Charles (9, 471 did not, however, report any initial forming process, unlike the situation in chalcogenide glass switches (Section 14.1.1). There was also some tentative indication of memory switching but this apparently was not substantiated. They interpreted their results in terms of a simple and qualitative electrothermal model involving the formation of a conducting filament. The work of Feldman and his colleagues (13, 461, which originated in the early 1970s, seems to be the only investigation of switching in a-Si until the rather later studies of Dey and Fong [48, 491. These authors reported results very similar to those of Feldman. They studied thin films of a-Si in the range 0.3-1.5 p.m thick, deposited by electron-beam heating in a vacuum evaporator. Titanium contacts were again used, either in the form of evaporated films or as probes. Dey and Fong reported only threshold switching, with I-V characteristics similar to those in Figure 14.3; they did not mention any evidence for memory behavior. In contrast to Feldman, however, Dey and Fong did observe forming effects, that is, the initial threshold voltage was relatively large but decreased to a more or less constant value after a number of switching cycles. In Dey and Fong's devices the threshold voltage varied systematically from about 6V for the thinner films (-0.3 pm) to about 9V for the thicker films (-1.2 pm). The delay time before switching was in the range 2-60 As, varying in a systematic way with pulse height, pulse duration, and repetition rate, again in a manner very similar to threshold switching in chalcogenide glasses. Dey and Fong also interpreted their results in terms of a simple one-dimensional electrothermal model, although developed a little more quantitatively than that by Feldman and Charles [9, 471. It should be noted that both teams (Feldman and Charles, and Dey and Fong) used a-Si films deposited by vacuum evaporation, which probably accounts for the relatively low OFF-state 533

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resistances (-100 kU) they both found. It is now well established that vacuum- evaporated a-Si is a very different material from the hydrogenated form of a-Si obtained, for example, by glow-discharge deposition of silane 1501. Three papers, concerned specifically with switching in amorphous silicon, by Gabriel and Adler [51J. den Boer [521, and Owen 1531 appeared almost concurrently early in 1982, each reporting very different effects observed in different amorphous silicon structures. Our own work [54-581, including recent results, will be described in detail in the following sections. Den Boerstudied n + -i-n structures of a-Si: H prepared by the glow discharge decomposition of SiH4 (1 stands for near-intrinsic, or undoped, material). The n + layers were 0.05 gm thick and were prepared by adding 1% PH 3 to the SiH4 gas flow; the i layer in different devices ranged in thickness from 2.5 to 5 JLm. Den Boer found that the n + ..i. + devices functioned as threshold switching devices with nonpolar characteristics similar to those in Figure 14.2(a). For the first switching cycle, the threshold voltage was in the range 40-10y, but for all subsequent operations it was only 10-35V, depending on the i layer thickness. (As the i layer thickness increased, the threshold voltage also increased.) The OFF-state resistance of the n f-i-n devices was about I MY], and the ON-state resistance was about 1 kfl. There was an observable delay time before switching, ranging from a few microseconds when the applied voltage was about 8V greater than V k to about a millisecond for voltages within 1V of Vm. The n + -i-n * switches could be cycled through at least 10 stable switching operations. Den Boer also compared structures with chromium or a combination of chromium and n contacts (i.e., Cr-n 4 -i-Cr and Cr-i-Cr). The Cr-n k-i-Cr contacts had rectifying characteristics, while the Cr-i-Cr contacts switched but were very unstable. Gabriel and Adler [511 prepared their films by sputtering from a polycrystalline silicon target in an argon-hydrogen plasma. In all cases their devices were notionally homogeneous thin films of intrinsic a-Si:H with molybdenum contacts. The samples were fabricated under a wide range of deposition conditions in two sputtering systems, and although results from some of the devices were rendered rather doubtful because of contamination problems, in no case did Gabriel and Adler observe any evidence of reversible switching. They concluded that, in contrast to the chalcogenide glasses, amorphous silicon does not have the electronic and structural properties required for reversible switching.

14.2 RECENT DEVELOPMENTS ON AMORPHOUS-SILICON SWITCHES 141.1 Digital Switching In a-Sl:Hp-n-I Devices

We now turn to the work on electrical switching carried out in the authors' laboratories. Although a number of different a-Si:H multilayer structures have been investigated, all the earlier results discussed in the following refer to metal- 534

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p-n-i-metal devices deposited in that sequence by the glow-discharge technique with gas-phase doping. In the early stages of the work, the a-Si:H layers were deposited onto a stainless-steel substrate. After completion of the a-Si:H deposition, a series of gold (Au), aluminum (Al), or nichrome (NiCr) dots, up to approximately 1 mm in diameter, was evaporated onto the surface of the samples. The top contact was completed by a probe or by a thin wire attached to the metal dots with conducting silver paste. In order to facilitate the investigation of device properties on a more reproducible basis, a 'pore" device structure was designed, with device areas down to a few jum in diameter. A set of photomasks was produced that allowed modern photolithographic technique to be combined with the normal a-Si:H deposition process. Figure 14.8(a) shows a cross section of one of the pore structures produced by photolithographic techniques, and Figure 14.8(b) represents a plan view of the same structure. The a-Si:H films were deposited on Coming glass substrates previously patterned with chromium bottom contacts. The a-Si:H was prepared in a layer sequence of p , n, I by the glow-discharge decomposition of silane using gas- phase doping. The p-n-i films were then patterned, and an insulating layer was used to define an active device area of 106 cm2. The metal used for the top contact was normally chromium but a number of other metals were also used, and their influence on the memory operation was thoroughly investigated.

Chromium Vanadium -

Insulator 1 ooOA a-Si:H

anadki Pore -+ flop o4act 1

Figure 14.8 Structure of metal-amorphous silicon-metal switch device. 535

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14.2.1.1 Static Current- Voltage Characteristics

Typical I-V characteristics for a freshly prepared (unswitched) device are illustrated in Figure 14.9(a) in both the forward and reverse directions; the forward direction is defined so that the substrate (and hence the p * region) is positively biased. It must be emphasized here that these measurements were taken "by hand," point by point, in a manner that required a few seconds for each measurement. (The significance of this remark will become apparent in the next sections.) In the forward direction there is a region of ohmic behavior over a limited voltage range followed by an abrupt change to a markedly nonohmic region until, at the point indicated by the arrow, the device is unstable and it becomes impossible to continue with point-by-point measurement. The change from ohmic to nonohmic behavior is more

(a) 1 01A)

110 4 J 5 160•CJ Jeoc Jx.rc

10 20 V(V)

-5

0 10 20 30 V(V)

Figure 14. (a) I-V characteristics of unformed (unswitchcd) amorpbous silicon device; (b) conductivity versus voltage plot. 536

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clearly apparent in the effective-conductivity-versus-applied-voltage plot of Figure 14.9(b). As the temperature increases, the onset of nonohmic behavior moves to lower voltages. In the reverse direction, corresponding to a negative potential applied to the p side, there is an initial ohmic region that is symmetrical for positive and negative voltages. However, the change to nonohmic behavior is much more gradual in the reverse direction, leading to eventual breakdown of the device. As noted above, during point-by-point measurements under forward bias, the a-Si:H p-n-i device tends to become unstable when the applied bias is about 24V at room temperature. At higher temperatures, the instability, indicated by the arrows in Figure 14.9(a), occurs at lower voltages. When we attempt to increase the voltage still further, the device switches into a low-resistance ON state. Typical I-V characteristics for both polanties in the ON state are shown in Figure 14.10. The I-V curve is ohmic; it extrapolates through the origin (i.e., the ON state is a permanent memory state); and it is slightly asymmetrical. Note that the current is now measured in milliamperes and the voltages across the device are small. On increasing the voltage in the forward direction, we find that the ON-state current continues to increase apparently indefinitely, subject only to any current-limiting resistor, and the device is eventually destroyed, presumably by Joule heating. In the reverse direction, however, another instability is observed, and at about - lv (typically) the device switches back into a high-resistance OFF state, as shown in Figure 14.11. The OFF-to-ON transition may now be repeated by biasing in the

I (MA) FWD+ -I--I- REV • S S

Ok 0 0.2 0.4 0.0 Q.5 1.0 V (Volt)

Figure 14.10 1-V characteristics of memory ON states. 537

662

FIgute 14.11 I-V characteristics of memory switching device for both polarities. forward direction, but on the second and all subsequent switching operations the forward threshold voltage V 1 occurs at a much lower voltage than the first operation (e.g.. at —5V compared with the 25V observed under the conditions of the measurements shown in Figure 14.9). The first OFF-to-ON transition, occurring at a relatively high voltage, seems unique and by analogy with the terminology in chalcogenide glasses, it is referred to as "forming." The formed p-n-i device may be cycled through ON and OFF states by a biasing in forward and reverse directions with critical points at V TV and, in the reverse direction, Va,, as illustrated in Figure 14.11. It is important to emphasize that these devices exhibit not only nonvolatile but polarity-dependent memory switching. On occasions the device appears to go through a number of intermediate states during the OFF to ON transition, and this is indicated in the figure. In addition, there is often an observable and appreciable region of negative resistance in the reverse-biased OFF-state characteristics of the formed device. A number of other dc experiments have also been carried out on formed devices in an attempt to provide additional information, primarily about the nature of the ON state. Figure 14.12(a) compares the area dependence of the OFF- and ON-state resistances for samples of different area (i.e., different pore sizes with diameters from 5 to 300 pm) from a single p4 -n-i deposition run [54J. These results were obtained on specimens that had previously been switched many times. Within

538

663

1012 10

G(Ci') ON

10-3

10' 10 1111iii i (a) - (b)

10' 10-6

iO 10 2.5 3.0 3.5 4.0 45 A(cm 2) 1000ff

FIgure 14.12 (a) Area dependence of ON and OFF state resistances; (b) temperature dependence of ON and OFF state conductances.

the experimental scatter, ROFF clearly scales with the reciprocal of the area A, demonstrating that in the OFF state the current flows throughout the whole area of the specimen. In complete contrast, the values of the ON-state resistances RON for the same specimens show no area dependence at all. This result can be understood only if the ON state has its origin in a highly conducting filament, less or equal to a few jum in diameter, that extends through at least part of the specimen thickness. The temperature dependence of both the ON- and OFF-state conductance has also been measured over the temperature range from about 230K to 400K. Results for a typicalp' -n-i specimen are shown in Figure 14.12(b). The OFF-state conductance GOFF varied slowly with temperature, increasing by less than a factor of three between 230K and 360K. The ON-state conductance was even less temperature-dependent, increasing by only 10% over this temperature range. The insensitivity of these device parameters to temperature is also observed in other properties. For example, the magnitudes of the voltages required to switch the device ON and OFF, measured under pulsed conditions, both increase by only a factor of 2.5 as the temperature is decreased from 400K to 200K. Clearly, the general temperature insensitivity of the switching must also be a feature of any theoretical model. 539

664

Transverse magnetoresistance pJp measurements of the ON state of a number of specimens have also been carried out at room temperature up to magnetic fields of B = 0.5 T [551. Within the experimental error. 1p/p was proportional to the square of the magnetic field strength B and was found to be positive. The values of 4plp(B2) ranged from 0.5 to 2.0 x 10 4T . Experiments on phosphorus- doped a-Si:H after thermal crystallization also gave a positive magnetoresistance with the same 4p/p(B) dependence. Unfortunately, no results have been reported; on homogeneous undoped glow-discharge a-Si films; it is therefore difficult to draw any definite conclusions about the amorphous or crystalline nature of the filament from the present results. However, it is probably correct to associate the B2 dependence of the memory ON state with a longer mean free path than is normal in amorphous solids. By conventional crystalline theory (581. the magnitude of the measured 1pfp(B2) would lead to an effective mobility —100 cm 2Vs' supporting of this suggestion. The importance of this measurement will be further discussed in Section 14.2.2.1 in connection with the very recent observation of electron quantization phenomena observed at low temperatures. Direct evidence for the existence of a filamentary ON state was obtained from thermal imaging [59J. The a-Si:H p-n-i device was covered with a thin layer of thermochromic liquid crystal. By passing current through the device in the ON state, it is possible to observe the current path from the changes produced by Joule heating in the reflected color of the liquid crystal. The resulting features for 20-tm pore diameter structures were viewed through a high-powered optical microscope. The change in the liquid-crystal appearance produced by the filament in the ON state could be seen clearly as a small circular "hot spot" approximately in the center of the pore. The visual observation of the current filament has also enabled us to establish that switching a device OFF and ON again produces the current filament in the same place and this implies that the switching processes are not destructive. In these experiments the specimens were covered by a thin layer of a liquid crystal. 4-cyano-4-alkylbiphenyl, which undergoes a nematic-liquid phase transition at 35.3°C. The phase boundary may be observed quite easily in cross- polarized light, and the transition was found to be fast and without hysteresis. If the sample temperature is fixed using a thermostatically controlled stage, the difference in temperature between a region of local heating (at temperature T) and the surrounding film (at T,) may be determined. The results indicate that no observable temperature rise occurred as a result of applying electrical power to the pore in either the unformed state just prior to forming or in the formed OFF state just prior to switching. However, as described above, in the ON state, local heating (which results from applying continuous power) could be clearly seen. The effect of changing the rms power applied to a 50-g.tm-diameter pore was studied using a continuous train of 300-ns pulses. The stage was maintained at 21°C; thus the phase boundary represented the locus of points (35.3° - 21° = 14.3°C) above the film 540

665

temperature. These isotherms were circular and in the particular case studied were symmetric about the center of the pore. A plot of the phase boundary diameter d, versus rms power PRMS is shown in Figure 14.13(a). Although there is considerable scatter, it can be seen that the relationship between d, and PRMS is substantially linear for d1 > 2 Am. Below this the accuracy of the measurements is limited by the resolution of the microscope used; the onset of observable effects occurs at PRMS = 2 mW. A linear dependence of d1 on PRMS is obtained as a solution of the heat conductivity equation for an idealized system of this kind, in which the source of local heating is assumed to be much smailer than d1 . Thus these data indicate that the diameter of the ON-state filament d, must be less than about 2 &m.

d,(i&rfl)

(a) 10

Pp.6 (MW)

AT - IC Pwlcn Isn. - subetal. beT. P ATS.25.5 MW 614-243

613.17.1

612. 153 at 611-10.3

d (ius boiiidwy Jaahl.l)

Figure 14.13 (a) Liquid crystal phase boundary diameter versus RMS power; (b) phase boundary diameter versus RMS power at various values of excess temperature AT. 666

The steady-state thermal properties of the 20-Mm pore structures with different top electrodes (Al, Cr, Ti) have also been studied, and the results are reprsented in Figure 14.13(b), where rms power is plotted versus the phase boundary diameter for different local temperature increases. It is found that the gradients GA and intercepts P0., scale linearly with the local temperature rise AT, with the constants of proportionality MG and M 0 depending on the top electrode metal (and possibly its thickness, although this has yet to be established). Therefore, the following relationships can be used to characterize the steady state thermal properties:

G = MG4T (14.7)

P0 = M,4T (14.8)

and therefore

P = MT + MGITd (14.9)

The values of MG and M 0 are summarized in Table 14.1. In the case of Al and Cr top electrodes, several different samples have been investigated, with the values obtained being similar to those in the table.

Table 14.1 Summary of Thermal Properties

M,JmWK') Top electrode MdWm'K') At 3 0.2 Cr 6 0.02 Ti 7 0.007

The physical significance of M0 is that it represents the surface thermal conductivity of the pore, and thus it is not surprising that the values obtained experimentally lie between the bulk values for glass (1 Wm - 'K - ') and metal (100 Wm - 'K -'). The significance of M,, is less clear. In the ideal case of a point heat source located on one thermally insulated surface of an infinite plate, the other surface being isothermal, Mp. should be zero. Thus it may be that the nonzero values in some way reflect the extent of the heat source. If this is the case, it would appear that the filaments in pores with Cr and Ti top electrodes are smaller than in those with Al electrodes. 542

667

14.2.1.2 Current Instabilities in Unformed State

Unusual current instability phenomena have been observed in a-Si:H p-n-i structures with Al top electrodes in which the i layer is either thin (bOA) or lightly n-doped (1-5 'tppm) [561. The principal features associated with the current instability are shown in Figure 14.14. A voltage pulse V. greater than a minimum threshold V (specified later) is applied so the p'-layer is positively biased. A displacement current occurs, which decays to a steady current I,. Some time td after the voltage pulse is first applied, the current rises abruptly. After reaching a maximum value l.. the current decays. A second pulse may be applied at time :, after the first pulse, which may or may not give rise to a second instability, depending on the values of V and t,. The dependence of the onset time id on pulse height V

I Imax —

is Ll o td r: Tf time flgwe 1414 Current instability in metal-amorphous silicon structure.

for three different temperatures is shown in Figure 14.15. The experimental data are summarized by the following empirical relation:

- VJ] Id = td. + toexp[ (14.10) V0 where V0 — 0.6-0.8V, V — 10-15V, to 10-100 As, and t — 100 ns. We note that V. is temperature independent over the range studied, but some or all of the other parameters clearly show a temperature dependence. 543

668

io 5

— 10.6

10 -1 12 14 10 10 Voltage (V) flgue 14.15 Onset time versus pulse height (current instability) at different temperatures.

The above relation applies only to the region where V is somewhat larger than Vi,. The detailed relationship between td and Vat V - V is difficult to establish; there is a tendency towards erratic behavior, but no current instabilities have yet been recorded with t4> 100 'as. It appears that t4 either increases almost discon- tinuously at V = V. or the effect simply does not occur under these conditions. As V is increased, several volts above V, t - t) but further increase in V causes an irreversible change in the device characteristics, culminating in a permanent low-resistance (1O - 1O'fl) state when the voltage pulse is removed. The observed minimum delay time td, may correspond to the transit time of an injected pulse of holes through the n-region. Taking typical values of td, n-layer thickness, and maximum applied voltage (0.1 ps, 0.1-0.3 m, and 10-20V, respectively) and assuming that a large proportion of the applied bias under these conditions appears across the n-layer, the hole-mobility values can be estimated to he in the range 10 to iO cm2Vs, which is not unreasonable. Measurements of the dependence of l., at constant t1 on device areas over the range 10-10' cm 2 have failed to reveal a direct proportionality; ! ranges over an order of magnitude in the various samples, but this seems to be primarily a random variation. As the smallest device examined in this study was about 10 JLM in diameter, this suggests that the current during the instability is transported through a region of these dimensions or less. This is an important observation, as it has been established 544

669

(Figure 14.13(a)) that conduction in the memory-ON state is localized within a filament of l-gm diameter or less. Thus it seems that the localized conduction occurring during the current instability may signify the formation of an incipient filament and is therefore a precursor to the memory-forming process. Additional indirect evidence supporting this view is that the forming delay time versus applied voltage relationship can be described by an expression similar to that given in (14.10). Certain features associated with the current instability are found to occur in the analogous crystalline-silicon mewi/i-n-p 1 Imetal (MISS) devices [101. In particular, the relationship between tj and V is similar. This suggests that the principles on which the theory of operation of the crystalline device is based may be applicable here. Although such theories differ somewhat in detail, the central concept is that sufficient holes, injected from the forward-biased p-n junction, accumulate to form an inversion region at the n-i interface. Buxo [591 demonstrated that the theoretical expression for the ta-versus-V relationship, based on the establishment of an inversion layer, is in good agreement with experiment. However, the behavior of the amorphous device differs in two important aspects. First, the current decays after reaching a maximum value, even when the voltage is maintained. The crystalline MISS device remains in its high-conductivity state provided a "holding" voltage is present. Second, there exists a voltage-dependent recovery time 1,, as described earlier and shown in Figure 14.14. This effect has no analog in the crystalline MISS device. The current decay may occur as a result of electron-hole recombination close to the n-i interface, which will increase the potential barrier to electrons tunneling from the metal into the conduction band in the n-layer. Ibis negative feedback will decrease the tunneling contribution to the total current; however, once the initial conditions are reestablished, the current should again rise, and this is not observed experimentally on a time scale of ps. The recovery effect may be the consequence of some longer-term structural change caused by recombination, or by Joule heating, as the local power density will be quite high.

14.2.1.3 Forming

The forming process does not occur instantaneously when a voltage step or pulse is applied to the device. Initially there is a delay time r0 during which the device current remains essentially constant at the OFF-state value appropriate to the voltage across the device. Only after this delay does the current begin to increase, and it then rises almost instantaneously to its ON-state value. The forming delay time is an extremely sensitive function of the applied forming voltage V,, and typical data, obtained at three temperatures, are given in Figure 14.16. The forming delay time varies over nearly 10 orders of magnitude, from a few hundred seconds at low forming voltages to about 10 us at high V,. In particular, at a temperature- dependent critical forming voltage V, there occurs a virtually discontinuous change 545

670

-2 F 1-4

[I [I I______C• Voltage (volts)

FIwe 14.16 Forming delay time versus voltage height.

in i1,. The critical voltages V indicated in Figure 14.16 are approximately the same as the voltages at the points of instability marked by the arrows in Figure 14.9(a); V, also corresponds to the forming voltage obtained in experiment with a curve tracer operated in ac mode at a frequency of 100 Hz. It can also be seen in Figure 14.16 that above and below V the delay time tends to a value that seems to be approximately independent of both voltage and temperature; for the particular results illustrated, m is in the range 102_1()3 s for V < V, and lies between 10 and l00ns for V> V. The results plotted in Figure 14.16 for V < V, correspond, of course, to voltages less than the point of instability indicated in Figure 14.9(a). There does appear to be a lower limit to the forming voltage, however, and present results indicate that the limiting voltage coincides with the bias at which the I-V characteristics change from their ohmic to nonohmic behavior (Figure 14.9(b)). Several experiments have shown that virgin devices fail to switch (form), even if held for many hours at a forward bias only slightly below the nonobmic region. In other words, forming occurs at any forward bias within the nonohmic region of the I-V 546

671

characteristics, but at voltages below the point of instability, To is comparatively long. It must also be reemphasized that the device current remains constant at its preformed magnitude during the delay time, even when r, is 100 s or more. Experiments have also been carried out to determine the effects of p'-n-i device geometry on the forming voltage V,. It was found that V, increases linearly with the thickness d. of the n-layer [54]. The charge Q = f Id:, which flows through or into the device during the forming delay time, has also been determined for V, > V. In this range the ratio (Q/dR) is approximately independent of V, and d. for n-layer thicknesses between 0.2 and 0.8 aim. This could mean that forming occurs when a critical volume charge has accumulated in the n-region. The current 1-V characteristics in the high-field preswitching region are plotted as logo versus E"2 in Figure 14.17, showing that the logarithm of conductivity prior to forming is proportional to the square root of the applied electrical field. There are at least two conduction mechanisms that can give rise to nonohmic behavior of this kmd: 1. The Poole-Frenkel effect, in which the potential barrier for the thermal excitation of trapped electrons into the conduction band is lowered by the applied external field; L The-Richardson-Schottky effect, which is associated with the lowering of the effective work function (or barrier) for charge-carrier emission from a metal electrode when an electric field is applied: The lowering is due to combined effects of the field and the image force. Assuming that the conductivity is bulk limited, the high frequency dielectric constant obtained by fitting the experimental data to the Poole-Frenkel model is 48.7, whereas a value of 12.7 is obtained using the Richardson-Schottky model. The dielectric constant for crystalline silicon is about 12, and this value does not change very much with frequency. The dielectric constant for amorphous silicon is probably similar. It is more likely therefore that the current flow in the high- field preswitching region (just prior to forming) is determined by the Richardson- Schottky effect at the metal/i-layer contact. This conclusion is further supported by the observation that the forming voltages are different in devices with different metal electrodes but otherwise identical thicknesses, indicating the importance of the Schottky barrier in the structure. However, it should be kept in mind that the values of E in the above analysis presuppose a uniform field whose magnitude is proportional to applied bias. It was shown in Section 14.1.1 that, under certain circumstances, conduction in p-n-i samples in the high-bias regime is spatially nonuniform, which suggests that the assumptions made concerning the field within the conducting region may become less valid as the bias is Increased. The rapid rise in conductivity under these conditions, as shown in Figure 14.17, may correspond to the onset of electron tunneling from the metal through the i-layer under a very high field. 547

672

Log (fl cm)

A I £ I - 10 A 1 0 I V

11 id 6 7 E½ (V/cm)½

Hgwe 14.17 Log conductivity versus square root of electric field.

14.2.1.4 Dynamic Switching Behavior The principal features of the pulsed operation of formed a-Si:H p + -n-i devices have been described by Owen [53) and LeComber [54). A representative diagram from an oscilloscope trace of the OFF - ON (WRITE) and the ON -, OFF (ERASE) transitions on applying a voltage pulse is shown in Figure 14.18. The oscillations on these traces are caused by ringing effects in the rather poorly matched electrical setup. The main points to note are: When biased with a pulse in the forward direction, the device switches from OFF to ON (WRITE), provided the pulse height exceeds the static threshold voltage Vm1, as defined in Figure 14.11. There is a delay time in the WRITE operation (Figure 14.18(a)) which is a strong function of the WRITE pulse magnitude as shown in Figure 14.19. These delay times are significantly faster than the delay times reported for chalcogenide switches (see for example [12, 14)). lithe results in Figure 14.19 V. are expressed in the form '1 = t. exp(—V/Vo) then to = 335 ns and = 4.5V. Similar results have been obtained for all the specimens investigated although the V0 values ranged from about 0.5 to 13V. 548

673

(a) (b)

V r

I(MA) dw

0 50 100 150 200 0 50 100 150 200 TIME (nsec) TIME (nsec)

Figure 14.18 (a) WRITE and (b) ERASE switching transient waveforms.

+ . I

60 • +taI +s\I * * 40 • tdw (ns) • 25 Pm - • &m 20 * 125 I&M + 2OOLm V m • +

I In I - 0 5 10 15 Vf (Volts)

Figure 14.19 WRITE delay time versus voltage height. 549

674

Provided the pulse is long enough, the ON state is permanent and the pulse duration required for switching to a memory state increases as the pulse.

height decreases toward V 1,,. In typical cases a permanent ON state is obtained with pulse durations of a few tens of nanoseconds and magnitude

—5V in excess of Va,. Similarly, on biasing in the reverse direction with a pulse of height >Vm the device switches from ON -. OFF (ERASE) and again there is a delay time of the order of nanoseconds in the response (Figure 14.18(b)). Both the OFF and ON states appear to be truly permanent. No detectable changes have been observed in devices stored at room temperature, either in the OFF or ON state, for a number of years.

14.2.2 Analog Memory Effects in a-SI:H Metal-p -Metal Structures

More recent experimental results [57,601 have demonstrated a new metal-p-metal amorphous silicon device which, rather than exhibiting a two-state digital operation, has a continuum of stable States that are nonvolatile and fully programmable by single 10-ns voltage pulses. It has also been suggested that the new analog memory devices can be used as nonvolatile and reprogrammable memory elements in analog neural networks (61). In this section we present a summary of the new results obtained on nonvolatile analog switching effects in amorphous silicon metal-p - metal devices and discuss the possible physical mechanisms responsible for the phenomena. The samples used for this work were amorphous silicon Cr-p-V thin- film structures. The p-layer was prepared by RF glow-discharge decomposition of SiH4 containing 10 vppm of B2H6. Films of I000A thickness were deposited onto Corning glass substrates previously patterned with chromium bottom contacts. The p' amorphous silicon was then patterned, and an insulating layer was used to define an active device area of 10 -"cm'. The metal used for the top contact was normally vanadium. However, a number of different metals were also used, and their influence on the memory operation will also be described. In accordance with our previous results (53), memory devices prepared in this way require an initial forming process. This means that the resistance of the as-deposited (unformed) device has to be lowered from R-10'17 to R-10 3-1O'fl (i.e., the typical value of an ON state). The forming can be achieved by biasing the sample with a single voltage pulse (duration 300 ns, magnitude —12V) with positive polarity applied to the top V contact. The metal-p-metal memory structures exhibit a forming step that is different from the previously investigated metal- p-n-i-metal structures [55]. The differences are demonstrated in Figure 14.0. In the case of metal-p-n-i-metal structures, the resistance suddenly drops from - 10'°12 (virgin state) to - iOfl after the critical voltage (forming voltage V,) has been applied. No change in the virgin resistance occurs when the sample is biased 550

'is

(a) (b) 20 20 V(V) masasW'Q VF 10 V!10 /C

l(mA) 3 2

0 0 100 200 200 0 200 4W Goo Wo TIME (nsec) TIME (nuic)

Figure 14.30 (a) Waveforms of a single forming pulse applied to amorphous silicon Cr-p'-n-i Al structure showing hard forming; (b) waveforms of single pulses of increasing magnitude applied to amorphous silicon Cr-p'-V structure showing soft forming (V and I represent voltage across device and corresponding device current, respectively). with voltages less than V,. This type of forming is termed hard forming. In contrast to this, the resistance of the unformed metal-p -metal structures can be lowered gradually by applying voltage levels with progressively increasing magnitudes. In this case, no sudden change of the current or voltage signal can be detected when the sample is biased with a voltage pulse, as seen in Figure 14.20(b). This process is called soft forming. Figure 14.21 shows the device resistance as a function of the soft-forming voltage (pulse. duration = 300 ns). On reaching a critical voltage (-14V in Figure 14.21) the device resistance suddenly drops from _lObfl to -10'- 1O'fl. This is the memory-ON state of the device. Once the memory device had reached its first (nonvolatile) ON state, all subsequent switching operations were performed with 10-100-ns pulses of 1-5V. An example of the analog switching effect is shown in Figure 14.22(a), where the sample resistance is plotted as a function of applied alternating WRiTE and ERASE pulses (100 as pulse duration each). It is important to emphasize the polarity dependence of the analog memory behavior. In the case of the WRITE pulses, positive polarity is applied to the Cr track (bottom contact), whereas ERASE pulses have opposite polarity. The sample was first switched to an ON state (RON = 2 x 10f2) and then a series of alternating WRITE and ERASE pulses were applied. The WRITE pulses were kept at a constant magnitude of 3.4V but the ERASE pulses were incremented by 0.05V steps from 1.2 to 3.4V after each WRITE pulse. It can be seen from Figure 14.22(a) that the sample resistance 551

676

101

00 .00 • 0 0 0 R(Q) 0

108 S

0 I 0

0 0

. 10€ • I A 6 S iU i 49 i.e Forming Voltage (V)

FIgure 14.21. Resistance of an amorphous silicon Cr.p-V memory structure as a function of forming vokage-

changes in an analog manner as the magnitude of the ERASE pulse increases, that is a function of the magnitude of the is, the difference between RON and R0,, ERASE pulses. A voltage range of AV (ERASE) = 1.6V resulted in a change in resistance from R-2 x iO1i to R-6 x 105fl. The shaded-area in Figure 14.22(a) indicates the reproducibility of the analogue memory switching, i.e., the scattering in the resistance during repeated experiments (data from 100 cycles are shown). Figure 14.22(b) shows another case where the ERASE pulses were maintained at a constant value of V - 3.4V, but the WRITE pulses were incremented from 1.2V to 3.4V in 0.05V steps. The value of the OFF-state resistance remained constant at-6 x IM that is,'it changed back to this constant level from every ON state, while the ON-state resistance decreased through a continuum of intermediate states shaded area in Figure 14.22(b) over a similar &V to the ERASE operation. The again represents the reproducibilty of the analog switching for 100 complete cycles. It is emphasized that the device will switch between any two resistance states within the range from about 1 W to 1 Mfl by selecting the correct polarity and magnitude of the WRITE and ERASE pulses. For all devices with a vanadium top contact, the values of 4V range from 1.5 to 2.OV for both the WRITE and ERASE operations. 552

677

le ERASE R (0) .. I • AvI.5van (a) •• - 10' WRITE

10 1 - 2 Era" Voflags M

10• ERASE R(Q) I', (b) le WRITE

lol l 2 3 W,Its Voltags (V)

l0• .. R(0) WRITE . - (C) — 40W O.2V

- 10 2 Voltags (V)

Igvre 14fl (a) Memory resistance ma function of ERASE voltage in aCr.f .V structure; (b) memory

g5$j$taflce as a function of WRITE voltage in a Cr.p-V structure; (c) memory resistance as a function of WRITE and ERASE voltages in a Cr-p*Cr structure.

We have repeated the above experiments on metal-p -metal devices with Cr as top metal and observed similar polarity dependent changes in the memory state resistance. However, in the WRITE and ERASE experiments, intermediate states were found to exist over only a narrow- AV of about 0.2V, as shown in Figure 14.22(c). Therefore, these devices are considered as digital devices. It is important to emphasize that both the analog (Figures 14.22(a) and (b)) and digital (Figure 14.22(c)) memory switching effects are nonvolatile. Devices set to ON or OFF states have been monitored for more than two years without any significant change 553

678

their resistance. Also, operation at temperatures up to 160°C shows little change in the threshold voltages or device stability. However, it is also found that devices with certain top metal contacts, such as Mo and Pd, show a volatile memory- switching effect. This is illustrated in Figure 14.23, where the current signal through the device is continuously monitored at a low voltage level (0.5V), which is below the voltage level of the programming pulses. The current signal decays rapidly after the end of the programming pulse, indicating that the memory state is volatile. The role of the top metal contact, therefore, has been thoroughly investigated by fabricating devices with a range of different top metals but with otherwsie identical physical parameters (i.e., - 1000A thickness of p4 -layer, Cr bottom electrode). With the analog switching voltage window AV as a guide, its value is found significantly dependent on the top metallization contact. This is illustrated in Table 14.2.

V11 lOnsWRlTEpulse 50$Js -

0.5 volt read level iOns ERASE pulse

50 100 150 TIME (psec)

Fipre 14.33 Volatile meniosy effect in a Cr-p-MO structure.

It can also be seen that the definition of analog (4V a 1V) or digital (UV) memory switching is somewhat arbitrary, because there is no sharp boundary between the two types of operation, that is, they are almost certainly associated with the same underlying physical phenomena. However, in the cases of Mo and Pd top contacts, a new type of volatile switching effect is observed and in the cases 554

679

Table 14.2 Effect of Top Metallization on Switching Behavior

Mew.! AV(V) Switching Characreriw.cs Ag, Al <0.1 Digital. nonvolatile Cr 0.2 Digital, nonvolatile Mn, Fe —0.5 Digital. nonvolatile

Ti - Unstable switching

Au, Cu - No switching W —1.0 Analog, nonvolatile V 1.8 Analog, nonvolatile Ni, Co 2.0 Analog, nonvolatile Mo. Pd 2.0 Analog, volatile

of Ti, Au, and Cu, no reproducible switching effects can be observed. These results suggest that the top metal contact plays a crucial role in determining the type of memory-switching phenomena observed in these devices. Here we will concentrate on the p 4 memory devices with vanadium top contact, because these show typical nonvolatile analog memory switching. The I-V characteristics of analog memory-resistance states were investigated systematically, both at room temperature and lower temperatures, and the following results were obtained. It was found that all the room temperature current- voltage characteristics show a linear-plus-power-law behavior in the various analog memory resistance states as shown in Figure 14.24. The i-V curves can be described by a simple nonlinear relationship:

I= * C'V + C2V (14.11) where C and C2 are constants and the exponent n increases with the low bias (linear region) resistance according to the relationship n = A + B logR. The observed power-law behavior could indicate the possibility of space-charge limited conduction at higher bias, although investigation of the thickness dependence has shown this to be unlikely. In accordance with the pulsed analog memory switching results, a continuous transition of states can be found between the ON (-10'S)) and OFF (1(6Ii) states. The terms ON and OFF seem to be somewhat arbitrary, therefore, and are used in this chapter only for practical reasons. If the curves in Figure 14.24 are extrapolated above IV, they meet in the region of 3 to 4V (i.e., at typical programming levels—see Figures 14.22(a)-14.22(b)). It was also found that the exponent n does not depend on the thickness or the diameter of the active device but it is primarily determined by the low bias (or linear region) resistance of the analog memory state. This indicates that the nature of the electrical conduction is quite similar in all memory states. However, repeatedly switching the device into the same resistance state need not always result in an identical value 555

680

1(A)

106

10 8

Voltage(V)

Fe 14.24 I-V characteristics of analog memory states plotted on log-log scale; characteristics are symmetrical about origin. of the exponent. This appears to indicate that the "same" resistance state can be achieved through different conduction paths within the same device. This is consistent with the suggestion that the conduction path (the filament) might have an inhomogeneous structure consisting of a small-scale dispersion of metallic-like particles embedded in an insulating matrix; this structure may provide a variety of conduction paths. It is possible to extend the range of the I-V characteristics up to the critical field where switching occurs using very short, single voltage pulses of varying polarities and pulse heights. Figure 14.25 shows the room-temperature "pulsed" current-voltage characteristics of an analog V-p-Cr memory device. These characteristics were obtained using 400-ns pulses of progressively increasing magnitude and of both polarities. The 400-as pulse length is long enough to observe a plateau in the pulse signal (i.e., RC effects are avoided). Positive polarity means that the Cr bottom contact is more positive than the top contact. Starting from a 9.5 x 10' (I OFF state (measured at 0.5 V. curve (a) in Figure 14.25), the onset of the strong nonlinear rise in the current occurs at about + 1. IV. Curve (a) is reproducible in that it can be repeated many times without a change in the device resistance, up to a voltage level of about + 1.7V. Further increase in the voltage height will result in a permanent decrease in the device resistance and, consequently, a change in the characteristics. The decrease of resistance is determined by the magnitude of the maximum voltage applied. 556

681

V (6).

0.4 .' .. + • * O2 . •' -2 -1 • B I • I I

BS S • 1 21(mA) • 0 S S S • S . S -02 o

@1' S•® S

I -0.4

Figure 14.25 Pulsed I-V characteristics of Cr-p-V analog stnicture.

Curve (b) represents a memory-ON state (R = 5.6 x 10fl at 0.5V) of the device. Curve (b) is reproducible up to a voltage level of —5V if positive polarity is applied. Further increase in voltage might destroy the device. However, if a negative voltage is applied to the same ON state (curve (c)). an ERASE process is observed at voltage levels greater than about 1.7V. On the other hand, if an OFF state (R = 9 x 1((i) is negatively biased (curve (d)), no change in the OFF- state resistance can be observed up to a voltage of about - 5V. The apparent polarity dependence suggests that the analog memory switching is not determined simply by the magnitude of the applied power or energy. This is further supported by a comparison of individual switching transients with opposite polarity. Figure 14.26 shows the waveforms of a single WRITE pulse (a) from OFF (R = 2 x 1Ofl) to ON (R = 3.8 x 1012) and of an ERASE pulse (b) from OFF (R = 2 x 1Ofl) to a slightly higher OFF state (R = 2.4 x 1O17). In the second case there is no change in the memory state resistance although similar voltage and current levels are applied. The calculated total charge flowing through the sample is also 1C for the similar: Qw = 8.53 x 10"C for the WRITE and QE = 7.24 x 10 ERASE pulse. These measurements suggest that the memory-state resistance is determined by a combination of applied voltage level and appropriate polarity. On the other hand, the fact that the memory resistance is not determined by the applied power or energy suggests that the memory switching does not depend significantly on the internal temperature of the device. Figure 14.27(a) shows a switching transient of 557

682

(a) (b) 4 V(V) V jrTT\

I(M) r j

0 50 100 150 200 0 50 100 150 200 TIME (nsec) TIME (nsec)

Fg.re 14.26 Waveforms of single-pulse memory switching: (a) OFF—ON transient; (b) ON–.OFF transient.

(a) (b)

8 V(V)

I(M) 4.

0 100 200 300 400 0 100 200 300 400 TIME (nsec) TIME (nsec)

Figure 14.27 Waveforms of single-pulse memory switching: (a) ERASE transient at 300K; (b) ERASE transient at 4.2K. 558

683

an ERASE pulse at 300K, where the device resistance is changed from RON = 3 x 10'fl to Ro,F = 6 x 10fl. Figure 14.27(b) shows a similar ERASE transient at much lower temperature (4.2K) where the device resistance has also changed from RON = 3 x 1Ofl to ROFF = 6 X lOfi. It can be seen that despite the large temperature difference, the current level and the threshold voltage required to achieve the same ERASE process as at room temperature increase by less than a factor of two. It should also be emphasized that the device continues to operate even at liquid helium temperatures. The analog switching effect is still observed at 4.2K without significant changes either in the threshold voltage or the current level. This suggests that the electrical conduction and memory phenomena are possibly connected to a temperature-independent physical process, which we propose may be associated with tunneling, for example, or with conduction through a channel of very small dimensions in which metallic-like inhomogeneities are distributed.

14.2.2.1 Low-Temperature Quantization Effects in a-Si:H Structures

The behavior proposed in the previous section is supported by recent results obtained during investigations of the low temperature conductivity of the analog memory states 1611, (621. Typical 1-V characteristics of a formed memory-ON state at 4.2K are shown in Figure 14.28(a). In the voltage region from 0 to 0.36V, the current around zero bias is of the order of -10 9A but increases to —10 6A at voltages approaching 0.3V (i.e., a strong nonlinear behavior is found). It is important to emphasize that the room-temperature 1-V characteristics of the memory- ON state are linear. The observed large increase in the resistance around zero bias is shown in the inset to Figure 14.28(a) and is consistent with tunneling conduction between metallic particles embedded in an insulating matrix (63). Further experimental evidence for tunneling conduction comes from the temperature dependence of the device current at low bias voltages. In this regime, V is smaller than the tunneling barrier 4' and the conduction is associated with field-assisted tunneling. The approximate form of the current density/voltage characteristics is (63]:

3 x 109d2T2 J(V, T) = J(V, O)(1 + ( 14.12) )

where d is in Angstrom units, # is in electronvolts and T is in Kelvin. The experimentally observed T2 dependence is consistent with d = 50A and 0 = 1 eV. At 0.36V, a current jump occurs and the resistance of the sample is lowered to the order of a few kfl. After the first current jump at 0.36V, further current steps can be observed at 0.47, 0.53, and 0.70V (curve A, Figure 14.28(a)). At 0.36V (henceforth called the critical voltage V,.) there is thus a dramatic change 559

684

I (mA)

0.4 'OIL R(ot I 0.3 10.1. 04 O2 0.2 (a) 0.1 0 ____i 0.1 0.3 0.5 0.7 0.9 Voltage (V) eU 300 Current (ILA) T 4.2K 200

100

0 0 200 400 600 800 Voltage (mV)

11g.,, 14.26 (a) I-V characteristics at 4.2K with (curve B) and without (curve A) magnetic field (curve B is dlaced vertically for clarity, with (inset) resistance-voltage characteristics at 4.2K showing zero bias anomaly); (b) IV characteristics at 4.2K showing hysteresis observed on tint increasing, then decreasing voltage. in the behavior of the sample. At voltages lower than V,. no discontinuities are observed but at voltages higher than V4, the resistance is lowered and the current increases in discrete steps. The current-voltage characteristics are symmetrical (i.e., the same behavior is observed for the opposite polarity. Curve B in Figure 14.28(a) depicts the I-V characteristics of the same sample under the influence of a B = 0.2 T magnetic field. The curve has been displaced by 50 A in the current scale for clarity. The direction of the magnetic field is 30° with respect to the conducting channel (the filament). Additional steps can be observed at 0.34, 0.42, 0.5, 0.59, 560

685

and 0.79V together with the steps observed in the zero magnetic field case (curve A). The effect of the magnetic field is reversible, and if the field is removed the I-V characteristics revert to the zero magnetic field case. A number of I-V characteristics have been obtained in which sharp and well-defined steps can be observed at 4.2K. However, it should be pointed out that the position of the observed steps is dependent on the direction of the voltage sweep (i.e., some hysteresis is observed as illustrated in Figure 14.28(b)). The critical voltage V,, at which the first current jump is observed and the magnitude of the first current jump are dependent on the resistance of the memory- ON-state investigated. Figure 14.29 shows the effect of changing the memory-ON- state resistance on the observed resistance steps. Resistance (in kfl) is plotted on the right hand vertical axis; on the left, resistance is plotted in the dimensionless quantized units of (1:12e2). The critical voltage and the magnitude of the first jump increase with increasing resistance (Ra > R, > Rb> R.), but after the first jump the characteristics are rather. similar, suggesting a similar conduction mechanism at higher voltages. The first jump appears to be associated with the formation of a highly conducting path within the structure whose characteristics are independent of the low-bias behavior (i.e., of the different memory stales). Therefore, we propose that the first large current step (at 0.36V) is associated with the formation of a narrow, highly conducting channel that significantly lowers the resistance of the sample. With further increase in the applied voltage the resistance is lowered

&ci) 1/2 A (h/2e2

1/3 4

1/4 115 1/6 2

0.4 0.5 V.0 0.2 voltage (V)

Fe 14.2 I.V characteristics of different memory states at 4.2K (Re> R.> RA). 561

686

in steps, corresponding to quantized resistance values R = ht2ie2 where i is an integer. In the voltage range from 0.36 to 0.8V, there are four steps, with i being 2, 3, 4, and 5. These correspond to the observed current rises in curve A, Figure 14.28(a). Higher voltages have not been applied to the sample because this could change the resistance of the particular memory state. If a magnetic field is now applied to the sample, further quantization of resistance is observed at values R = h12(i + 1/2)e2. This is illustrated in Figure 14.30(b), in which the data correspond to curve B in Figure 14.28(a). It can be seen that extra steps in the resistance occur at i = 2.5, 3.5, 4.5, and 5.5. The effect of temperature is illustrated in Figure 14.31. The curves have been shifted along the current axis for clarity. The observed current steps gradually decrease with increasing temperature, finally disappearing at the remarkably high temperature of 190K. In our more recent work [641, using samples that have undergone a further conditioning step, similar quantization effects have been observed at temperatures up to 400K.

R p I 8

1, 112

1/3 1/3 $ 1/1 1/4 its iie 1/6

0.3 0.5 0.7 0.9 0.3 0.5 0.7 0.9 Vollop (V) Vage (V)

Flpre 14.3S (a) Resistance versus voltage at 4.2K with no magnetic field (dashed line indicates idealized contribution from ballistic transport channel); (b) resistance versus voltage at 4.2K with 0.2-T magnetic field.

In analyzing the main results of this work, two important facts should be emphasized. First, the observation of quantized electron transport provides a vital clue to the structure of the analog memory element. Second, the programmability of the analog memory provides information about the possible mechanism of the switching process itself. The analog memory effect in amorphous silicon metal-p' - metal structures can only be observed if the sample is subjected to an initial forming 562

EIA

190K I (MA) 170 K

-r 124 K 0.1 90K

70 K

50K 4.2 K

0 240 480 720 960 Voltage (mV)

Ilgwe 14.31 I-V characteristics as function of temperature (cuzves displaced vettically for clarity). process. The forming process is characterized not only by the breakdown of the high-resistance state of the structure but, more importantly, by the presence of a positive feedback mechanism that provides a low-resistance ON state so that the breakdown is nondestructive and repetitive switching is possible. Furthermore, the experimental results suggest a strong influence of the choice of the top metal contacts (summarized in Table 14.2) on the type of memory switching observed (digital or analog switching) and on the success of obtaining stable and reproducible switching. These results are in good accordance with the previous observations 1651 modification of the that the first switching event (forming) causes a local structural pP amorphous silicon layer, producing a highly conducting filament that does not revert to the original amorphous material when the device is switched OFF. After OFF resistance than the forming process, the p devices usually exhibit a lower the unformed device resistance, in contrast to our original data for p-n-i devices 1551. The temperature dependence of the conductivity is also greatly reduced by the forming process. The area independence of RON (Figure 14.15) suggests localized electrical conduction after forming. These results, together with the newly observed quantized electron transport phenomena (Figures 14.28-14.31), provide strong experimental evidence that the forming process creates a filamentary region consisting of a new material whose properties have changed significantly compared to those of the unformed original material. It is feasible that the high fields and 563

688

current densities present during forming result in high temperatures developing locally, which could lead to enhanced diffusion of metallic particles from the electrode into the thin amorphous film. Such a region would become the preferred current path carrying the electron current in the ON state. The I-V characteristics in the ON state (Figure 14.10) suggest that any material rearrangement within the filament ocurs so as to destroy the rectifying properties of the original metal-p Schottky contact at room temperature; it should also be noted that this is the case even in a typical OFF state. Lowering the temperature of the memory device reveals further information about the possible structure. The low-temperature I-V characteristics (Figure 14.28(a)) show that we have observed a zero-bias anomaly and quantized resistance in the ON state of amorphous silicon Cr-p-V structures. The phenomenon of quantized electron transport is usually observed in the case where the mean free path of the electron A is larger than the length of the conducting channel, resulting in ballistic conduction 166, 671. The usual approach to the fabrication of devices based on ballistic conduction is to use a very high mobility material (usually high- purity GaAs). where the mobility can reach values in the order of - 1O' cm 2V - s -' leading to the values of electron mean free path:

A - rv - (3kTm) - 1 p.m (14.13) q

where i is the momentum relaxation time, v, is the thermal velocity, Tis the lattice temperature, in is the effective mass of the electron, q is the electronic charge and k is the Boltzmann's constant. For the above estimate, m - 0.068 in. (m is the free electron mass) has been used in the case of GaAs. The value of A —1 p.m is certainly longer than the device dimensions that can be achieved by modern submicron technology. The structure in which ballistic transport is most widely investigated is a GaAs-AIGaA5 heterojunction with a split gate fiddeffec: transistor (FET) configuration, usually less than 0.5 pm in length, with a gap of about 0.7 p.m 165. 661. As the voltage on the gate is made increasingly negative, the width of the conducting channel decreases and becomes sufficiently small that one-dimensional quantization occurs as the channel width becomes comparable to the electron wavelength. This is sufficiently short that electrons may pass through the conducting channel without appreciable scattering. Our structure is quite different and one possible mechanism that would explain the quantized transport in amorphous silicon structures is outlined below. The observed I-V characteristics at low temperatures show two distinct regimes, as shown in Figures 14.28-14.31. Below the critical voltage V.,, the curves are nonlinear but there are no discontinuities; however, above V., sudden jumps in current are observed associated with quantized resistance. These observations suggest that the current flow below and above Y, is determined by different mechanisms. 564

The I-V characteristics below V,, (see inset, Figure 14.28(a)) suggest that the filament has a relatively large resistance around zero bias (i.e., a barrier for electron flow exists). The observed zero-bias anomaly is consistent with tunneling conduction between metallic particles embedded in an insulating matrix 1631. Below V, the I-V characteristics show a continuous but strongly nonlinear behavior due to the creation of free carriers by the high electric field across the small tunneling distance. In such a system the creation of free carriers is an activated process related to the increase in the electrostatic potential of a particle when a free electron is added to it. The activation energy can be provided entirely by thermal energy, hence the effect diminishes at higher temperatures; or; in the presence of an applied field, part or all of it can be provided by the field itself. Experimentally, a P dependence of the current at a constant voltage (less than I') is observed in good accordance with the assumption that the electron transport is dominated by field-activated tunneling processes in this region. Figure 14.32 illustrates an idealized model having a single permanent inclusion extending from the top contact, with a narrow channel connecting it to the bottom contact. The evidence [551 is that the overall diameter at the top contact is less than 0.5 sun. The length. of the channel must be consistent with tunneling. With increasing applied voltage, the tunneling current increases exponentially. The sudden increase in current at V. implies that the tunneling

Lw_~'_]

-u _

Figire 1432 Schematic description of filaumt showing proposed metallic indusion and ooedimen- sional cmdoWn channel 565

barrier effectively breaks down. This is not a destructive effect, however, as the process is completely reversible and no material changes ensue. At present we have no firm explanation for the steps in the current above V., which we have associated with quantized resistance. Experiments on other one- and two-dimensional systems show electrical properties that are associated with quantized resistance, but these are only observed at very low temperatures. For example, in a two-dimensional electron gas [66J, the quantum Hall effect is related to e2Ih, and in a one-dimensional electron gas [67,68] in the ballistic regime, resistance is quantized in terms of ht2e2. However, it is not expected that quantized resistance states associated with ballistic transport should be observed when, as in the present case, the applied voltage is much greater than kT—if, for example, all the applied voltage were to appear across the critical part of the structure, the equivalent temperature would be in excess of 1000K. We have suggested elsewhere 164J that the quantized phenomena might be related to electrical transport through a quantum point contact. Assuming that the inclusion of modified material created during forming has a tapered shape (Figure 14.32), the resulting current now could be restricted to a very localized area. The device resistance could thus be determined by the contact area where the tip of the modified region and the lower metal electrode are in close proximity. At such distances the resistance might be associated with either a single atom or a small number of contact atoms (69J. It has been shown theoretically [701 that the resistance associated with a single contact atom reaches saturation with a minimum value given by R = 1i/2e2 , provided that no elastic deformation occurs. This is the constriction resistance (= 129060) associated with an ideal conduction channel. It should be noted that this quantized resistance value is predicted assuming current flow through a single atomic orbital only. If more orbitals (conduction paths) are involved, these would reduce the quantized resistance value in integer steps according to the number of paths involved. Exper- imental results from a scanning tunneling microscope (STM) using an Ii lip showed a resistance jump to a value R -4 x 1073 at close contact, rather larger than was predicted theoretically [71J. In this experiment the tunneling current is recorded as a function of distance D. from the conducting surface. The jump occurs it the transition from the tunneling to the one-atom point contact regime in the STM. Our measurements show that the resistance of conditioned a-Si:H sandwich structures can be quantized under some circumstans. The voltages at which this occurs correspond to energies that are greatly in excess of the thermal energy kT. Furthermore, the quantized resistance values reflected in the I-V characteristics do not involve all possible integer or half integer values. For these reasons we believe that existing theories of quantized resistance associated with ballistic transport are not applicable to our structure. However, the results obtained so fir do not appear incompatible with current flow through atomic scale point contacts, and it is possible that this could explain the room temperature quantization observed. The most important feature of the observed quantized behavior is that it can be observed up to -400K, much higher than previously reported. 566

691

The model outlined above describes one possible mechanism for the quantized electron transport and provides important information about the structure of the filament, but as it stands it does not explain the' memory switching. The most important difference between the two phenomena lies in the fact that the quantized electron transport observed in memory devices is seen at applied voltage levels lower than —1V, that is, at voltage levels where no memory switching would be expected to occur. The observed quantized jumps in the device resistance are threshold-switching type: if the voltage is lowered, the sample resistance reverts to the original low-bias case and there is no permanent change in the I-V characteristics of the device. On the other hand, memory switching occurs at voltage levels from —1.5 to —4V, resulting in a permanent change in the I-V characteristics of the device (i.e., a different resistance state of the memory).

14.2.2.2 Models for Memory Switching

Some of the possible mechanisms that might explain the memory-switching behavior are discussed below.

Thermal Models

The thermal models 126, 291 used to explain the behavior of the amorphous chalcogenide memories might appear to offer a basis for explaining the a-Si:H switching process. In the chalcogenide memories the ON state is associated with a filament of crystalline material that is formed after sufficient power has been applied to the layer to melt a small area of the material. Switching OFF is achieved by burning out this filament using a number of relatively short high-power pulses and allowing rapid quenching to reform the highly resistive amorphous phase. However, there are a number of important differences between the amorphous silicon and, the chalcogenide memories: It has been established that the a-Si:H memories do not form or WRITE at constant power; in general, forming occurs at much lower energy (<10 J) than in the chalcogenides (10-10 J). The forming, WRITE, and ERASE operations for the a-Si:H memories are generally polarity dependent. No rise in the temperature of the a-Si:H specimens can be observed prior to switching. The a-Si:H WRITE and ERASE times are many orders of magnitude shorter than those for the chalcogenides (e.g., 10 s for the WRITE operation compared with 10 s). It is thus unlikely that the crystalline/amorphous thermal model is applicable to a-Si:H memories. 692

Models Based on Trapped Space Charge

In many respects the behavior of the a-Si:H layers appears to be closely related to that of crystalline-silicon MISS structures in that both show fast polarity-dependent switching, both show current instabilities, and both have high conductance states associated with current filaments. However, the crystalline MISS structures are threshold switches that always revert to the OFF state when the power is removed, whereas the a-Si:H structures have the additional feature (and complexity) of nonvolatile memory behavior. It is nonetheless possible that the initiation of memory switching in the amorphous silicon devices is closely related to the mechanism proposed to explain threshold switching in crystalline MISS structures. Essentially two models have been used to explain MISS behavior [10]. These are generally referred to as the "punch-through" and "avalanche" modes, and both mechanisms require that high fields be developed across space-charge barriers in the films. In addition, in both models the low-impedance (ON) state is produced by injected charge, causing inversion of the Si at the Si/insulator interface. It is tempting to suggest that the "permanent" memory of the a-Si:H layers may be produced by a similar mechanism in which the charge is trapped in deep gap states at the insulator semiconductor interface for which the probability of release is very small. However, the a-Si:H devices retain their ON-state memory conductance without any observable change for at least 1 year at room temperature and at least 24 hours at 95°C. Using thermal release rates from deep midgap states of energy E as a measure of the persistance of the trapped space charge, the average thermal release time (x exp(—(E - Ej/kTJ) indicates that the capture cross section of these centers would have to be less than 10 - " cm' in order to agree with the experimental data. Although extremely small, such values would be consistent with Coulomb repulsive centers identified in crystalline materials. However, the problem is that recombination of the trapped charge distribution through tunneling or diffusion may well invalidate the above estimate by leading to a much faster decay of any trapped space charge distribution. All that can be said at present is that a model in which the observed memory is associated with a trapped space charge cannot be excluded, but in view of the remarkable nonvolatility of the memory states, it is unlikely to be the basis of anything more than a partial explanation.

Hydrogen Motion in a-Si:H

It is known that significant amounts of hydrogen are incorporated in the random network of the a-Si:H layer. The possibility exists, therefore, that memory switching may be associated with some atomic motion of hydrogen in the material. For instance, it has been reported that in n-type crystalline-silicon-Cr Schottky barrier 568

093

structures, hydrogen plays an important role in lowering the contact barrier 1741. Also, the polarity dependence of the threshold voltages for the a-Si:H memories could be understood on the basis of field-assisted diffusion.

Tunneling Method

One possible explanation for the quantized electron transport and the zero-bias anomaly observed at lower temperatures is that tunneling processes are involved and hence must be taken into account when suggesting a model for memory switching. In order to explain the different and permanent resistance states, the model should embody the following features: Changes in the conductivity of the whole filament; Local changes in conductivity at a certain point(s) in the filament; Changes in the filament geometry. Thermal models would presumably require an average energy for switching of an order similar to that established for forming, but is has been found that ON -. OFF -. ON switching can, be achieved using single-pulse energies of —lO' J (Figure 14.26). In addition, if it is assumed that the filament is homogeneous, there is no obvious way of introducing a polarity dependence. However, if there are inhomogeneities in the filament it may be possible to substantially increase the local energy density and field strength. Consider, for example, a simple one-dimensional filament consisting of alternating regions of material A and B. The observed resistance of the filament will depend on the relative contributions to the filament resistance of A and B type material, that is, if R, < RA, the properties of the A type will be observed, and if RA < R 8, the properties of the type B material will be observed. If highly conducting regions are separated by small gaps, it is also necessary to consider the possibility of inter-island tunneling. In order to establish such a model, a more detailed description of regions A and B would have to be included, for example: Region A is metallic air a degenerately doped material whose conribution to the filament is fixed and sr. Of(V)whereVistheappliedvoltageandois the conductivity. Region B is a gap between A-type material that can be sufficiently small to allow significant tunneling to occur; ae is some function of the voltage. In the ON state, R1 < R, and the measured conduction properties of the filament are those corresponding to material A. namely, ohmic I-V char8cteristies and small thermal activation. In the OFF state, RA

694

(4) the density of states g(E) in the metallic regions, and (5) the occupation of g(E) (i.e., the temperature dependence). In this case, the functional form of the current density I versus voltage V at different temperatures AV, T) can be obtained under two limiting conditions. First:

V c 4', and 44' 4', constant T

In this case the transmission probability P = c exp( - d4') may be treated as a constant for electrons near the Fermi energy E,. Provided that g(E) variations near E, are small, I will increase linearly with V as more empty states become accessible in metallic region A (i.e., conduction is approximately ohmic and symmetric). This is consistent with the experimental data shown in Figure 14.12. Second:

V < 0, and the temperature is varied

In this case P is essentially independent of T because usually kT - 10 'eV. Thus only, a few electrons are promoted thermally to levels where P is significantly larger than at E,. The approximate form of thermal I-V characteristics in this regime is as given in Equation (14.12). How might switching be achieved in terms of the tunneling model? The low- bias isothermal resistance of region B-like parts of the filament is determined by geometric factors (included in constant c) and the transmission probability. There- fore one obtains:

I = cV cxp(—dO"2) ( 14.14) or

R = cexp(dvY' 2) .. '( 14.15)

The exponent is almost numerically correct if d is in A and 0 is In eV. Taking d= 5OA and 4'- leVthcnR1 = cexp(50) and increasing dbysA gives R2 " exp(55) (i.e., R has increased by about a factor of 150). An equivalent Increase in resistance is obtained by keeping d constant and increasing Ofrom MV to1.2eV. Changes in the tunnel barrier width on an atomic scale, or small (20%) changes in barrier height, could therefore aunt for the presence of switching. It Is not dear, however, how these could occur in a manner that would explain the systematic dependence of device resistance on the magnitude and polarity of the switching voltages as shown, for example, by the data in Figures 14.22(a) and 1422(b). Finally we should note that in this model the cross section of the conducting channel is considered to be constant. The possibility exists that memory switching 570

•95

might be associated with some permanent change to the cross section of the conducting channel (i.e., the high currents involved might cause small-scale material rearrangements). In summary, we can say that, based on the available experimental evidence, it is possible that memory switching in the amorphous silicon metal-p 1 -metal structures might be associated with a tunneling barrier within a permanent filament produced by the forming process. A change in the particle size or spatial distribution as a result of localized heating or high-field effects could explain the differences between high- and low-resistance states. However, the high switching speeds (-10 us) and the insensitivity of switching to temperature suggest that large-scale structural changes are unlikely. It is possible though, that a structure might arise with small variations at a "weak link" in the filament that could produce the observed changes in a short time. As tunneling is a quantum mechanical effect, the nanosecond time scale for the switching is feasible. Finally we emphasize that the effect has been observed in what is initially a metal-amorphous semiconductor structure. It is not known with any certainty what effect the forming process has on the structure of the conducting channel, but the observed quantized behavior suggests the importance of the very small dimensions of the conducting channel rather than other physical parameters. The significance Of using amorphous silicon sandwich structures lies in the forming process, that is, in the process that allows the fabrication of such small structures. The strong influence of the metal contact on the observed memory switching behavior indicates the importance of alloying during the forming process. It has been reported, for example, that solid-phase amorphization or glass formation occurs in V-Si reactions induced by rapid thermal annealing (i.e., at conditions similar to forming) but does not occur in Co-Si and Cr-Si (751. Therefore, the presence of a very small tunneling conduction path in amorphous silicon V-p-Cr structures might be due to a . new type, of solid-phase reaction, induced by forming, that creates a homogeneous (possibly amorphous) V-Si silicide.

14.3 CONCLUSION

Research into electronic Switching in amorphous semiconductor thin films has a long history, with several distinct stages involved. Early work was concentrated on chalcogenide thin-film devices in which the basic switching took place between highly conducting ON and highly resistive OFF states (that is, an essentially digital mode of operation was involved). Memory switching in chalcogenide glasses is based on a crystalline/amorphous phase transition, whereas threshold switching in these glasses can be described by purely electronic mechanisms for films si pm thick or by electrothermal considerations for film thickness from I to 10 pm. More recent observations on amorphous silicon thin-film structures have shown that, in contrast to chalcogenide thin films, these devices can exhibit an 571

analog behavior under certain conditions. This analog switching mechanism appears to be associated with interatomic processes such as tunneling, and, as a consequence, is significantly faster than switching in chalcogenide glasses. Finally, our most recent results have demonstrated that in certain a-Si memory structures, quantized electron transport phenomena occur. These results are most surprising considering the magnitude of the effect and the high temperatures involved and appear to be a consequence of the extremely small (atomic scale) dimensions of the structures involved.

REFERENCES

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SYMBOLS

Symbol Description

B ' Magnetic field strength d Layer thickness d n-Layer thickness d, Phase boundary diameter of phase transition e Electronic charge E Electric field (Vcm) Ec, Critical field for switching

EF Fermi energy Energy life Recombination current density 57

Symbol Description I Current density k Boltzmann's constant g(E) Density of states (DOS) GoFF OFF-state conductance G. Gradient of phase boundary diameter IN Critical holding current I. Steady current 1. Maximum current M* Electron-effective mass Free electron mass N Average number of phonons of frequency w P RMS power P Transmission probability P. Intercept of phase boundary diameter qe4 Injected charge Q Charge ROFF OFF-state resistance RON ON-state resistance td Switching delay time tow WRITE delay time DE ERASE delay time T Temperature Ti Electron temperature V. Critical forming voltage

V 1 Critical voltage V, Forming voltage V,,, Holding voltage V Threshold voltage V1 , Forward bias threshold voltage V71. Reverse bias threshold voltage âpfp(B) Transverse magnetoresistance 4V Voltage window for analogue memory programming a- Conductivity A Electron mean free path Mobility Tunneling barrier height Relaxation time m Forming delay time Mean time between electron-lattice collisions W Frequency (1 Resistance 576

701

ACRONYMS

CCNDR Current-controlled NDR CFO Cohen-Fritsche-Ovshinsky FET Field-effect transistor I-V Current-voltage MISS Metal/i-n-p*/metal NDR Negative differential resistance TONC Transient ON characteristics VCNDR Voltage-controlled NDR

Appendix - List of Publications August 1994

(Where there are co-authors, the number in parenthesis indicates my position in the list of authors).

(1) The Electrical Properties of Vitreous Silica, with R W Douglas, J Soc Glass Tech, Vol. 43, 159-178 (1959).

The Dielectric Properties of Glass. Thesis, University of Sheffield, Oct 1959, 163 pp. Properties of Glasses in the System CaO—B 203 --Al203 . Part I. The D.C. Conductivity and Structure of Calcium Boroaluminate Glasses. Phys Chem Glasses, Vol. 2, 87-98 (1961).

Properties of Glasses in the System CaO—B 203 —A1203 . Part H. The Dielectric Properties in Relation to Structure. Phys Chem Glasses, Vol. 2, 152-162 (1961)

(1) Dendritic Growth of Indium Antimonide, with N. Albon. Electrochem Soc. Electronics Div., Abs Vol. 10, 95-99, (1961).

Properties of Glasses in the System CaO—B 203 —A'203 . Part ifi. Thermal Expansion in the Cabal Glasses and its Correlation with Structure and Electrical Properties. Phys Chem Glasses, Vol. 3, 134-138, (1962).

Comments on the Volume Conductivity of Borosilicate Glass. J Appl Phys, Vol. 34, 705-706, (1963).

(2) The Influence of Twin Structures on Growth Directions in Dendritic Ribbons of Materials having the Diamond or Zinc-Blende Structure, with N Albon. J Phys Chem Solids, Vol. 24, 899-907 (1963).

Electrical Conduction and Dielectric Relaxation in Glass. Review article in "Progress in Ceramic Science", Vol 3, 77-196. Ed J E Burke, Pergamon Press (1963). Invited Review.

(2) Doped Anodic Oxide Films for Device Fabrication. Part I. Controlled Incorporation of Phosphorus into Anodic Oxide Films on Silicon, with P F Schmidt. J Electrochem Soc, Vol. 111, 682-686, (1964)

(3) Doped Anodic Oxide Films for Device Fabrication on Silicon, Part H. Diffusion Sources of Controlled Composition and Diffusion Results, with P F Schmidt, T W O'Keefe and J Oroshnik. J Electrochem Soc, Vol. 112, 800- 807, (1965).

The Conduction Mechanism in CABAL Glasses. Letter to the Editor Phys Chem Glass, Vol. 6, 253-254, (1965).

(1) Electrical Properties of Some Chalcogenide Glasses, with N Clare and S Frank. Proc Conf on Electronic Processes in Low-Mobility Solids, pp 109-113 (Sheffield), (1966).

(2) Electronic Processes in Low Mobility Solids, with I 0 Austin. Bull Inst Phys, Vol. 17, 298-301, (1966).

Electronic Conduction Mechanisms in Glasses. Glass Industry Vol. 48, (11), 637-642 and Vol. 48, (12), 695-699, (1967). Based on an invited lecture at a Symposium on Glass in Electronics, Rensselaer Polytechnic Institute, Troy, USA, Mar 28-29, (1967).

Ap2

(1) Diffusion from a Plane Finite Source into a Second Phase, with Special Reference to Oxide-Film Diffusion Sources on Silicon, with P F Schmidt. J Electrochem Soc, Vol. 115, 548-553, (1968)

(1) Electronic Properties of Some Simple Chalcogenide Glasses, with J M Robertson. J Non-cryst Sol, Vol. 2, 40-51, (1970).

Band or Hopping Conduction in Chalcogenide Glasses. J Non-cryst Sol, Vol. 4, 78-79, (1970).

(2) Electronic Conduction in Vanadium Phosphate Glasses, with G S Linsley and F M Hayatee. J Non-cryst Sol, Vol. 4, 208-219, (1970).

Semiconducting Glasses. Part I: Glass as an Electronic Conductor. Contemp Phys, Vol. 11, 227-255, (1970). Invited Contribution.

Semiconducting Glasses. Part II: Properties and Interpretation. Contemp Phys, Vol. 11, 257-286, (1970). Invited Contribution.

The Electrical Properties of Glasses. Phys Education, Vol. 5, 97-100, (1970). Also published in the Students' Sourcebook, Nuffield Foundation Physical Sciences Project (Penguin), (1973). Invited Contribution.

(2) Current Noise in Vitreous Semiconductors, with C Main. Phys Stat Sol, Vol. 1(a), 297-306, (1970).

(2) Drift Mobility Studies in Vitreous Arsenic Triselenide, with J M Marshall. Phil Mag, Vol. 24, 1281-1305, (1971).

(2) The Hole Drift Mobility of Vitreous Selenium, with J M Marshall. Phys Stat Sol, (a), Vol. 12, 181-191, (1972).

(2) Electronically Assisted Thermal Breakdown, with J M Robertson. J Non-cryst Sol, Vols. 8-10, 439-444, (1972).

(3) Trap-limited Processes in Some Chalcogenide Glasses, with J M Marshall and C Main. J Non-cryst Sol, Vols. 8-10, 760-767 (1972).

(1) Electronic Conduction and Switching in Chalcogenide Glasses, with J M Robertson. Proc IEEE Transactions on Electron Devices. Vol. ED-20, 105- 123, (1973). Invited Contribution.

(2) D.C. Conduction in Glasses in the Selenium-Germanium System, with S Frank. Phys Stat Sol, (a), Vol. 16, 623-632, (1973).

Preparation of Amorphous Materials and the Glass Transformation. Chap 4 (pp 161-191) of "Electronic and Structural Properties of Amorphous Semiconductors". Ed P G Le Comber and J Mort, Academic Press, (1973). Invited Contribution.

(2) Photoconductivity and Noise in Chalcogenide Glasses, with C Main. Chap 16 (pp 527-549) of "Electronic and Structural Properties of Amorphous Semicon- ductors". Ed P 0 Le Comber and J Mort, Academic Press, (1973).

(2) The Conductivity and Dielectric Properties of Glasses in the As-S and As-Se systems, with R B South. "Amorphous and Liquid Semiconductors", Vol 1, 305-312. Ed J Stuke and W Brenig, Taylor and Francis, (1974).

Ap3

(2) Trapping Effects in Amorphous Semiconductors, with C Main. "Amorphous and Liquid Semiconductors", Vol 2, 783-790. Ed J Stuke and W Brenig. Taylor and Francis, (1974).

(3) Transport Properties and Localised States in Vitreous Chalcogenides, with J M Marshall and F Fisher. "Amorphous and Liquid Semiconductors", Vol 2, 1305-1310. Ed J Stuke and W Brenig. Taylor and Francis, (1974).

(2) Microelectronic Structure for the Measurement of Photoconductivity in Dyes, with S Smithies and J Mayor. J Phys E, Vol. 7, 427-429, (1974).

(3) The Electron Drift Mobility in Arsenic-Selenium Glasses, with J M Marshall and F D Fisher. Phys Stat Sol, (a) Vol. 25, 419-428, (1974).

37.(2) The Mobility of Photo-induced Carriers in Disordered As 2Te3 and As30Te48Si12Ge10, with J M Marshall. Phil Mag, Vol. 31, 1341-1356, (1975)

(2) Field Effect Measurements in Disordered As 30Te40Si12Ge10 and As2Te3 , with J M Marshall. Phil Mag, Vol. 33, 457-474, (1976).

(3) Transport Properties and Electronic Structure of Glasses in the Arsenic- Selenium System, with F D Fisher and J M Marshall. Phil Mag, Vol. 33, 261- 275, (1976).

(2) The Thermal Activation Energy of Crystalline As2Se3 , with C Wood. Solid State Commun, Vol. 20, 329-331, (1976).

(1) Localised States and Electronic Properties of Amorphous Semiconductors, with W E Spear. Phys Chem Glasses, Vol. 17, 174-192, (1976).

(3) The Raman Spectra and Structure of Glasses in the As-S and As-Se Systems, with P S Ewen and M J Sik. 'The Structure of Non-crystalline Solids", pp 231- 235, Ed P H Gaskell. Taylor and Francis, (1977).

The Electrical Properties of Glass. (Invited Review). "Glass 1977" 52 pp. (Keynote reviews from XIth International Congress on Glass, Prague, July 1977). Also published in 3 Non-cryst Sol, Vol. 25, 370-423, (1977).

(2) Dye-sensitised Photoconductivity in Amorphous Si0 2 Films, with B Flynn and J Mayor. J Phys C, Vol. 10, 4051-4059, (1977).

(3) Microelectronic Ion-Selective Electrodes, with R G Kelly and J R Jordan. Proc of the Analytical Division of the Chemical Society, Vol. 14, 338-340, (1977).

(1) Electronic States and Transport Mechanisms in Chalcogenide Glasses, with J M Marshall. Proc of the 7th International Conf on Amorphous and Liquid Semiconductors, pp 529-540. Ed W E Spear, CICL, Edinburgh University, (1977).

(3) Local Temperature Measurements in Chalcogenide Switching Devices, with R B South and J M Robertson. Proc of the 7th International Conf on Amorphous and Liquid Semiconductors, pp 722-726. Ed W E Spear, CICL, Edinburgh University, (1977)

(2) Electronic Conduction in Vanadium Tellurite Glasses, with B W Flynn and J M Robertson. Proc of the 7th International Conf on Amorphous and Liquid Semiconductors, pp 678-682. Ed W E Spear, CICL, Edinburgh University, (1977).

Ap4

(2) Oxide and Chalcogenide Glasses for Microelectronic Ion-Selective Electrodes, with R G Kelly and J R Jordan. Proc of SIRA International Symposium on In- dustrial and Clinical Applications of Electroanalytical Sensors, 4 pp. London, (December 1977).

(2) Electrical Contact Properties of Semiconducting Chalcogenide Glasses, with A Wallace and J M Robertson. Phil Mag, Vol. 38, 57-70 (1978).

(3) Novel Light-Modulated MOS Transistor, with B W Flynn and J Mayor. Solid State and Elect Devices (WE), Vol. 2, 94-96, (1978).

(2) The Characterisation of Metal-Thin-Insulator—n—p Silicon Switching Devices, with J Buxo, G Sarrabayrouse and J-P Sebaa. Rev de Physique Applique, Vol. 13, 767-770 (1978).

(1) The Threshold Characteristics of Chalcogenide Glass Memory Switches, with J M Robertson and C Main, J Non-cryst Sol, Vol. 32, 29-52 (1979).

(2) Growth of As2Se3 Single Crystals by an Open-Tube Vapour Transport Technique, with C Hampton and P McA Dryburgh. 2nd European Conf on Crystal Growth, Abstracts Book C132 (September 1979).

Chalcogenide Glasses as Ion-Selective Materials for Solid State Electrochemical Sensors. J Non-cryst Sol, Vols. 35 and 36 (Part II), 999-1004, (1980).

(2) Resonance Raman Scattering in As-S Glasses, with P J S Ewen. J Non-cryst Sol, Vols. 35 and 36, (Part II), 1191-1196, (1980).

(4) Dynamic Properties of Switching in MISS Structures and Applications to Charge Transfer Devices, with G Sarrabayrouse, J Buxo, A Munoz-Yague and J-P Sebaa. "Insulating Films on Semiconductors, 1979, pp 179-185, Ed G G Roberts and M J Morant. Inst of Phys Conf Ser No 50 (1980).

(3) The Inversion Controlled Switching Mechanisms of MISS Devices, with G Sarrabayrouse, J Buxo, A Munoz-Yague and J-P Sebaa. Proc WE, Part I, Solid State and Elect Devices, Vol. 127, 119-125 (1980).

(2) Dye-sensitised Charge Injection into Amorphous Si0 2 Films, with R Hoiwill and J Mayor. J Non-cryst So!, Vol. 40, 49-59, (1980).

(3) A Note on the Raman Spectra and Structure of AsS 100_(x ~!40) Glasses, with P J S Ewen and M J Sik. Solid State Commun, Vol. 33, 1067-1970, (1980).

(2) Dye-sensitized Photodischarge of Metal-Dye-Oxide-Silicon (MDOS) and Metal- Dye-Nitride-Oxide-Silicon (MDNOS) Capacitors, with I A M Wilson. "Insulating Films on Semiconductors", Eds: M Schulz and G Pens!, pp 145-148, Springer Series in Electrophysics, 7, (Springer Verlag, Berlin), (1981).

(2) Laser-Raman Study of the Structure of Silver Doped As 40S60 Glasses, with A P Firth and P J S Ewen. J de Physique, Colloque C4, Suppl No 10, Tome 42, 903-906, (1981).

(2) The Electronic and Optical Properties of Vanadium Tellurite Glasses, with B W Flynn. J de Physique, Colloque C4, Suppl No 10, Tome 42, 1005-1008, (1981).

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*64. (1) New Amorphous Silicon Electrically Programmable Non-Volatile Switching Device, with P G Le Comber, G Sarrabayrouse and W E Spear. TEE Proc, Part I, Solid-State and Electron Dev, Vol. 129, 51-54, (1982).

* THIS PAPER WAS AWARDED THE MAXWELL PREMIUM, FOR 1982, BY THE INSTITUTION OF ELECTRICAL ENGINEERS.

(3) Structural Changes in Amorphous Arsenic Sulphide Films on Photodoping with Silver, Studied by Raman Spectroscopy, with A P Firth and P J S Ewen. 'The Structure of Non-Crystalline Solids, 1982", pp 286-293, Ed P H Gaskell, J M Parker and E A Davis, Taylor and Francis, (1983).

(3) A Model for Photostructural Changes in the Amorphous As-S System, with M Frumar and A P Firth. J Non-cryst Sol, Vols. 59 and 60, 921-924, (1983).

(2) The Electronic Properties of Glasses in the Cu 1(As04Se06) System with x Between 5 and 30, with M I Fraser. J Non-cryst Sol, Vols. 59 and 60, 1031- 1034, (1983).

(1) Memory Switching in Amorphous Silicon Devices, with P G Le Comber, W E Spear and J Hajto. J Non-cryst Sol, Vols. 59 and 60, 1273-1280, (1983). An invited paper presented at the 10th mt Conf on Amorphous and Liquid Semiconductors, Tokyo, August 1983.

(4) A Raman Study of Photochemical Reactions in Amorphous As 2S3 on Polycrystalline Ag2S Substrates, with P J S Ewen, W Taylor and A P Firth. Phil Mag B, Vol. 48, L15-L21, (1983).

Electron Transport in Chalcogenide Glasses, in "Coherence and Energy Transfer in Glasses", Eds P A Fleury and B Golding, pp 243-279, Plenum Press, NATO Conf Ser VI (Materials Science), (1984).

(3) A Raman Study of Reversible Photostructural Changes in the Amorphous As-S System, with M Frumar and A P Firth. Proc Conf on "Amorphous Semiconductors", Gabrovo, Bulgaria, September 1984, Vol I, 48-51, (1984).

(3) The Mechanism of Photo-enhanced Silver Dissolution in Amorphous As-S Layers, with M Frumar and A P Firth. Proc Conf on "Amorphous Semiconductors", Gabrovo, Bulgaria, September 1984, Vol I, 216-219, (1984).

(3) Reversible Photodarkening and Structural Changes in Amorphous As 2S3 Thin Films, with M Frumar and A P Firth. Phil Mag B, Vol. 50, 463-475, (1984).

(2) Applications of Amorphous Semiconductor Materials in Electronics, with J Hajto. Materials and Design, Vol V, 221-227, (1984).

(2) Electronic Switching in Amorphous Silicon Junction Devices, with P G Le Comber, W E Spear, J Hajto and W K Choi. Chapter 15, pp 275-289, of "Hydrogenated Amorphous Silicon, Part D - Device Applications", Ed J J Pankove, Vol 21, Part D of the series "Semiconductors and Semimetals", Ser Eds R K Willardson and A C Beer, Academic Press, (1984).

(3) New Observations on the Electrical Properties and Instabilities of Pure and Metal Doped Si02 Films, with K V Krishna, J J Delima and F C Eze. Physica, Vol. 129B. 245-248 (1985).

Electronic Transport in Amorphous Chalcogenide Semiconductors. In "Glass - Current Issues", pp 376-398, Eds A F Wright and J Dupuy. Proc NATO ASI Series E: Applied Sciences - No 92, Martinus Nijhoff Publishers, (1985).

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(3) Inorganic Resists Based on Photo-Doped As-S Films, with A P Firth, P J S Ewen and C M Huntley. Proc SPIE, Vol. 539, (Advances in Resist Technology II), 160-165, (1985).

The Glass Transition. Lecture Course Presented at the Spring College on Amorphous Solids and the Liquid State, mt Centre for Theoretical Physics, Trieste, Italy, June 1982. Published in "Amorphous Solids and the Liquid State", Chap 12, pp 395-432, Eds N H March, R Street and M Tosi, Plenum Press, (1985).

(1) Photo-Induced Structural and Physico-Chemical Changes in Amorphous Chalcogenide Semiconductors, with A P Firth and P J S Ewen. Invited paper in Phil Mag B, Vol. 52, (Special Festschrift issue for Sir Nevill Mott), 347-362, (1985).

(2) Microelectronic Ion Sensors: A Critical Survey, with R G Kelly. lEE Proc, Part I, Solid-State and Electron Dev, Vol. 132, 227-235, (1985).

(3) An Optical Reflectivity Study of Ag Photo-Dissolution into As-S Films, with A P Firth and P J S Ewen. J Non-cryst Sol, Vols. 77 and 78, 1153-1156, (1985).

(3) Electronic Instabilities in Transition Metal Doped Amorphous Silicon Dioxide Films, with K V Krishna and J J Delima. J Non-cryst Sot, Vols. 77 and 78, 1321-1324, (1985).

(2) The Switching Mechanism of Amorphous Silicon Junctions, with P G Le Comber, W E Spear, J I-Iajto, A J Snell, W K Choi, M J Rose and S Reynolds. J Non-cryst Sol, Vols. 77 and 78, 1373-1382, (1985).

(3) Electronic Conduction and Instabilities in Thin Films of Amorphous Silicon Dioxide, with J J Delima and K V Krishna. Phil Mag B, Vol. 53, 115-131 (1986).

(2) Solid-State Ion Sensors: Theoretical and Practical Issues, with R G Kelly. J Chem Soc, Faraday Trans 1, Vol. 82, 1195-1208, (1986).

(3) A Model for the Variations in the Field-Dependent Behaviour of the Poole- Frenkel Effect, with W K Choi and J J Delima. Phys stat sol (b), Vol. 187, 352-359, (1986).

(3) Silver in Photodoped Arsenic Sulphide Films: An EXAFS Study, with A.T. Steel, A.P. Firth, P.J.S. Ewen and G.N. Greaves. J. de Physique, Colloque C8, Suppl. No 12, Tome 47, 395-398, (1986).

(2) Model for the Switching Voltage of MISS Devices Based on an Analogy with the Thyristor with W K Choi, J J Delima and S M Gage. Electronics Letters, Vol. 23, 1-2 (1987).

(2) Electronic Conduction in Basalt Glasses and Glass-Ceramics: Correlation with Magnetite Crystallization, with J.R. Jurado-Egea and A K Bandyopadhyay. J. Mater. Sci., Vol. 22, 3602-3606 (1987).

(4) IR and RBS Studies of RF Sputtered Films of Vanadium Doped Silicon Dioxide, with J J Delima, A J Snell and K V Krishna. J. Non-cryst. Sol., Vol. 90, 291-294 (1987).

(4) Transient Current Instabilities in a—Si:Hpni Structures, with W K Choi, S Reynolds, J Hajto, A J Snell, M J Rose, P 0 Le Comber and W E Spear. lEE Proc, Part I, Solid-State and Electron Dev., Vol. 134, 1-6 (1987).

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(2) Thick Film Hybrid pH Sensors, with R E Belford. Sensors and Actuators, Vol. 11, 387-398 (1987).

(2) Temperature Dependent AC Impedance Studies of Glass to Metal Contacts in Solid State Glass pH Sensors, with R E Belford, J. Non-cryst. Sol., Vol. 92, 73-88 (1987).

(4) Compositional Dependence of Photodissolution Kinetics in Amorphous As-S Films, with P J S Ewen, A Zakery and A P Firth. J. Non-cryst. Sol., Vols. 97 and 98, 1127-1130 (1987).

(5) Optical Properties of Spin-Coated Amorphous Chalcogenide Films, with E Hajto, P J S Ewen, R E Belford and J Hajto. J. Non-cryst. Sol., Vols. 97 and 98, 1191-1194 (1987).

(5) Preformed J-V and C-V Characteristics of a—Si:Hpni Junctions with W K Choi, S Reynolds, J Hajto, S M Gage, A J Snell and I M Flanagan. J. Non- cryst. Sol., Vols. 97 and 98, 1331-1334 (1987).

(3) Thick Film Devices, with R E Belford and R G Kelly. Chap 11, pp 236-255 in "Chemical Sensors". Ed: T E Edmonds, pubi: Blackie (1988).

(4) Optical Monitoring of Photodissolution Kinetics in Amorphous As-S Films, with P J S Ewen, A Zakery and A P Firth. Phil.Mag. B, Vol. 57, 1-12 (1988).

(2) Interfacial Electrochemical Aspects of Glass in Solid State Ion-Selective Electrodes, with R E Belford. Chap. 9, pp 316-335 in "Glass and Glass- Ceramics". Ed: M H Lewis, publ: Chapman and Hall (1988).

(5) Chalcogenide Gratings Produced by the Metal Photodissolution Effect, with A Zakery, C Slinger, P J S Ewen and A P Firth. J. Phys. D, (Applied Physics), Vol.21, S78-S81 (1988).

(4) Electrical and Optical Characteristics of Vanadium Doped Amorphous Silicon Dioxide Films Prepared by CVD, with K V Krishna, J J Delima and A J Snell. In 'The Physics and Technology of Amorphous Si02", pp.231-235. Ed. R A B Devine (Plenum Press) (1988).

(4) Photodissolution of Silver in Arsenic Sulphide Films - An EXAFS Study, with A T Steel, G N Greaves and A P Firth. J. Non-cryst. Sol., Vol.107, 155-162 (1989).

(4) Spectroscopic Investigation of Spin-coated As-S Amorphous Films, with E Hajto, P J S Ewen and P 0 Hill. Phys. stat. sol.(a), Vol.114, 832-843 (1989).

(3) The Selective Removal of the Negative High Resolution Photoresist System Ag-As-S, with R E Belford and E Hajto. Thin Solid Films, Vol.123, 129-137 (1989).

(3) A Phenomenological Model of Switching in Metal-Thin Insulator- Semiconductor-Semiconductor Devices: A Development of the Analogy with the Thyristor, with W K Choi, J J Delima and S Reynolds. J. Appl. Phys., Vol.65, 2102-2110 (1989).

(4) Device Applications of Vanadium Doped Silicon Dioxide, with J J Delima, A J Snell, K V Krishna, A Hawryliw and R Thompson. J. AppI. Phys., Vol.65, 4082-4086 (1989).

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(3) As-S/Ag System for Integrated Optics, with M N Kozicki, Y Kharwaja, P J S Ewen and A Zakery. Proc. 6th Intnl IEEE VLSI Multilevel Interconnection Conf. pp.251-257 (1989). (5) Optical Constants of Ag-Photodoped As-S Amorphous Films, with A Zakery, A Zekak, P J S Ewen and C W Slinger. J. Non-cryst. Sol., Vol.114, 109-111 (1989).

(4) Model of Laser Induced Regular and Chaotic Oscillations in Amorphous GeSe 2 Films, with J Hajto, I Janossy and W K Choi. J. Non-cryst. Sol., Vol.114, 304-306 (1989).

(4) Dry Etched High Resolution Positive and Negative Inorganic Photoresist, with E Hajto, R E Belford and P J S Ewen. J. Non-cryst. Sol., Vol.115, 129-131 (1989).

(7) Amorphous Silicon Analogue Memory Devices, with M J Rose, J Hajto, P G Le Comber, S M Gage, W K Choi and A J Snell. J. Non-cryst. Sol., Vol. 115, 168-170 (1989).

(8) Anomalous High Zero Bias Resistance in Metal-Amorphous Silicon-Metal Structures, with S M Gage, J Hajto, S Reynolds, W K Choi, M J Rose, P G Le Comber and A J Snell. J. Non-cryst. Sol., Vol.115, 171-173 (1989).

(3) The Application of Vanadium Doped SiO 2 in EEPROM Devices, with A J Snell and K V Krishna. Appl. Surface Science Vol.39, 368-373 (1989).

(2) Exploratory Observations of Random Telegraphic Signals and Noise in Homogeneous Hydrogenated Amorphous Silicon, with W K Choi, P G Le Comber and M J Rose. J. Appl. Phys. Vol 68, 120-123 (1990).

(4) Observations of Quantised Ballistic Transport in Amorphous Silicon Memory Structures, with J Hajto, M J Rose, P G Le Comber and A J Snell. "Amorphous Silicon Technology - 1990", Materials Research Society Symposium Proceedings, Vol 192, 347-352. Eds: P C Taylor, M J Thompson, P G Le Comber, Y Hamakawa and Arun Madan, publ: MRS, Pittsburgh, Pennsylvania (1990).

(5) The Programmability of Amorphous Silicon Analogue Memory Elements, with J Hajto, M J Rose, A J Snell and P G Le Comber. "Amorphous Silicon Technology - 1990", Materials Research Society Symposium Proceedings, Vol 192, 405-410. Eds: P C Taylor, M J Thompson, P G Le Comber, Y Hamakawa and Arun Madan, publ: MRS, Pittsburgh, Pennsylvania (1990).

(2) A Thyristor Model of Switching in Metal-Thin Insulator-Semiconductor Devices: The Influence of Insulating Layer and Illumination, with W K Choi. J.Appl.Phys, Vol.68, 6447-6452 (1990).

(3) Optical Absorption in As-Se Glasses, with Jon Peturrson and J M Marshall. Phil.Mag.B, Vol.63, 15-31 (1991).

(5) On the Kinetics of Ag Photodissolution in As 2 S3 Chalcogenide Glass Films: Oscillatory Behaviour of the Reaction Rate, with E Marquez, R Jiminez- Garay, A Zakery and P J S Ewen. Phil. Mag. B, Vol 63, 1169-1179 (1991).

(2) Structural Characterisation of Pure and Vanadium Doped Films of R.F. Sputtered Si02 , with J J DeLima. Thin Solid Films, Vol 195, 159-174 (1991).

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(4) Thin Film Electrode: A New Method for the Fabrication of Submicrometer Band Electrodes, with B J Seddon, M J Eddowes, A P Firth and H H J Girault. Electrochem. Acta, Vol 136, 736-771 (1991).

(4) Interference Grating Fabrication in Spin-Coated As 2 S3 Films, with E Hajto, P J S Ewen and R E Belford. Thin Solid Films, Vol 200, 229-237 (1991).

(1) Copper Sensing Ion Selective Electrodes Based on Chalcogenide Glasses, with M C Kennedy. "Sensors - Technology, Systems and Applications", pp 15-21, Ed: K.T.V. Grattan. The Adam Huger Series on Sensors, Series Ed: B E Jones, pubi: Adam Hilger, Bristol (1991).

(5) Amorphous Silicon Analogue Memory Elements, with M J Rose, J Hajto, P G Le Comber, A J Snell and I S Osborne. MRS Symp. Proc., Vol.219, 525-530 (1991).

(2) Quantized Electron Transport in Amorphous-Silicon Memory Structures, with J Hajto, S M Gage, A J Snell, P G Le Comber and M J Rose. Phys. Rev. Letters, Vol.66, 1918-1921 (1991).

(2) Analogue Memory and Ballistic Electron Effects in Metal-Amorphous Silicon Structures, with J Hajto, A J Snell, P G Le Comber and M J Rose. Phil. Mag. B, Vol 63 (Special Festchrift issue for Professor W.E. Spear FRS), 349-369 (1991).

(5) Quantised Electron Effects in Metalla-Si:HlMetal Thin Film Structures, with J Hajto, M J Rose, A J Snell, I S Osborne and P G Le Comber. J. Non-cryst. Sol. Vols 137 and 138, 499-502 (1991).

(4) Electrical Properties of Silver-Doped As-S Glasses, with E Hajto, R E Belford and P S Ewen. J. Non-cryst. Sol., Vols 137 and 138, 1039-1042 (1991).

(5) Analogue Memory Effects in Metalla-Si:H/Metal Memory Devices, with A J Snell, P G Le Comber, J Hajto, M J Rose and I S Osborne. J. Non-cryst. Sot. Vols 137 and 138, 1257-1262 (1991).

(5) Application of Chalcogenide Glasses in Integrated and Diffractive Optics, with A Zakery, P J S Ewen, C W Slinger and A Zekak. J. Non-cryst. Sol. Vols 137 and 138 (1991).

(3) PASS - A Chalcogenide-Based Lithography Scheme for I.C. Fabrication, with M N Kozicki, S W Hsia and P J S Ewen. J. Non-cryst. Sol. Vols 137 and 138, 1341-1344 (1991).

(4) Photodoped Chalcogenides as Potential Infrared Holographic Media, with C W Slinger, A Zakery and P J S Ewen. AppI. Optics, Vol 31, 2490-2498 (1992).

(2) Photo-Induced Changes in Chalcogenide Glasses and their Applications, with P J S Ewen, Chapter 14, pp 287-309 in "High Performance Glasses". Eds: M Cable and J M Parker, publ: Blackie (1992).

(2) Electronic Switching in Amorphous Semiconductor Thin Films, with J Hajto, A J Snell, P G Le Comber and M J Rose. Chap 14, pp 641-701 in "Amorphous and Microcrystalline Semiconductor Devices: Volume II, Materials and Device Physics". Ed: J. Kanicki, publ: Artech House, Boston, USA (1992).

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136. (6) Calculation of the Thickness and Optical Constants of Amorphous As-S Films from their Transmission Spectra, with J Ramirez-Malo, E Marquez, P Villares, R Jiminez-Garay and P J S Ewen. J.Phys. D - Applied Physics, Vol.25, 535- 541 (1992).

137. (6) Aspects of Non-volatility in a-Si:H Memory Devices, with M J Rose, A J Snell, P G Le Comber, J Hajto and A G Fitzgerald. MRS Symp. Proc., Vol.258, 1069-1075 (1992).

138. (6) The Role of the a-Si:H Layer in Metalla-Si:HlMetal Memory Structures, with I S Osborne, J Hajto, M J Rose, A J Snell and P G Le Comber. MRS Symp. Proc., Vol.258, 1076-1080 (1992).

139. (7) Application of Analogue Amorphous Silicon Memory Devices to Resistive Synapses for Neural Networks, with A A Reeder, I P Thomas, C Smith, J Wittgreffe, D J Godfrey, J Hajto, A J Snell, A F Murray, M J Rose, I S Osborne and P G Le Comber. MRS Symp. Proc., Vol.258, 1081-1086 (1992).

140. (7) On the Optical Properties of Ag-photodoped Amorphous As 30S70 Films of Non-uniform Thickness, with E Marquez, J B Ramirez-Malo, J Fernandez- Peña, R Jiminez-Garay and P J S Ewen. Optical Materials, Vol.2, 143-50, (1993).

141. (5) Use of a-Si:H Memory Devices for Non-volatile Weight Storage in Artificial Neural Networks, with A J Holmes, R A G Gibson, J Hajto, A F Murray, M J Rose and A J Snell. J. Non-cryst. Sol., Vols 164-166, 817-82 (1993).

142. (5) DC and AC Measurements on Metalla-Si:H/Metal Thin Film Devices, with J Hajto, A J Snell, J Hu, A J Holmes, M J Rose and R A G Gibson. J. Non- cryst. Sol., Vols 164-166, 821-824 (1993).

143. (3) Linear and Nonlinear Optical Properties of Chalcogenide Glasses, with E Hajto and P S Ewen. J. Non-cryst. Sol., Vols 164-166, 901-904 (1993).

144. (5) Nonlinear Optical Properties of Silver Doped As2S3 , with T I Kosa, R Rangel- Rojo, E Hajto, P J S Ewen, A K Kar and B S Wherrett. J. Non-cryst. Sol., Vols 164-166, 1219-1222 (1993).

145. (7) On the Influence of Ag-photodoping on the Optical Properties of As-S Glass Films, with E Marquez, J B Ramirez-Malo, J Fernandez-Pena, P Villares, R Jiminez-Garay and P J S Ewen. J. Non-cryst. Sol., Vols 164-166, 1223-1226 (1993).

146. (6) Diffractive Infrared Optical Elements in Chalcogenide Glasses, with P J S Ewen, A Zekak, C W Slinger, G Dale and D A Pain. J. Non-cryst. Sol., Vols 164-166, 1247-1250 (1993).

147. (5) Kinetics and Reaction Products of the Photo-induced Solid State Chemical Reaction Between Silver and Amorphous As33S67 Layers, with T Wagner, M Vlcek, V Smrcka and P J S Ewen. J. Non-cryst. Sol., Vols 164-166, 1255-1258 (1993).

148. (5) Metal-semiconductor Transition in Electroformed Chromium/Amorphous Silicon/Vanadium Thin-film Structures, with J 1-Iajto, A J Snell, J Hu, A J Holmes, M J Rose and R A G Gibson. Phil.Mag. B, Vol.69, 237-251 (1994).

149. (5) Near-infrared Optical Nonlinearities in Amorphous Chalcogenides, with R Rangel-Rojo, T Kosa, E Hajto, P J S Ewen, A K Kar and B S Wherrett. Optics Communications, Vol.109, 145-150 (1994).

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150. (4) Index of Refraction Measurements in Photodoped Ag—As 33S67 Films, with T I Kosa, T Wagner and P J S Ewen. Phil.Mag. B. In press.

Patents

Improvement in and relating to photo-conductive cells: Ultrasonic stirring during lead sulphide photocell depositions. British Patent 29557/56.

(1) Improvements in relating to photo-conductive cells: Silver lead sulphide etc photocells, with R W A Gill. British Patent 29558/56.

(1) Improvements in and relating to dielectric materials, with R W Douglas. British Patent 27628/58.

(1) Improved semiconductor device, with J Mayor. British Patent 24081/1972.

(1) Improved semiconductor device, with 0 Sarrabayrouse, P G Le Comber and W E Spear. British Patent 8124248/ 81, European Patent Appin 0072221. Also filed in USA and Japan.

(3) Improvements relating to memory devices, with J Hajto, W K Choi, P G Le Comber and W E Spear. British Patent Appin. 2144911/85.

(2) Memory device threshold switching, with J Hajto, P G Le Comber and W E Spear. British Patent Appin. 2153147/85.

(1) Non-volatile semiconductor memories, with J West, K V Krishna and J J Delima. British Patent Appin. 8521600/85.

(1) Ion selective field effect transistor, with P 0 Le Comber and H Reehal. British Patent Appin 8926/29295.0/89.

(2) Analogue device, with J Hajto, P 0 Le Comber, A J Snell and M J Rose. British Patent Appin 8910854.2GB. Also filed in USA, Japan, Australia and European countries.

(3) Amorphous silicon devices: pulse width control/resistive memory element, with J Hajot, P G Le Comber, M J Rose and A J Snell. UK Patent Appin 9122362.8GB. Also filed under Patent Co-operation Treaty, PCT Nos: GB92/01929; W093/08575. Ap12

Invited Speaker at Numerous Conferences, Summer Schools, Colloquia, etc, eg, since 1982 at: (abbreviated list)

International Centre for Theoretical Physics, Trieste (Italy). Lectures on 'The Glass Transition" at Spring College on Amorphous Semiconductors and the Liquid State, May- June 1982.

NATO Summer School on "Coherence and Energy Transfer in Glasses", Clare College, Cambridge University, England, September 1982. Lectures on "Electronic Transport in Chalcogenide Glasses".

SERC Postgraduate School on "Non-Crystalline Materials", University of Leicester, March 1983.

10th Tnt Conf on Amorphous and Liquid Semiconductors, Tokyo, August 1983. Invited Lecturer.

NATO Advanced Study Institute on "Glass - Current Issues", Tenerife, April 1984.

University of Reading, Department of Physics, November 1984.

University of Nottingham, Department of Physics, Feb 1986.

XIV hit Congress of Glass, Delhi, March 1986. Special invited lecture on "Glass in Electronics".

Indian Institute of Science, Bangalore, Departments of Materials Science, and Solid State and Structural Chemistry, March 1986.

International Conference on Solid State Chemistry, Karlovy Vary Czechoslovakia (Czech. Academy of Sciences), October 1986. Invited speaker.

International Conference on Amorphous Metals and Semiconductors, University of Hyderabad, India, March 1987. Invited speaker.

TEE Meeting on Sensors, London, March 1988. Invited speaker on chemical sensors.

University of East Anglia, Department of Physics, April 1988.

Texas A&M University, Department of Physics, March 1989.

Jet Propulsion Laboratory (JPL), California Institute of Technology, Pasadena, California, April 1989.

Bellcore Research Laboratories, New Jersey, USA, August 1990.

A.F. loffe Physico-Technical Institute, St Petersburg (Leningrad), June 1991.

State University of New York, Alfred, New York, USA, July 1992.

University of Leeds, Houldsworth School of Applied Science, April 1993.

Royal Society of Chemistry, International Conference on Materials Chemistry, University of Aberdeen, July 1993. Invited Lecturer.

Kreidl Symposium, Liechtenstein, July 1994. Invited Lecturer.