Preparation and Characterization of (Gesbte)) Semiconductors for Phase-Change Optical Storage Hessa Ali Humeid Al-Shamisi

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2008 Preparation and Characterization of (GeSbTe)) Semiconductors for Phase-Change Optical Storage Hessa Ali Humeid Al-Shamisi

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PREPARATION AND CHARACTERIZATION OF (GeSbTe) SEMICONDUCTORS FOR PHASE-CHANGE OPTICAL STORAGE

By Hessa Ali Humeid AI-Shamisi

Supervised by Dr. Hassan Ghamlouche Physics Department Faculty of Science, UAEU

Dr. Naser Qamhieh Dr. Saleh Thaker Physics Department Physics Department Faculty of Science, UAEU Faculty of Science, UAEU

A Thesis Submitted to the Deanship of Graduate Studies in Partial Fulfillment of the Requirements fo r Degree of Master of Science in Materials Science and Engineering

2008 Acknowledgments

First, always and fo remost, I thank almighty Allah fo r rus blessing and fo r providing me the capability to successfullycomplete trus work.

I would like to express my heartiest thanks and sincere gratitude to my supervisors, Dr.

Hassan Ghamlouche, Dr. Naser Qamrueh and Dr. Saleh Thaker fo r their guidance, support, encouragement, patience, unlimited assistance, critical reviewing and providing me the required fa cilities during the work. 1 fe el both honored and privileged to work with such a group.

Many thanks to Mr. Mohammed Elshaer (lab technician in the physics department) and Mr. Esam Attia (technician in the Central Lab Unit), fo r their help in preparing some experiments and analyzing the samples. Special thanks to Ms. Sadeeqa Ahmed, from the physics department, fo r her assistance and support.

Special appreciation is expressed to my colleagues and friends in UAE University, especially in the physics department.

No words can ever express my great gratitude and special thanks to my great fa mily, especially my parents fo r their great support and my husband fo r his patience and great help throughout my study.

II Abstract

In the last decade, the phase change optical recording based upon chalcogenide glasses has advanced remarkably at full-scale commercial applications in the fo rm of rewritable compact disks (CD-RWs) and digital video disks (DVD-RWs). The principle of phase-change optical recording is based on a thermally induced reversi ble transformation between amorphous and crystalline phases. The search fo r recording materials possessing fa st crystallization rate and long data retention has been pursued. Research results have revealed that Ge-Sb-Te compounds exhibit fa st crystallization, which has been explained in terms of their single-phase structure.

In the present work, electrical properties of Ge-Sb-Te thin films using Impedance

Spectroscopy, Current-Voltage, and Capacitance-Voltage techniques is presented. First, we studied the effect of the electrical contacts, deposited on top of Ge-Sb-Te films, on the measured electrical signal (Impedance Spectroscopy). To do this, three different configurations of the electrical contacts were prepared. The results show that the electrical contacts have no effect when the silver paste is placed on the gold electrodes away from the thin film under test. This guarantees no interaction between the silver paste and the film under test during the heating process. Also the results show that the order of depositing layers (material and the gold) is not important and what matters is the location of the silver paste on the gold electrodes.

Second, the Impedance Spectroscopy and Current-Voltage data shows similar resistance and relaxation frequency temperature dependence with activation energies 0.36 and 0.39 eV, respectively. Therefore the corresponding energy gap is estimated to be 0.72 eV and 0.78 eY. This result agrees with the optical gap (0.7 eV) reported in literature.

This agreement co nfirms that Ge2Sb2 Te5 belongs to the chalcogenide fa mily. In addition,

III the I-V curves show that the amorphous-crystalline transition temperature (Tc is around

135 °C

Third, the Capacitance-Voltage measurements were performed on amorphous samples of Ino.3Ge2Sb2 Te5 and Ge2Sb2Tes thin films in order to study the effect of doping on the electrical properties of Ge2Sb2 Te5. The results show that inducing Indium to

Ge2Sb2 Tes decrease the capacitance of the sample as the bias voltage increases fo r the whole range of temperatures. However, the same behavior is observed fo r undoped sample

(Ge2Sb2 Te5) only at temperatures close to the amorphous-crystalline transition temperature and at high applied bias voltages (� 15 volts). The behavior is attributed to the increase in electron concentration as the bias voltage and temperature increase. For lno.3Ge2Sb2 Te5 sample, the indium can be considered as an additional source of free electrons. This would contribute to a furtherand significant decrease in the capacitance ofIno.3Ge2Sb2 Te5 sample, and eventually the capacitance becomes negative. The nonlinearity in the capacitance and conductivity could be related to the nucleation mechanism as the temperature becomes clo e to the amorphous-crystalline transition temperature. Finally in the conclusion 1 proposed some study to be conducted on Ge2Sb2 Te5 films.

IV TABLE OF CONTENTS

LIST OF FIGUR ES ...... VII

LIST OF TABLES ...... IX

CHAPTER I ...... 1

INTRODUCTION ...... 1

CHAPTER II ...... 5

THEORETICAL BACKGROUND ...... 5

2. 1 Introduction ...... 5

2.2. Glass fo rmation and the amorphous phase ...... 6

2.3 Phase-change materials ...... '" ...... 9

2.3.1 Chalcogenides ...... 10

2.4 Density of states in amorphous semiconductors ...... 13

2 5 Conduction in amorphous semiconductors ...... 16

2.5.1 Extended state conduction...... 17

2.5.2 Hopping conductivity ...... 19

2.6 AC conductivity ...... 20

CHAPTERll ...... 24

SAMPLE PREPARATION AND EXPERIMENTAL TECHNIQUES ...... 24

3. 1 Sample preparation ...... , ...... 26

3.1.1 Preparation of bulk materials ...... · ...... · ··· · ...... 26

3 1.2 Thermal Evaporation Technique (TE) ...... 27

3. 1 . 3 Sputtering Technique ...... · ...... · ...... · 28

3 .1.4 Deposition of electrodes ...... ········ ··································· 28

v 3 2 Mea uring Technique ...... 29

3 2.1 The impedance pectroscopy ...... 29

3 .2.2 Current-Voltage technique ...... 31

3.2.3 Capacitance-Voltage technique ...... 31

CHAPTER IV ...... 32

RESULTS AND DISCUSSIONS ...... 32

4. 1 DC Measurements ...... 32

4.2 Impedance spectroscopy measurements ...... 35

43 Capacitance-Voltage measurements of Ge2Sb2Tes and Ino.3Ge2Sb2Tes ...... 42

...... CONCLUSION ...... 51

REFERENCES ...... 53

VI LIST OF FIGURES

Fig. 1 � Sc ematic of the temperature-time �rofiles associated with recording (leftpan el) and ...... erasure (nght ...... panel) of amorphous marks In a crystalline layer...... 5 Fig. 2 Volume as function of ...... a temperature fo r a liquid, a glass and a crystal. . . 7 Fig. 3 Schemat ions ...... ic representat of a crystalJine and an amorphous structure .. 8

Fig. 4 ...... Composition triangle of Sb, Te and Ge ...... 9 Fig. 5 The periodic table of elements. The chalcogens appear in the

...... sixth group ...... of the table...... 11 Fig. 6 Two crystalline phases of Ge2Sb2 Tes. On the right hand side the pure fc c crystal, and in left hand side the pure hcp crystal...... 12

Fig. 7 ...... Density of states in amorphous semiconductors . .. 15

...... (a) CFO model ...... 15 (b) With bands of donor (E) and acceptor (Es) arisi ng from the same defect (Mott and

...... Davis model) ...... 15 Fig. 8 Illustration of the density of states near the band edge, together with the carrier mobility �(E) the occupation probability fCE) and the conductivity cr(E)...... 16 Fig. 9 Schematic illustration of the temperature dependence of conductivity expected fo r an amorphous semiconductor ...... 20

Fig. 10 Impedance vector representation ...... 21 Fig. 11 Impedance plots fo r series and parallel combinations of Rand C...... 22 Fig. 12 X-ray diffraction spectrum of Ge2Sb2Tes film asdeposited ...... 25

Fig. 13 EDX of Ge2Sb2 Tes ...... 26 Fig. 14 Schematic illustration of experimental arrangement used fo r producing amorphous

Ge2Sb2 Tes bulk samples...... 27

Fig. 15 Schematic diagrams of the electrical contacts . ..· ...... 29

Fig. 16 Connections fo r an impedance measurement ...... 30 Fig. 17 1-Y curves fo r Ge2Sb2Tes (Sample 4) films atdiff erent temperatures a) in the amorphous phase and b) around and afterthe transition to the crystalline phase ...... 33 Fig. 18 Resistance of Ge2Sb2Tes film (Sample 4) as a function of temperature. The dashed line is the linear fitof the resistance in the amorphous state ...... 34 Fig. 19 Resistance measurements as a function of temperature fo r samples prepared at Aachen, Leuven and UAE. The amorphous-crystalline transition temperatures Tc are 145 D D D C, 140 C and 135 C, respectively ...... 35

Fig. 20 Impedance spectra fo r a Ge2Sb2Tes (Sample 4) filmat different temperatures . . . . 37 3 Fig. 21 Frequency at the peak of the semicircle as a function of 10 fT (K1). The slope of the dashed line determines the activation energy ...... 37 Fig.22 Imaginary impedance as a function of frequency fo r sample 1 at different temperatures in the amorphous state...... 39 Fig.23 Imaginary impedance as a function of frequency fo r sample 2 at different temperatures in the amorphous state...... 40

Fig. 24 Imaginary impedance as a function of frequency fo r sample 3 at different...... temperatures in the amorphous state ...... 40

Fig. 25 Imaginary impedance as a function of frequency fo r sample 4 at different...... temperatures in the amorphous state ...... 41 D Fig. 26 Impedance spectra fo r samples 1 and 4 at T = 80 C ...... 41 Fig. 27(a) Capacitance variation of sample S5 as a function of applied bias voltage (-20V to +20Y) fo r temperatures: 45, 65, 85, 105 and 115°C. The frequency is 100 kHz...... 43

Vll Fig. 27(b) Capacitance variation of sample S5 as a function of applied bias voltage (-20V to +20V) fo r temperatures: 45, 65,85, 105 and 1 15°C. The frequency is 1 MHz...... 43 Fig. 28(a) Capacitance variation of sample S6 as a function ofappJied bias voltage (-20V to +20V) fo r temperatures: 65,85, ] 05, 115 and 125 °C. The frequency is 100 kHz. .... 44 Fig. 28(b) Capacitance variation of sample S6 as a function of applied bias voltage (-20V to +20V) fo r temperatures: 65,85, 105, 1] 5 and 125 °C. The frequency is ] MHz...... 44 Fig. 29(a) Capacitance variation of sample S5 as a function of temperature fo r different applied bias voltages, ranging from 0-20 volts. The frequency is 100 kHz. Solid lines are to guide the eye...... " ...... 45 Fig. 29(b) Capacitance variation of sample S5 as a function of temperature fo r different applied bias voltages ranging from 0-20 volts. The frequency is 1 MHz. Solid lines are to guide the eye ...... 46 Fig. 30(a) Capacitance variation of sample S6 as a function of temperature fo r different applied bias voltages, ranging from 0-20 volts. The frequency is 100 kHz. Solid lines are to guide the eye ...... 46 Fig. 30(b) Capacitance variation of sample S6 as a function of temperature fo r different applied bias voltages, ranging from 0-20 volts. The frequency is IMHz. Solid lines are to guide the eye ...... 47 Fig. 31(a) Normalized capacitance of samples S5 and S6 as a function of temperature fo r applied bias voltage of 0 volts. The data is shown fo r both 100 kHz and 1 MHz. Solid lines are to guide the eye. Error bars are systematic and estimated to be about 5 % ...... 48 Fig. 31(b) Normalized capacitance of sampJes S5 and S6 as a function of temperature fo r applied bias voltage of 20 volts. The data is shown for both] 00 kHz and 1 MHz. Solid lines are to guide the eye. Error bars are systematic and estimated to be about 5 % ...... 48

VIII LIST OF TABLES

Table 1: Requirements fo r phase change media ...... 6

Table 2: General information about the samples under investigation ...... 24

LIST OF PUBLICATIONS

1. Sale h T. Mahmoud, H. Gbamlouche, N. Qamhieh, Hessa AJ-Sbamisi "Novel preparation methods of electrical contacts in Ge-Sb-Te thin fi lms", Applied urface Science Vol. 253, (2007), p. 7242-7245.

2. H. Ghamlouche, S. T. Mahmoud, N. Qamhieh, Sadiqa Ahmad and Hessa AI Shamisi "Capacitance-Voltage Measurements of Ge-Sb-Te Thin Films", Physica Scripta Vol 77, p. 1-5 (2007)

3. N. Qamhieh H. Ghamlouche, S. T. Mahmoud, H. AI-Shamisi and S. Ahmad "Electrical Conductivity Measurements in Ge2Sb2Te5 films", J Optoelectronics and AdvancedMaterials Vol. 9, No. 10, p. 3157-3160 (2007).

4. Hessa AI Shamisi, H. Ghamlouche, N. Qamhieh, S. T. Mahmoud, "Electrical Measurements of Ge-Sb-Te Films Using Impedance Spectroscopy", Proc. of The 8th Annual UAE University Research Conference, Al Ain (2007)

IX CHAPTER I

INTRODUCTION

The ability to store information has been an important aspect of human development.

Many events, knowledge and information in the world are preserved fo r future generation.

Optical data storage, is one of the most successful new techniques fo r storing huge amounts of data [Erwin, 2006], [Libera, 1990] and [Ovshinsky, 1968]. Whereas the first optical storage products only allowed reading of the encoded information, nowadays both recordable and rewriteable storage media have been developed.

Phase-change media is a rewriteable optical storage media; recently it is used in many rewriteable products. This success is based on the discovery [Yamada, 1987] of alloys containing elements such as Ge, Sb and Te, and the fo rming of phase-change media to meet

DVD specifications [Satoh, 2001]. Studying the characteristics of this material led to the development of three generations of optical storage products: compact disks (CD), digital versatile disks (DVD), and Blue-ray disks (BD) or high-definition digital versatile disks (HD

DVD).

The fo rmation of small amorphous marks in a crystalline matrix corresponds to writing of information, whereas the re-crystallization of the amorphous spots leads to the erasure of information. In recent years phase-change materials have also become of interest fo r developing non-volatile memories with a number of attractive features [Wuttig, 2005] and

[Luo, 2004]. In applications as a non-volatile memory, the pronounced difference in electrical resistivity is used. The amorphous state has a high resistance. Applying a long voltage pulse locally heats the amorphous region and leads to re-crystallization. Applying a higher voltage pulse to the crystalline state with low resistance leads to local melting and the fo rmation of an amorphous region on rapid quenching. The phase transitions from the amorphous to

o 1 crystalline states, and ice versa, of Ge-Sb-Te and Se-Sb-Te films by applying electrical

pulses have been observed [Tanaka, 2002] and [Nakayama, 2003].

The first materials used were good glass fo rmers such as Te-based eutectic alloys, represented by Te85Gel5 doped with an element such as Sb, S and P [Ovshinsky, 1968].

Although these materials already showed electrical switching that could be used fo r electronic storage, the time fo r crystallization was of the order of microsecond, partly because the first alloys did not crystallize in a single-phase material. The first materials to show fa st re

crystallization and good optical contrast were Ge-Te [Chen, 1986] and Gel J TC6oSfl4Au25

[Yamada, 1986] and [Ohno, 1989]. This give rise to the discovery of a new alloys, such as

Ge1Sb4Te7, GelSb2Te4 and Ge2Sb2Tes [Yamada, 1987 and 1991]. Nowadays Ge2Sb2Tes and related materials such as GeSbTeN, GeSnSbTe, GeB1SbTe, GeBiTe and GeInSbTe [Kojima,

1998 and 2001], [Yusu, 2005] and [Kusada, 2006] have been tried, some of which are frequently used in commercial products. The Ge-Sb-Te composition has been used in commercial optical and electrical devices due to its fa st crystaJJization. The most important properties of Ge2Sb2 Te5 is the fa st transformation between amorphous and crystalline phases.

Ge2Sb2Te5 alloy exhibit best performance in data storage devices due to it's high thermal stability at room temperature, high crystalline rate at high temperatures, and extremely good reversibility between amor phous and crystalline phases [Zhiwei, 2006]. Recently, these materials attracts the attention of the scientist to study and understand the crystallization phenomena [Yamada, 1991], [Tomigava, 1999], [Weidenholf, 1999], [Friedrich, 2000],

[Gonzales, 2000], [Kolobove, 2004], [Wuttig 2005], and [\Vang, 2005]. The importance fo r this research is to achieve a (i) high recording density, (ii) high data transfer rate and (iii) high direct overwrite cyclability. The minimum recorder bit dimension can be significantly decreased by using short wavelength lasers, which results in higher recording density [Liqiu,

1998]. Optimizations of the physical properties of the phase change material; particularly its

2 cry tallization behavior is essential to increase the data transfer rate and overwrite cyclability

[Martijn 2003]. These studies have pointed out that the crystallization of the amorphous

materials requires some incubation time to fo rm the critical nuclei, which limits the speed of devices [Weidenholf, 2001].

Most of the experimental methods used to investigate the amorphous to crystalline transition have shown that resistance measurements are among the most sensitive [Friedrich,

2000]. However, results of DC measurements frequently depend on the nature of the electrical contacts. Morales-Sanchez al. 2003 proposed AC impedance measurements to separate the - et contribution of contacts from the measured electrical response. The impedance measurements in amorphous Ge2Sb2Tes thin films as a function of temperature, have shown that the contacts have a strong influence on their electrical properties. New configurations of electrical contacts have been used in order to minimize their effect on the total measured response of chaJcogenide a110 Ge2Sb2 Te5 using AC impedance spectroscopy technique [Saleh, 2007].

The rapid and reversible amorphous-to-crystalline phase transformation IS accompanied by increase in the optical reflectivity and the electrical conductivity. However, uncertainties about the optical band gap and electrical properties of this material have persisted. Hereafter the optical properties of Ge2Sb2Tes phase change material had been investigated and it is shown that the optical absorption in all phases (amorphous and

crystalline) fo llows the Touc relation (a = B(l1aJ - Eg)2 ,where B is a constant and Eg is often taken as a definition of the optical gap, the so-called Tauc-gap), [Tauc, 1974], which corresponds to the optical transitions in most amorphous semiconductors, [Bong Lee, 2005].

The common fe ature of these glasses is the presence of localized states in the mobility gap.

This is due to the absence of long range order as well as various inherent defects. In chalcogenide glasses, it is assumed that the localized states in the mobility gap are the charged defects D+ and D- with negative effective correlation energy [see section 2.4]. This type of

3 defects u ually pins the Fermi energy level in the middle of the fo rbidden energy gap and result in the absence of electron spin resonance signal [Street, 1978] and [Okamoto, 1984].

The investigation of electrical conductivity in chalcogenide glasses is a valuable tool fo r determining the position of the Fermi energy level in the energy gap of semiconductor materials

The motivat ion of the present work is to investigate the electrical properties and the indium doping effects of Ge2Sb2 Tes phase change materials using different techniques. The electrical measurements such as Current-Voltage (I-V) characteristics, DC conductivity, AC -- conductivity (Impedance Spectroscopy), and Capacitance-Voltage (C-V) analysis will be conducted. The data gathered from these measurements can give us a clear idea about the phase transition temperature, the nucleation process and the energy gap fo r these materials.

This thesis is organized as fo llows; after the introduction in chapter I, a basic

phase-change materials, specially, chalcogenide (Ge-Sb-Te) is theoretical 0 erview of presented in chapter II. In chapter TIl, the preparation of the bulk alloy and thin films of

Ge2Sb2 Tes using sputtering and thermal evaporation techniques in addition to impedance spectroscopy, I-V and C-V techniques are presented. The experimental results are discussed in chapter IV. The conclusion of thework is summarized at the end of the thesis.

4 CHAPTER II

THEORETICAL BACKGROUND

2.1 Introduction

In a rewritable disc, infonnation is stored in the so-called phase-change layer. This is often a chalcogenide alloy, which can be reversibly converted from the crystalline to the amorphous tate by a laser pulse. Applying short consecutive write pulses that cause melt quenching of the initially crystalline recording film controls writing of amorphous marks. .- Erasure of amorphous marks is achieved by heating the phase-change film to intennediate

temperature levels above glass transition temperature (T g) and below melting point (T m) to induce re-crystallization. Characteristic temperature-time profiles that are associated with the write and erase processes are given in figure 1.

Write: melt-quenching Erase: recrystallization

------�

Time Time

crystalline molten amorphous amorphous crystalUne

Fig. 1 Schematic of the temperature-time profiles associated with recording (left panel) and

erasure (right panel) of amorphous marks in a crystalline layer [Erwin, 2006].

5 The phase-change materials used today are a result of a 30-years and still continuing period of empirical optimization of materials. A large number of phase-change materials have been proposed, but only a fe w materials meet the fo llowing requirements:

1- Writability· they have to enable writing of data.

2- Archiving: the stored information has to be stable.

3- Readability' easy to read.

4- Erasability' the information should be erasable.

5- CycJability: the storage medium should allow numerous write/erase cycJes. - These data storage requirements can be translated to media requirements (see Table 1).

Table 1: Requirements fo r phase change media [Erwin, 20061- Storage requirement Material requirement Material property Writability Glass fo m1er Melting pointllayer design, appropriate optical absorption Archival storage Stable amorphous phase High activation energy, high crystallization temperature Readability Large signal to noise ratio High optical contrast Erasability Fast re-crystallization Simple crystalline phase, low viscosity CycJability Stable layer stack Low stresses, low melting temperature

2.2. Glass fo rmation and the amorphous phase

In phase-change optical recording, information is stored by writing amorphous marks in a crystalline phase-change layer. This section will deal with the basics of the process of mark fo rmation and the structure of the resulting amorphous phase. In figure 2 the volume of a liquid is considered that is cooled to below the melting temperature. At the

melting point (Tm ), crystallization may occur. As illustrated in figure 2, crystallization is

accompanied by an abrupt change in volume at (Tm ) . It is, however, also possible that the liquid will become 'undercooled', getting more viscous with decreasing temperature, and

6 ultimatel a solid pha e will [orm. This solid is called a glass and the temperature region in

which the undercooled liquid acquires the properties of a solid is called the glass transition

temperature (Tg). Glass fo rmation is characterized by a gradual break in the slope of the

volume vs. temperature diagram. For a given composition, the value of the glass transition

temperature depends on the cooling rat e. When cooled slowly (dashed line in figure 2), the

glass transition temperature will shift to lower values, as the undercooled liquid has more

time to adjust its propertie to its metastable equilibrium values. However, cooling down

slowly also increases the chance fo r crystallization.

v Liqui I Undercooled liquid I

, � r -- -- � _ Glas�------

j Crystal

Fig. 2 Volume as a functionof temperature fo r a liquid, a glass and a crystal [Elliott, 1990].

The ability to fo rm glasses is almost a universal property of condensed matter. In

order to produce an amorphous material crystallization should be bypassed. Crystallization

7 takes time and, therefore, the amorphous phdse can be reached by cooling ra pidly to below the gJass transition temperature. In a phase-change optical disc, rapid cooling is made possible by the small volume that is amorphized and the special stacking of layers comprising the disc.

Amorphous solid, is a solid like any other if macroscopic properties like shape, shear stiffiless etc. are considered. But, on an atomic scale there is a difference between crystalline and amorphous solids. The main difference between them is the length scale over which the atoms are related to one another by periodicity. A crystal material has an - atomic structure that repeats periodically across its whole volume on the other hand, amorphous materials, like glass, have no long-range order at all This is illustrated in figure

3. For example, all atoms in figure 3(b) satisfy their coordination number, each have three nearest neighbors at nearly the same distance. Nevertheless at short range there is variation in bond strength and fluctuation in bond angles. On the contrary, it has been discovered recently, that the well-known phase-change material Ge2Sb2 Tes shows a substantial difference in short-range order between the amorphous and the crystaJJine state. It still needs to be clarified if this is a generic fe ature of phase change materials.

(a) (b)

Fig. 3 Schematic representations of a crystalline and an amorphous structure [Elliott,

1990].

8 2.3 Pbase-change materials

uitable phase-change materials fo r optical recording have the ability to fo rm glasses at time scales and temperatures that are appropriate fo r given conditions such as data transfer rate and available laser power. This requires that the glass fo rmation takes typically less than 100 ns within an adequately cooled recording stack, and that the melting point is below 1000 DC, typically around 600 °C. Furthermore, the material should possess sufficient optical contrast between the amorphous and crystalline state. Some materials that fulfill these demands and that are currently applied in rewritable CDs and DVDs, ar& indicated in figure 4. The main constituents of these materials are antimony, tellurium and germanium [Erwin, 2006]

Te Sb

Fig. 4 Composition triangle of Sb, Te and Ge [Erwin, 2006].

•

9 Phase-change memory uses the unique behavior of chaJcogenide glass, which can be "switched" between two states, crystalline and amorphous with the application of heat.

early all memory devices make use of a chalcogenide alloy of germanium, antimony and tellurium (Ge-Sb-Te) called GST. It is heated to a high temperature at which point the chalcogenide becomes a liquid. Once cooled, it is frozen into an amorphic glass-like state which has a high electrical resistance. By heating the chalcogenide to a temperature above its crystallization point, but below its melting point, it will transform into a crystalline state with a much lower resistance. This phase transition process can be completed in as quickly as five nanoseconds, which makes chaJcogenides suitable fo r memory devices.

2.3.1 Chalcogenides

Chalcogens are the group 6 elements in the periodic table, as shown in figure 5. they are sometimes known as the oxygen fa mily. They consist of the elements oxygen (0),

sulfur (S), selenium (Se), tellurium (Te), and the radioactive polonium ( Po). The compounds of the heavier chaJcogens (particularly the sulfides, selenides and tellurides) are collectively known as cha1cogenides. Unless grouped with a heavier cha1cogen, oxides

are not considered chalcogenides.

Chalcogenides have special electrical properties. They have no electron spm

resonance and they cannot be doped since they have negative correlation energy defects.

Chalcogenide alloys are glasses containing a chalcogenide element (sulphur, selenium or

tellurium) as a substantial constituent [Mott, 1979].

•

10 cbaJcogeos

IA /0 1 H IIA Periodic Table IliA IVA VA VIA VilA He 3 4 5 6 7 8 9 10 2 Li Be of Elements B C N 0 F He 11 12 15 16 7 18 3 Ha Mg P S CI Ar

19 20 4 K Ca 37 38 5 Rb Sr

55 56 8{) 6 Cs Ba Rn

87 88 7 Fr Ra

It Lanthanide 58 59 60 61 62 63 6<1 65 66 67 68 69 70 71 Series Ce Pr Nd Sm Eu Gd 1b 0)' Ho Er Tm Yb lu + Actinide 90 91 92 93 94 95 96 97 98 99 100 101 102 103 Series Th Pa U

Fig. 5 The periodic table of elements. The chalcogens appear in the sixth group of the table.

Germanium-Antimony-Tellurium (Ge-Sb-Te) GST, is a phase change material from

the group of chalcogenide glasses, used in rewritable optical discs and phase-change

memory applications. The new phase-change materials are likely p-type Ge-Sb-Te

semiconductors. The melting point of this alloy is about 600 °C (900K). At 150 °C Ge-Sb-

Te becomes pure fcc crystal, as shown in figure (6a), then it becomes pure hcp crystal at

400 °C as shown in figure(6b).

�Sb2Te5, whose trigonal crystal structure is depicted on the right-hand side in

figure 6, consists of a sequence of hexagonal layers of Ge, Sb and Te. However, if short

laser pulses are used to crystallize this material, a metastable structure is fo rmed, which is

characterized by six fo ld coordination of the atoms with the cubic arrangement

characteristic fo r the rocksalt lattice.

11 Thi structural arrangement is shown on the le ft-hand side in figure 6. The Te atoms

'occupy each ite of their fc c sub-lattice in the NaCI structure. Alternating Ge, Sb and vacancies occupy the sites of the other sub-lattice. Recent studies find compelling evidence fo r a pronounced local distortion away from the six fo ld coordinated sites, which considerably lowers the energy of the solid, [Erwin, 2006].

(a) (b)

Fig. 6 Two crystalline phases of Ge2Sb2 Tes. On the right hand side the pure fc c crystal,

and in left hand side the pure hcp crystal [Erwin, 2006].

As mentioned before, selenium (Se) and tellurium (Te) are considered semiconductors. In the amorphous phase lack of periodicity or long range order makes the

-> wave vector k no more a good quantum number to describe the electrical properties. The only valid concept in amorphous phase is the density of state.

12 2.4 Density of tate in amorphous semiconductors

everal models were proposed fo r the density of states of amorphous

emiconductors. AJI used the same concept of localized states in the band tails which is different than the sharp bands in a crystaL The difference between these models of the amorphous semiconductor lies in the estimate of the extent of this tailing. An early model, namely Cohen Fritzsche-Ovshinsky (C FO), supposed that the amorphous structure would lead to overlapping band tails of localized states as shown in figure (7a). Those states derived from the conduction band (CB) would be neutral when empty and those from the valence band (VB) would be neutral when full. Such overlapping states would pin the

Fermi-level energy (EF) near the middle of the gap, a fe ature required by the electrical properties of these materials. The other principal fe ature of this model is the existence of

the "mobility edges" at energies Ec and Ev, in the CB and VB tails respectively. The

mobility edges were introduced earlier by Mott, which is why the model is sometimes called the Mott CFO model. The difference between the two energies is called the mobility gap. evertheless, one of the major objections against the CFO model is the high transparency of amorphous chalcogenides below a well defined absorption edge [Mott,

1979]

An alternative suggest ion to the CFO model fo r states in the gap is shown in figure

(7b) [Mott, 1975]. This model assumes that the tails of localized states should be rat her

narrow, and they extend a few tenths of an electron-volt into the fo rbidden gap. The model

also proposed the existence of a band of states near the middle of the gap, arising from

defect centers, e.g. dangling bonds. These stat es may act both as deep donors (D-) and

+ acceptors (D ) (charge defects). Single and double occupancy conditions lean to two bands

separated by an appropriate correlation energy or Hubbard U .

•

13 Adopting the abo e model fo r gap states in tetrahedral amorphous serruconductors such as Ge or i clearly explains many of their electrical properties. For example, the observed electron spin resonance (ESR) signal reveals the presence of unpaired spins ascribed to electrons in dangling bond states. Moreover, at low temperatures, conduction occurres by variable range hopping, indicating that transport properties are dorrunated by a high density of states at EF [Qamhieh, 1996].

For the chalcogenides, the situation is different. There is plenty of evidence that chalcogenide glasses contain high density of gap states with EF pinned near rrud-gap.

However, none of the mentioned phenomena in the previous paragraph is normally observed (unless one first illurrunates the material at low temperatures) [Mott, 1979].

Therefore, a rather different model is required fo r gap states in chalcogenides which gives a good description of many electrical and optical properties of these glasses. The model proposed that localized gap states are charged defects with negative effective correlation energy between electrons. The origins of this energy were soon understood in terms of the

Valence-Alternation-Pair (V AP) model [Kastner, 1976].

•

14 (a) Valence Conduction band b d an

Mobility Mobili edge edge

(b) V l ce a e n Conduction band band

Band tails

Fig. 7 Density of states in amorphous semiconductors

(a) CFO model

(b) With bands of donor (E) and acceptor (Es) arising from the same defect (Mott

and Davis model) [Mott, 1979].

15 2.5 Conduction in amorphous semiconductors

AJthough chalcogenides are p-type (conduction by holes), standard usage is to discuss equivalent conduction properties in terms of electrons. Conductivity is a macroscopic quantity which represents an average property of carriers as they move from site to site. Calculation of the conductivity therefore involves the transfer rate, scattering and trapping processes, as well a the appropriate averaging over the distribution of states. Conductivity is the product of the carner density and the carner mobility

1) a = neJ.1 (

The contributions to (J are summed over the density of states,

a = f N(E)eJL(E)f(E)dE (2)

Where f{E) is the Fermi-function. Figure 8 shows schematically the variation of N(E), �(E) and f{E) with energy E and their contribution to the conductivity fo r states near the band edge.

a(E) N(E) �(E) feE)

Fig. 8 Illustration of the density of states near the band edge, together with the carrier

mobility 11(E), the occupatir)O probability f{E)and the conductivity aCE)·

When there is a very high defect density in the middle of the band gap, as in tetrahedrally coordinated semiconductors like Si and Ge, conduction takes place by hopping at the Fermi

16 energy Hopping conduction can also take place in the band tails where the density of states again becomes large, but the carrier concentration is lower. The much lower defect density in amorphous chalcogenides prevents tills mechanism from contributing significantly at moderate temperatures and instead conduction takes place by electrons or holes at the band edges where both the density of states and the mobility increase with energy [Qamrueh,

1996]. Since our study is at relatively illgh temperatures (above room temperatures), the mechanism of extended states conduction is reviewed in the fo llowing section.

2.5. 1 Extended state conduction The conductivity of any semiconductor can be expressed in the fo rm of Eq.2. The integral contains contributions from el ectron transport above EF and hole transport below

EF When conductivity takes place far from the Fermi level, Boltzmann statistics can be applied to describe the occupancy. The conductivity due to a single type of carrier excited beyond the mobility edge into the extended states is given by

(3)

For the particular case in which the mobility increases abruptly at the mobility edge energy, Ee, according[ to Matt's view,] then Eq. 3 gives E -E (4) a = a rrun exp - C KT F

Omin is referred to as minimum metallic conductivity and is given by

(5) where is the free carrier mobility at Ec. Mott calculated a value fo r given by the !lc, Omin, expressIOn

e2 - (6) a rrun = const. na o

17 where a i the interatomic distance and the constant lies in the range 0026 and 0. 1;

The conductivity in amorphous semiconductors usually activated, IS and IS

described by

(7)

where a. is the conductivity prefactor and Eo is the activation energy. Comparing Eq. 4

and 7, one might conclude that a measurement of

the mobility edge (Ec-EF), and the prefactor gives the conductivity at the mobility edge.

However, the measurements are unhel pfuJ in this regard as there is a huge variation in the

values of a..

The most obvious mechanism by which the prefactor varies is when the activation

energy is temperature dependent,

(8)

where the constant value 8 represents the first order approximation to the temperature

dependence, and (Ec-EF)o is the value extrapolated to zero temperature. Substituting Eq . 8 into Eq. 7, the expression[ fo r the conductivity).] becomes (£ -E (9) a = a . exp(o / k)exp - C kT F

Therefore, within the linear approximation, the conductivity activation energy Eo measures

the value of (Ec-EF) and the prefactor

3 I I They fo und values clustering round 10 n- cm- fo r most amorphous semiconductors. An

estimate fo r 8 can be obtained from the temperature dependence of the optical gap.

18 2.5.2 Hopping conductivity

We have seen that structural defects glve nse to localized states in the gap of amorphous semiconductors. However one of the definitions of localized states is that the average conduction associated with the states at energy E is zero. Thus, conduction involving localized states can take place by hopping of electrons from full states to neighboring empty states, with phonon assistance. Hopping conduction dominates when the Fermi occupation factor in Eq. 2 gives the states nearest to the Fermi energy a sufficiently strong weighted contribution to the total conductivity. There are two ranges of localized tates where conduction by hopping can take place:

1) Conduction in band tails: When carriers are excited into localized states at the

band edges hopping conduction can occur at energies close to the conduction band

edges EA and EB, (figure 7).

2) Conduction in states around EF: If the density of states at the Fermi level is

finite, then there is contribution from carriers with energies near EF which can hop

between localized states by a process analogous to impurity conduction that takes

place in heavily doped crystalline semiconductors. Hopping is to sites with the

smallest distances and is therefore sometimes called nearest-neighbor hopping.

19 Ln (J 1. Extended state conduction Ec-/EF 2. Conduction in band tails

. -- .. , --- / , " 3. Conduction in states around EF -/

Iff

Fig. 9 Schematic illustration of the temperature dependence of conductivity expected fo r an

amorphous semiconductor.

This behavior has been observed in several amorphous semiconductors at low temperatures, where the activated nature of band conduction ensures that it is smaller than the contribution from variable-range hopping. Figure 9 shows schematically the three conduction mechanisms and the change in it on lowering the temperature. If the density of defect states at EF is high then conduction in the band tails will not be dominant at any temperature range and a direct transition from extended state to hopping conduction at EF will result [Qamhieh, 1996]. In the present work, onJy the first part of figure 9 at high temperature is to be considered, where, extended state conduction takes place.

2.6 AC conductivity

AC conductivity is one of the studies done on solids in order to characterize the bulk resistance of the crystalline sample. Measurement of AC conductivity can be done by different techniques. The currently used technique is the complex impedance spectroscopy.

This study also gives information on electrical properties of materials and their interface with electronically conducting electrodes. The complex impedance spectroscopy

20 mea urement of AC conductivity is based on studies made on the measurement of sample impedance/admittance over a range of temperatures and frequencies and analyzing them in complex impedance plane. This is particularly characterized by the measurement and analysis of impedance (2) , admittance (11 and plotting of these functions in the complex plane which is known as Nyquist diagrams. Impedance is a more general concept than resistance because it takes phase differences into account. In AC measurements, the resistance, R, is replaced by the impedance, Z, which is the sum of resistance and reactance.

Impedance can be written as - Z= Z' +Z", (10)

I where Z ' is the real part and Z I the imaginary part of Z. Z is a vector quantity and may be plotted in the plane with either rectangular or polar coordinates as shown in figure 10.

The two rectangular coordinate values are

(1 I) Re(Z) = Z ' = IZI cos

I I (12) Im(Z) = Z = IZI sin

I ")2 ]1/2 = = gnitude of [(Z + (Z with phase angle

Xe I------�

Fig. 10 Impedance vector representation.

21 In polar fo rm, Z may be written as

Z(O) = IZl exp(j 0) , (13)

Where, 0) is the angular frequency exp(j 0)) = cos( 0)) + j sine0)) (14)

In general, Z is frequency dependant. Impedance spectroscopy consists of the measurement of Z(O)) over a wide frequency range. It is from the resulting structure of Z(O)) vs 0) that one derives information about the electrical properties of the electrode material system [padma,

2002]. Impedance plane plots fo r the sample which can be considered as a capacitance (C) in series with the resistance (R), or (C) in parallel with (R) are shown in figure 11.

R (' R

----/'VV � I I _ •• • .-[ -11 ---' (

ZIt (Q) Z" (.0)

1 High freq uency

R R Z'(Q) Z'(Q)

Fig. 11 Impedance plots fo r series and parallel combinations of R and C.

22 Figure 11 also shows, the impedance spectra (Imaginary part of the impedance Z" versus real part Z' of the impedance fo r a certain temperature), where the resistance is equal to the diameter of the semicircle in Nyquist plot. The semicircle starts at high frequency then ends to a low frequency fo r a certain temperature but the shift on this semicircle depends on the temperature as we will see in the next chapters. At the semicircle peak one

at which (1)RC can get the relaxation frequency = 1 .

Impedance spectroscopy technique was used to study the AC conductivity and the capacitance of the samples with a series combination of R and C which is explained in the - next chapters

23 CHAPTER III

SAMPLE PREPARATION AND EXPERIMENTAL TECHNIQUES

Thin films of b2 Tes were prepared by Ge2 two different techniques (Sputtering &

Thermal evaporation techniques) at different places as shown in table 2. (The bulk alloys of glassy Ge2Sb2 Te5 were prepared by rapid melt quenching technique). These alloys have been used as a source in the thermal evaporation technique to deposit filmsat two places,

UAE University and the University of Leuven (Belgium).

- Table 2: General information about the samples under investigation

Sample Sample Contact Co mposition Deposition Tc Prepared at type Technique 1 Aachen Fig. 15-a Ge2Sb2Te5 Sputtering 145 °C University

2 Aachen Fig. 15-c Ge2Sb2 Te5 Sputtering 145 °C University

3 Leuven Fig. 15-c Ge2Sb2TeS Thermal 140 °C University Evaporation

4 UAE Fig. 15-b Ge2Sb2Te5 Thermal 135 °C University Evaporation

5 UAE Fig. 15-b Ge2Sb2Te5 Thermal 135 °C University Evaporation

UAE Fig. 15-b l no.3Ge2Sb2 Tes Thermal 150°C 6 University Evaporation

The amorphous structure and stoichiometery of the films were identified by X-ray diffraction and Energy Dispersive X-ray analysis EDX as shown in figures (12 & 13). X- ray spectroscopy is a technique which allows checking whether the material is amorphous or crystalline. Using this technique, the amorphicity of evaporated Ge2Sb2 Tes film was

24 confirmed. Sample was characterized by the appearance of diffraction lines due to the

fo rmation of a crystalline state. To check the amorphicity of our sample X-ray diffraction

spectroscopy were performed using Philips diffractometer with Cu.:Kt X-radiation source.

Figure 12 shows the diffraction pattern in the range of 28 == 0-80°. The absence of any

sharp peaks indicates the amorphous nature of the quenched material .. EDX is an analytical

technique used fo r the elemental analysis, as shown in figure 13 three peaks appear fo r the

three elements in the sample used Ge2Sb2 Te5 which are characteristic of an element's

atomic structure

Gold electrodes were evaporated fo r electrical contacts usmg three different

configurations. Three different techniques were used to analyze the thin films. 1) The DC

fo ur probes technique has been used to measure the I-V curves and hence the resistance of

the samples. From this technique one can determine the amorphous-crystalline transition

temperature. 2) The impedance spectroscopy technique was used to study the AC

conductivity and the capacitance of the samples. 3) The capacitance-voltage technique was

used to study the capacitance of the film as a functionof temperature and frequency.

1 000

800

600

200

0 0 20 40 60 80 28

Fig. 12 X-ray diffraction spectrum of Ge2Sb2Te5 filmas deposited

25 Counts

2000- Te

1500-

1000- s

500-

....

3.1 Sample preparation

3. 1.1 Preparation of bulk mate rials

Rapid quenching technique has been used to prepare the bulk alloys of the phase- change materials. This technique is historically the most established one and it is still the most widely used in the preparation of amorphous chalcogenide materials [Qamhieh,

1996].. The method consists of weighing and mixing the high purity constituents fo llowed by melting in sealed evacuated quartz ampoules. The melt is continuously rotated to ensure homogenization. Subsequently, the melt will vitrify when cooled byquenching in either air or ice water. More rapid quenching has also been used in preparing amorphous chalcogenides, e.g. roller quenching fo r Sb3S3 and melt-spinning fo r Te-Se, Ge-Se and Te-

Ge alloys.

The bulk alloys of glassy Ge2Sh2 Te5 were prepared at UAE University from

99.995% pure Ge, Sb, and Te by rapid melt quenching technique, as shown in figure 14. 3 The materials with proper constituents were sealed in an evacuated quartz tube (at 10- Pa)

and heated at 1050 °C fo r 24 hours in a rotating cylindrical furnace to ensure

homogenization.

26 r Te G s I --:-Jr

1) Materials in 2) Sealed under -3 a quartztube vacuum =10 Pa

, • " Tube holder " , \ , ,

3) Melts in cylindrical furnaceat 1050°C for 24 hour

Ice-water

4) Quenched in . ----� iced-water

Fig. 14 Schematic illustration of experimental arrangement used fo r producing amorphous

3.1.2 Thermal Evaporation Technique (TE)

This technique is perhaps the simplest and possibly the most widely used method fo r producing amorphous solids in thin film fo nns. TE is normally performed in vacuum and involves resistive or electron-beam heating of a boat containing the material to be evaporated. The material is melted then evaporated and finally condensed onto a cold substrate. This method is suitable fo r relatively low melting-point compounds, such as chalcogenides.

27 3.1.3 puttering Technique

puttering is a physical vapor deposition, process whereby atoms in a solid target material are ej ected into the gas phase due to bombardment of the material by energetic

Ions

The process can be thought of a large cluster of atoms, colide by energetic ion. The first collision pushes atoms deeper into the cluster, subsequent collisions between the atoms can result in orne of the atoms near the surface being ej ected away fr om the cluster. The

number of atoms ej ected from the surface per incident particle is called the sputter yield.

3.1.4 Deposition of electrodes

Gold electrodes were evaporated fo r electrical contacts in three different ways, as

shown in figure 15. In the first method figure (1Sa ), gold .is evaporated on top of Ge-Sb-Te film. In the second method figure (I5b), gold is evaporated first on the glass substrates and then Ge-Sb-Te film is deposited. In the third method the material is deposited on a part of the glass and then the gold is deposited on top of it such that part of the gold electrode

surface touches only the substrate, as shown in figure(] 5c).

28 (a) electrodes

Ge-Sb-Te fihll Glass substrate

(c)

Fig. 15 Schematic diagrams of the electrical contacts.

3.2 Measuring Technique

3.2.1 The impedance spectroscopy

Impedance is an important parameter used to characterize electronic circuits, devices, and the materials used to make devices. Impedance (Z) is generally defined as the total opposition a device or circuit offers to the flow of an alternating current (Ae) at a given frequency, and is represented as a complex quantity which is graphically shown on a vector plane. An impedance vector consists of a real part (resistance, R) and an imaginary part (reactance, X) as shown in figure 10, [Kazunari, 2003].

29 Impedance spectroscopy (IS) is a powerful method of characterizing many of the electrical properties of materials and their interfaces with electronically conducting electrodes It may be used to investigate the dynamics of bound or mobile charge in the bulk or interfacial regions of any kind of solid or liquid material: ionic, semiconducting, mixed electronic-ionic and even insulators (dielectrics) [Ross 1987].

Impedance spectroscopy measurements were performed in this work using 1260

Solartron impedance analyzer in the range of 1Hz-1 0MHz, with amplitude of 0.5 volt and zero bias. Silver paste has been used to make contact between the coaxial cable and the gold electrodes. The measurements were performed at different temperatures from 30-130 dc. The temperature was measured by a K-type thermocouple, and the samples were heated in Argon gas environment. Figure 16 shows the circuit connections fo r an impedance measurement. In this figure the current (I) is applied across the sample and the voltage (V)

then measured across it. The impedance is then given by: is Z = V I

Z60 and Zview have been used fo r data acquisition and analysis. These pieces of software were supplied by Solartron especially fo r the 1260 solartron impedance analyzers.

:------·------FaradayShf�d------: Test Equipment r Pa tc GPIB electrode

ub trate

bTc film

------_.------

Fig. 16 Connections fo r an impedance measurement

30 3.2.2 Current-Voltage technique

Four probes technique was used to measure the DC resistance of three samples which were prepared from the same batches of samples 2, 3 and 4. The samples were heated from room temperature up to 170°C at a heating rate of 5 °C/min.

3.2.3 Capacitance-Voltage technique

Capacitance-Voltage measurements were performed on amorphous and crystalline samples using KeithJey 590 CY meter at two operating frequencies, 100 kHz and 1 MHz.

The circuit fo r the CV Analyzer is similar to the impedance measurement connection shown in figure 16.

The samples were heated inside a Faraday-cage and Argon gas environment. The measurements were performed fo r a sweep of voltages from -20 to +20 volts at different temperatures. K-type thermocouple has been used to measure the samples temperatures.

This measurement is used to find the dependence of both capacitance and resistance on applied bias voltage, temperature and frequency.

For the I-V and C-V measurements, we have written our own programs fo r data acquisition using Labview. This later is a graphical programming tool used mostly in instrument automation.

31 CHAPTER IV

RESULTS AND DISCUSSIONS

4. 1 DC Measu rements

Four probes technique has been used to measure I-V characteristics of sample 4 u mg the sample configuration shown in figure (15b), in addition to two other gold electrodes.

Figure 17 shows the current versus voltage (I-V) curves fo r Ge2Sb2Te5 film at different temperatures (70- 1 50°C). The I-V curves are straight lines at the measured temperature and voltage ranges. This indjcates that the material has an ohmic resistance, which can be calculated from the slope of the I-V curve at each temperature [Qamhieh,

2007].

The resistance of sample 4 is plotted as function of temperature as shown in figure

l 18. A sharp drop in the resistance is evident at T � 135°C Uri T= 2.45 K ). This temperature is the amorphous-crystalline phase transition temperature (Tc) of the material,

[Wang, 2005]. Below this temperature the In R versus liT curve is a straight line in the measured temperature range. The resistance R can, therefore, be expressed by the well known Arrhenius relation [Mott, 1979],

( 15)

the activation energy fo r dark conductivity and is the where a is the conductivity, £.01 is kB

Boltzmann constant. From figure 18, the value of Ev is estimated to be 0.36 eV.

32 3 , I I I ° • T = 70 C (a ) = x 0 T 80 °C - 2f- X ° A T = 90 C * ° x * 'V T = 100 C x "* x * 1 � = ° \l * T 110 C * --. x * V v *' v ... 4

-2 - x '\(

, -3 I I I -10 -5 0 5 10 Vo ltage (V)

. , I I

Ib• 'lI + T = 130 DC - 4 f- .. T = 140 DC

"" = 150 DC • T • a - ")� • f- :It .. ill • - .. Df-... ••••• .. ··tiJrl� ••• ••• • ir- •

I iO

Fig. 17 J-V curves fo r Ge2Sb2 Tes (Sample 4) films at different temperatures a) in the

amorphous phase and b) around and afterthe transition to the crystalline phase.

33 - - ' , -.

T 1 0

• ,-... I �______------___ C 6 � --- 1 0 --y,- Q) Amorphous 0 c: ro � 5 (/J 10 f/) OJ Cl:: Crystal I 4 10 � . ' •

1 0 3 2.3 2.4 2.5 3.0 3.1

Fig. 18 Resistnce of Ge2Sb2 Te5 film (Sample 4) as a function of temperature. The dashed

line is the linear fitof the resistance in the amorphous state.

The amorphous-crystalline transition temperatures (Tc) of Ge2Sb2Tes fi lms have been roughJy determined using resistance measurements as listed in table 2. Sample 2 3 and 4 were heated from room temperature up to 170 °C at a heating rate of 5 °C/min ..

Figure ] 9 shows the sample resistance as function of temperature fo r three samples prepared from the same batch of samples 2, 3 and 4. The sharp drop in the resistance appears at temperatures of 145 °C (sample 2), 140 °C (sample 3) and 135 °C (sample 4).

The temperature at which the sample resistance drops by several orders of magnitude is the amorphous-crystalline transition temperature [Wang, 2005].

34 5 10

' 10

• Sam p ie 2 - • Sam p ie 3 ... Samp le 4

60 80 100 120 140 160 180

o Ternpe rature ( C)

Fig. 19 Resistance measurements as a function of temperature fo r samples prepared at

Aachen, Leuven and UAE. The amorphous-crystalline transition temperatures Te are

° ° 145 C, 140°C and 135 C, respectively.

4.2 Impedance spectroscopy measurements

The impedance spectrum at different temperatures (T= 80- 1 20°C) fo r sample 4 are

shown in figure 20 each point in the impedance spectrum represent the imaginary part of

the impedance Z" versus its real part Z' at a certain frequency and temperature. It can be

observed that at the same temperature, the distribution of the data points fit with one

semicircle from low to high frequency, the so caJled Nyquist plot. A parallel resistor and

capacitor (RC) equivalent circuit accurately fit the frequency data [Barsoukov, 2005].

Hence the electrical properties of Ge2Sb2 Te5 film can be associated with a simple RC

equivalent circuit. R is determined from the radius of the semicircle and C is determined by

35 using the frequency at the peak of each semicircle (relaxation frequency,fo ), where 2 1t fo

RC = 1 hould be satisfied (see figure 1 1). As expected, the diameter of the semicircle keeps decreasing with increasing the temperature. Moreover, the semicircle peak shifts toward higher frequency as the temperature increases. These two behaviors are due to the thermal activation process that occurs in semiconductor materials. The relaxation frequency obtained is plotted against temperature, as shown in fi.gure 21. It is obvious from the figure that the Log of the frequency is increase as the temperature increase according to the

Arrhenius relation. This shows similar phenomena of the conductivity dependence on the - temperature.

(16)

The activation energy, Ev, calculated from the slope of the straight line fit is fo und to be 0.39 e V. This value is relatively close to the one obtained by I-V measurements.

The temperature behavior of the DC and AC measurements indicate that the conduction in these glasses, Ge2Sb2 Te5, is through an activated process having single activation energy in th e investigated temperature range, where both resistance and relaxation frequency have the same temperature dependence. The calculated activation energy values from figures 18 and 21 are close to each other, 0.36 and 0.39 eV, respectively. To first approximation, the value of the activation energy determines the position of the Fermi energy level with respect to the conducting mobility edge. If

Ge2Sb2Te5 film has chalcogenide characters, this means that the Fermi energy level is pinned in the middle of the energy gap, Eg [Mott, 1979]. Therefore the energy gap can be estimated to be Eg= 2Ey. The corresponding energy gap obtained from I-V measurements and AC impedance spectroscopy are 0.72 and 0.78 eV, respectively. This value fo r the energy gap is in good agreement with the optical gap Eo= 0.7 eV measured by [Bong Lee,

2005]. This can be reserved as an evidence fo r the chalcogenide nature defe cts in

36 amorphous Ge2Sb2 Te5 films, which is characterized by the negative effecti e correlation energy charge centers D · and D-.

7x10

.---- 6x10 . -- .. /- • • ___T=BO ·C rJ/ -.-T=90'C T= I OOoC / �T=I IOaC • T= l 2:tC c: � • qu NI n e\'- •

2x10 • • • I • • 1x10

1.2X 10

Fig. 20 Impedance spectra fo r a Ge2Sb2Te5 (Sample 4) filmat different temperatures .

...... '- ...... � 10' ...... - .... o ......

- .2 60 6'5 � ;0 : 90 : 95 3 00 3 05 0001T ( 1) 3 Fig. 21 Frequency at the peak of the semicircle as a function of 10 fT (JC l). The slope of

the dashed line determines the activation energy.

37 Figures 22, 23, 24 and 25 show the imaginary part of the impedance as function of frequency at different temperatures below Tc fo r samples 1, 2, 3 and 4 respectively. Figure

22 shows the presence of two peaks. The peak at low frequency is attributed to the electricai contacts, whereas the peak at high frequency is attributed to the contribution of the material under test [Morales, 2003]. The appearance of the contacts effect in figure22 is due to the fa ct that these measurements have been made using the contact type shown in figure (l5a). Figures 23, 24 and 25 show only one peak at high frequencies that is related to the material contribution, whereas the contribution of the contacts disappears at low ...,.. frequencies. The di appearance of the contacts effect in figures 23, 24 and 25 is due to the fa ct that these measurements have been done using the contact type shown in figures (15c),

(15c) and ( l5b), respectively. Figure 26 shows a typical example of the impedance spectra of samples 1 and 4. It can be seen that the spectrum of sample 1 consists of two well defined semicircles, whereas the spectrum of sample 4 consists of onJy one semicircle. As has been shown above, the low frequency semicircle describes the electrical properties of contacts and the one at high frequency describes the electrical properties of the material. It is evident from the above results that the place where the silver paste is put on the sample plays a crucial role in electrical measurements. In the configuration of figure (l5a), the silver paste is placed on the gold that is deposited over the tested material. During the heating process the silver paste might penetrate the gold and interact with the material.

Such interaction could be the reason fo r having similar behavior fo r the peaks due to the contacts and the one due to the material, as shown in figure22. The decrease in both peaks amplitudes is due to the decrease in the resistance of the material as the sample heats up.

Moreover, both peaks shift toward higher frequency as the temperature increases. This is. due to the activation phenomena of the carriers in the semiconductor materials [Barsoukov,

2005].

38 It is worth tv note that there are no preferences in the order of deposition between the gold and the materials since the effect of the contacts disappears in the configurations shown in figures (ISb) and (1Sc). Beside that, the silver paste should be placed on the gold film only to avoid any direct contact between the silver paste and the material. This will guarantee that there is no interaction between the silver paste and the material during the heating process.

D & • T = 80 C 1 2x10 • T=9 01leo � 0 D & o. ... T = 100 C 1 Ox1 0 � D <> T = 110 C - = D 5 ..,. T 120 C � 8.0x1 0 D *" T = 130 C • N E .. �0 . . I 5 o· 6.0x1 0 o 0 •

0 •

5 4 Ox 10

2. 0x1 05

00 10D 10' Frequency (Hz )

Fig.22 Imaginary impedance as a function of fr equency fo r sample I at different

temperatures in the amorphous state.

39 1.2x1lt T D • - eo C T=9 D o 0 C ... 1.0x10s • .& T = 100 DC • • T = 110 DC o •

8.0x10' T = 120 DC • • ..----, T 130 DC • C n = -- 6.0x10' • cP:> N o • E • 0 0 I • 0 • 4.0x10' .t. ° • • 2. 0x10'

0.0 0 ;2 10 10

Frequency (Hz)

Fig.23 Imaginary impedance as a function of frequency fo r sample 2 at different

temperatures in the amorphous state.

• • 80 • T = Cc • • 4x 1 0 ' 90 0 T = Cc 100 A I = DC 0 110 0 0 T = DC • 3x 10' 120 0 = - T T Cc • 130 a = - t:l T Cc 0 • N 0 M E 2x 10' • I • • �<>a • 1 x l0' .0 0 . � o° .to .... o ..:r o 0 .... � <:P it 0 1 10 0z 10J 10' 1 ti 10' let 1 O� l O Frequency (Hz)

Fig. 24 Imaginary impedance as a function of frequency fo r sample 3 at different

temperatures in the amorphous state

40 2 Ox1 0& 80 + • T = D C . .

= 90 + • 0 T DC 100 .6.. T = DC • • 1 5 xl0& = 110 <> T DC 120 + 00 = D 0 0 - ... T C 130 0 C = - * T DC • <> • N 1 O x10& 0 E • �� • £ • o 4 �<> • £ 0<> � 5 Ox1 05 0 4 <> <> • £ <> • 0 · 0<> .. • 0 £ <> · 0 .- · 0 {> 0 .0 100 10' 1 O� 103 10' 105 10& 10' Frequency (Hz)

Fig. 25 Imaginary impedance as a function of frequency fo r sample 4 at different

temperatures in the amorphous state

00 ,T 1 r I 5 -2.0x10 1 • cP 5 0 Sample -4 Ox l 0 01% 4 • 00 - � 0 • Sample o o a .0 5 0 -6.0x1 0 0 .0 0 a 5 • 0 0 0 0 o� -8 Ox 1 0 0 0 0 0 o I- • • 0 0 -1.0x 1 Oli l- - • 0 0 0 0 c:: fi 0 • - - 1 . 2x 10 f- 1i N -1.4x 10 • f- • -1.6x 1 li • O f- • -1 8x l01i f- • • 1i -2. 0x l0 • • • -2 2x l I Oli 1 1i 0 1 xl li 2x l0· 3x l 0 4 x l 0 O Z· (0)

Fig. 26 Impedance spectra fo r samples 1 and 4 at T = 80 °C

•

41 4.3 Capacitance-Voltage measu rements of Ge2Sb2 Tes and Ino.3GezSbz Tes

Influence of indium doping on the crystallization kinetics of Ge2Sb2 Tes has been investigated by [Wang, 200S]. The results indicate that indium might play an important role in modifYing the crystallization kinetics of Te-based phase change materials.

Therefore, we investigate below the influence of indium doping using Capacitance-Voltage measurements.

Amorphous thin films ofGe2 Sb2 Tes (SS) and lno.3Ge2Sb2 Tes (S6) are prepared by thermal evaporation technique. Both films have a thickness and a separation between the gold electrodes of about 0. 8 � and 1 mm, respectively. Pre-deposited gold electrodes on glass substrates are used fo r the electrical contacts as shown in figure (ISb).

The capacitance variation of Ge2Sb2 Tes (SS) and Ino.3Ge2Sb2 Te5 (S6) with applied bias voltage ranged from -20V to +20V, as shown in figures 27 and 28 at different temperatures fo r 100 kHz and 1 MHz. The two figures (27 and 28) show a bell shape curve where the capacitance attains a maximum value, and it is more pronounced at 100 kHz

frequency and at temperatures close to amorphous-crystalline transition temperature (Tc ).

However a negati e capacitance is observed fo r ample S6 at 100 kHz-frequency and

12SoC temperature, as shown in figure (28a) .

•

42 12 • T = 45DC a D 0 T = 65 C *� *' * 10 * .... T = 85DC * *' * ...... T '* = 105DC * * lL *' ** --Q. 8 * T = 115DC ;* , CD u c: / �, ., 6 == u ., Q. * � ., 4 / #* U �

2 *

-20 -10 20 o 10 Applied bias (V)

Fig. 27(a) Capacitance variation of sample S5 as a function of applied bias voltage (-20Y

to +20Y) fo r temperatures: 45, 65, 85, 105 and 115 °C. The frequency is 100 kHz.

• T = 45 °C 5.0 b 0 T = 65 °C

.... T = 85 °C ...... lL '* T = 105°C --Q. � 4 .0 * T= 115 ° C CD u c: ., � == u ., Q. 3 .0 0.,

-20 -10 0 10 20 Applied bias (V)

Fig. 27(b) Capacitance variation of sample S5 as a function of appliecl bias voltage (-20Y

to +20Y) fo r temperatures: 45, 65, 85, 105 and 1 15 °C. The frequency is 1 MHz.

43 0.0 LL a...

Q) o c � -0.4 ·u (U a... (U U -0.8

-20 -10 o 10 20 Applied bias (V)

Fig. 28(a) Capacitance variation of sample S6 as a function of applied bias voltage (-20V

to +20V) fo r temperatures: 65, 85, 105, 115 and 125 °C . The frequency is 100 kHz.

0.42 • T = 65 °C b 0 T = 85 °C

0.40 A T = 105 °

* T = 115 °c

Q) o c ...... (U 'u (U a... (U U

-20 -10 o 10 Applied bias (V)

Fig. 28(b) Capacitance variation of sample S6 as a function of applied bias voltage (-20V

to +20V) fo r temperatures: 65, 85, 105, 115 and 125 °C. The frequency is 1 MHz.

44 Figures 29 and 30 show, respectively, the capacitance variation of samples S5 and

S6 a a function of temperature fo r different applied bias voltages (0-20 volts) at 100 kHz

(figurers (29a) and (30a» and ] MHz (figures (29b) and (30b) . In figure (29a) the

capacitance increases with temperature fo r bias voltages less than 15 volts and it shows a

maximum at -105 °C fo r voltages greater than 15 volts. In figure (30a) the capacitance

decreases fo r all bias voltages and becomes negative at temperatures greater than 1 15°C. At

high frequency (1MHz) figures (29b) and (30b) show that the capacitance increases with

temperature fo r all bias voltages, and no negative capacitance was observed at high

frequency(1M B).

• Bias = 0 V a 10 Bias = 3 V -0- Bias = o 6 V ----.. Bias = LL -*- 9V / a.. 8 Bias = -*- 12 V Bias = 15 V u -'\l C Bias = 18 V co -T- T5...... 6 Bias = 20 V / : * co a.. �...... *---- co � ��=- e�� v o 4 � ! � r: e � O ;;;:g;;;; � � =-- � '-.�

� 2�� -L�� __ L- -L � � �� --� 40 60 80 100 120

Temperature (C)

Fig. 29(a) Capacitance variation of sample S5 as a function of temperature fo r different

applied bias voltages, ranging from 0-20 volts. The frequency is 100 kHz. Solid lines

are to guide the eye.

45 5.0

Bias = 0 V 4 .5 Bias = b -0- 3 V Bias = .. 6 V -(r- Bias = 4 . 0 9 V Bias = 12 V Bias = v-- 15 V

3.5 Bias = �c T- 18 V -- Bias = co Q 20 V '0 3.0 � co o 25

Temperature (oC)

Fig. 29(b) Capacitance variation of sample S5 as a function of temperature fo r different

applied bias voltages, ranging from 0-20 volts. The frequency is 1 MHz. Solid lines

are to guide the eye.

a 04 O � �

,---. g��$""� LL - Bias =OV Q. 00 Bias = 3 V �� -0- Q) Bias = V u 6 c Bias = 9 V ...... co -0 4 Bias = V "[> 12 co Bias = V Q. ---v-- 15 co Bias = V 0 -0.8 ---T- 18 Bias = 20 V -Q-

-1 .2 20 40 60 80 100 120 140 Temperature CC)

Fig. 30(a) Capacitance variation of sample S6 as a function of temperature fo r different

applied bias voltages, ranging from 0-20 volts. The frequency is 100 kHz. Solid lines

are to guide the eye.

46 040

Bias = 0 V

Bias = -0- 3 V b

Bias = 6 V

Bias = 9 V ...-... 036 Bias = • 12 V • I LL -0 Q '---'a.. Bias = -<;J- 15 V

Bias = 18 V () p� Bias = c 20 V -cu '0 cu 0 . 32 a.. cu 0

Temperature (oC)

Fig. 30(b) Capacitance variation of sample S6 as a function of temperature fo r different

applied bias voltages, ranging from 0-20 volts. The frequency is 1 MHz. Solid lines

are to guide the eye.

To compare the capacitance variation with temperature variation of the normalized capacitance (C/ C ) of S5 and S6 fo r 0 and 20 volts bias are shown in figures (3 1a) and 4_� o c

These figures summarize the three effects: temperature, applied bias -,1b), r -.;spectively. voltage and frequency. A decrease toward negative values of the capacitance fo r S6 at 100 kHz and 0 bias voltages is observed in figure (31a). A more pronounced decrease in the capacitance of S6 at 100 kHz is observed at higher bias (20 volts), as shown in figure(3 1 b).

The capacitance of S5 starts decreasing at high temperature at 20-volts bias, as shown in figure(3 1 b).

47 -- S5 ( 100kHz) 3 S5 (1 MHz) --*- (1 00 kH z) -<>- S6 S6 (1 MHz) 2 �

o 3

-1 a

L-� ____ �____ �______�______�______-2 � 40 60 80 100 120 140

Te mperature (aC)

Fig. 31(a) Nonnalized capacitance of samples S5 and S6 as a function of temperature fo r

applied bias voltage of 0 volts. The data is shown fo r both 100 kHz and 1 MHz. Solid

lines are to guide the eye. Error bars are systematic and estimated to be about 5 %.

-- S5 (100kH z) 0 � u S5 (1 MHz) ----A- � (100 kH z) U -0- S6 _1 (1 MHz) 0 �S6

-2 b

-3 40 60 80 100 120 140

T emperatu re (aC)

Fig. 31(b) Nonnalized capacitance of samples S5 and S6 as a function of temperature fo r

applied bias voltage of 20 volts. The data is shown fo r both 100 kHz and 1 MHz.

Solid lines are to guide the eye. Error bars are systematic and estimated to be about 5

48 The capacitance variation and negative capacitance effect in homogenous semiconductor (barrier free) has been studied by [penin, 1996]. In the case of application of an alternating voltage across a certain semiconducting material, the equivalent capacitance measured by a CV meter is given by [Majeed Khan, 2005]:

_ A aIX r - ( ) rm ] (16) e d [ - 1 22, +UIr where, A is the area of the plate capacitor and d is the dielectric thickness. e ) (rm = 47raIX is the Maxwellian dielectric relaxation time, is e the dielectric constant and r is the dielectric relaxation time. After substituting r m' Eq. (16) can be written as:

(17)

At low temperatures (T« Tc ), the conductivity of Ge-Sb-Te film is low, hence the second term ofEq. (17) can be ignored and the capacitance is given by:

( 1 8) Ce = � 47r d

As the temperature increases and gets close to T crystallization occurs by the c, mechanism of nucleation and growth. In such a mechanism, small crystalline nuclei fo nn initially, which subsequently grow [Erwin, 2006]. In this mechanism, the area of the nuclei increases and the separation between the islands decreases and hence the equivalent capacitance of Eq. 18 increases.

The conductivity of a semiconductor can be increased either by doping or increasing the temperature. Doping with indium can be considered as a source of free electrons that increase the conductivity of the sample. The nonlinear behavior of the sample's capacitance with the applied bias voltage might be related to the nucleation mechanism. Due to nucleation, two phases will be fo rmed in the sample, amorphous film

49 and crystalline islands The interface Uunction) between the two phases might behave as a potential barrier and the charge carriers are accumulated at the junction. At low bias voltage, the electric filed is not sufficient fo r the charge carriers to overcome the barrier.

Therefore, the conductivity of the film is not affected. As the applied bias voltage increa es, the charge carriers gain enough energy from the applied electric field and overcome the barriers and hence the conductivity increases. Therefore, at high temperature, close to T and at high bias voltage the second term of Eq. (17) becomes more dominant c, over the first term and hence the capacitance decreases, as shown in figure(30 a). Since the conductivity of indium doped film (S6) is much greater than the undoped one (S5), the second term of Eq. (17) increases. This leads to a direct decrease of the capacitance fo r all applied bias voltages, as shown in figure (30a). Eventually, the capacitance becomes

. 4 Jrr o-DC·greater t h an one. The p h enomena f·negatIve capacIt ance negati e when IS 0 2 2 £(1 + (u r ) has been observed in other semiconducting materials such as Cd3AS6 [Gould, 1999], CdTe

[Ismail, 1996] and CdSe [Oduor, 1997].

At high frequency (1MH z), the second term of Eq. (17) decreases and the first term is dominant fo r the whole range of temperature and applied voltage. Therefore, at high frequency the capacitance increases as shown in figures(2 9b) and (30b).

50 CONCLUSION

Investigation of the electrical properties of Ge-Sb-Te thin films using Impedance

Spectroscopy, Current-Voltage and Capacitance-Voltage techniques has been presented.

First, the electrical contacts effects on the electrically measured signal have been studied using Impedance Spectroscopy technique. Three different configurations of the electrical contacts are studied. The results show that the effect of the electrical contacts disappears when the silver paste is placed only on the gold electrodes away from the thin film under test. This is related to the fa ct that the silver paste will not interact with the material during the heating process. The results show that the order of depositing layers

(material and the go ld electrodes) is not important, whereas the position of the silver paste on the gold electrode is the more significant.

Second the electrical properties of Ge2Sb2Tes thin films usmg Impedance

Spectroscopy and Current-Voltage measurements have been presented. The data shows similar resistance and relaxation frequency temperature dependence with activation energies of 0.36 and 0.39 eV, respectively. Therefore the corresponding energy gap is estimated to be 0.72 eV and 0.78 eV (This value is in good agreement with the values reported in literature [Bong Lee, 2005]). This agreement between electrical and optical measurements confirms that Ge2Sb2 Tes belongs to the chalcogenide fa mily. The measured amorphous-crystalline transition temperature (Tc) is fo und to be 135 °C.

Third, Capacitance-Voltage measurements have been performed on amorphous samples of I no.3Ge2Sb2 Tes and Ge2Sb2 Tes thin films in order to study the indium doping effect on the electrical properties of Ge2Sb2Tes . The bias voltage, temperature and frequency effects have been investigated. The results show that the capacitance of

Ino.3Ge2Sb2 Tes sample decreases as the bias voltage increases fo r the whole range of

Ge2Sb2 Tes sample at temperatures. However , the same behavior is observed fo r

51 temperatures close to the amorphous-crystalline transition temperature and at high applied bias voltages (� 15 volts) only. The dependence of the sample's capacitance on bias voltage and temperature is attributed to the increase in the free electron concentration as the bias voltage and temperature increase. For lno.3Ge2Sb2 Tes sample, the indium can be considered as an additional source of free electrons. This would contribute to further decrease in the capacitance of l no.3Ge2 Sb2 Tes sample. The negative capacitance effect might be attributed to a significant increase of the film's conductivity due to temperature and applied bias voltage. The nonlinearity in the capacitance and conductivity could be related to the - nucleation mechanism as the temperature gets close to the amorphous-crystalline transition temperature.

Chalcogenide glasses are versatile semiconductor and dielectric functional materials.

They possess a number of well known peculiar properties like photo-induced change of optical parameters and electrical switching. Research options still abound to identifyal loys with very low switching voltages, and to demonstrate a sufficient number of read-write and erase cycle Seeking new materials with very low voltage switching and investigating the optical and electrical properties of these materials are attracting much attention.

Moreover, investigations of photo-physical processes in amorphous chalcogenide layers were extended towards the nanostructure. The nano-Iayered films were in the fo cus of

development of a new photosensitive, optical recording media. These topics provide future research opportunities.

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