The Semiconductor Field-Emission Photocathode

IEEE TRANSACTIONS ON ELEOTRON DEVICES, VOL. Y>D-21, NO. 12, DECEMBER 1974 ' 785 TheSemiconductor Field-Emission Photocathode DIETER K. SCHRODER, MEMBER, IEEE, R. NOELTHOMAS, JAMES VINE, AND H. c. NATHANSON, MEMBER, IEEE Absfracf-The recently developed large-area field-emission photo- dense, needle-like emitters on flat silicon substrates [S]. cathode is described. It consists of a finely spaced array of point We haverecently succeeded infabricating large-area emitters fabricated by etching ofp-type silicon or other semicon- arrays (up to 6-7 cm2) of p-type silicon field emitters ductor. Uniform emission over areas of 6-7 cm2 have been obtained. [SI. For Si, the spectral response extends from 0.4 to 1.1 pm. Quantum They are photosensitive and emit electrons into vacuum yields of 25 percentat 0.86 pm havebeen measured, which is with an except'ionally high degree of uniformity. Broad- about five times the value reported for the extended S-20 photo- band photoemission has been observed with responsivities cathode and comparable to the best 111-V photoemitters. Calcula- at visible andnear infraredwavelengths comparable tions indicate that quantum yields of up to 40 percent at 0.86 rm and to those of the 111-V negative-electron-affinity photo- 28 percent at 0.9 pm are attainable with the present photocathode structures. Forlow dark current densities, photocathode cooling emitters. In the transmissive mode of operation, primary to temperatures approaching77 IC must be employedat present. quantum efficiencies of 25 percent at 0.86 pm were mea- The dark current is shown to be dominated by surface-generated sured.The high photoresponseis at'tributed Do a high electrons in the space-charge region of the emitters. Effects of phos- tunnelingprobability at the surface and long minority phorus gettering and annealing treatments on dark current are dis- carrier diffusion lengths in the high-resistivity p-silicon. cussed, and the spatial frequency response of the device is determined. The results of a computer study showthat the field The field-emission photocathode is unique in that photo- intensification factor of p-semiconductor field emitters behaves emission is observed without the use of cesium or cesium- quite differently from that of metallic emitters. oxide activat'ion,unlike other types of photosurfaces. Consequently, high-processing tenlperatures and stringent high-vacuum conditions arenot required, thus making I. INTRODUCTION the silicon-array field-emitter conlpat'ible with present-day image-tube fabrication methods. LECTRON emission intovacuum from semi- In this paper, theprinciple and operat'ing characteristics E conductling field emitters under the influence of an of the field-emitter photocathode a,re given. In Section external electric field has been extensivelyinvestigated 11, the physics of photosensitive field emission from during the past few years. These studies were performed p-typesemiconductors is described. The field-intensi- using single-field emitters and indicate clearly that field fication factor, /?,is analyzed in Section I11 by computer emission fromp-type Si [l], Ge [a], andother semi- studies andit is shown that it behaves very differently from conductors [3],[4] is stronglyphotosensitive. Photo- that of metallic emitters. /? is high when the p-emitter electric yields approaching and even exceeding unity have operates in the Fowler-Nordheim region and drops when beenreported for Si [S] and Gc [2],[5]. While single the operating point shifts into the saturation regime, thus emittersare useful for investigativestudies, they are establishinga self-ballasting feedbackmechanism. The hardly practica,l as field-emission photocathodes. However, dark current is discussed in Section IV and is shown to be an array of field emitters such that the electron current the result, of thermalgeneration of electron-hole pa,irs from each emitting tip is proportional to the light falling in the depleted space-charge region a,t the emitting sur- onit,, presents a novel approach to photoemission, and face, domina,ted by surfacegeneration. The effects of such a device could be of considerable importance since phosphorus gettering and annealing treatments showing phot,oemission using the field-emission effect does not the importance of surface states are treated in Section V. havea long wavelengththreshold as encountered in In Section VI, the spectral response and quantum yield cesiated 111-V photocathodes [6]. are given, both experimen.tally and t'heoretically.It) is Attempt's have been made in the past todevelop photo shown thatthe field emitterphotocathode is the only field-emission arrays. Ribik et al. [7] ha,ve reported existing cathode with eflicient photoresponse beyond 1.1- efficient photoemission from small-area (0.5 cm2) field pm wavelength. Finally, in Section VII, the spatial fre- emittersmade of Si a,nd Ge, andArthur and Wagner quency response is analyzed by considering both lateral employed the VLS epitaxialgrowth technique to form minority carrier diffusion and the discrete nature of the device. It is shown t'hat modulationtransfer fun.ction &Innuscript received ?;lay 17, 1974; revised August 8, 1974. This of 40 percentat 15-20 fine pairs/mmare possible work was sttpported in part by the Advanced Research Projects A~~~,~~and mo,litore(~by the ~Tjsion Laboratory, Fort with existing devices. For opt'imized designs, 65 percent at Belvoir, Va., under Contract DAAK 02-72-C-0399. 20 line pairs/mm and limiting resolutions of SO line pairs/ The authors are with the Westinghouse Research Laboratories, Pittsburgh, Pa. 15235. predicted. are mm Authorized licensed use limited to: IEEE Xplore. Downloaded on March 6, 2009 at 10:39 from IEEE Xplore. Restrictions apply. 786 IEEE TRAXSACTIONSON ICLECTRON DEVICES,DECEMBER 1974 11. FIELD EMISSION FR.01LI p-TYPE t SEMICONDUCTORS Field emission from metallic emitters is well described bythe Fowler-Nordheim (F-N)theory [lo] andthe emission current is given by fl'2 I, = 1.54 X 10-6-AA, exp (-6.83 x lo7~$~'~v(y)/E) 4 (1) where E is the electric field, C$ is the work function, A, Inverse Voltage is the emitt'er area, and v(y) is a slowly varying funct,ion Fig. 1. Current,-voltage charactjerktics of a p-typesemiconductor [11] that represents the Schot,tky lowering of the poten- field emitter showing the effects of light and temperature. The tial barrier. The preexponential term is the supply func- energy band diagrams correspond to regions I, 11, and 111, tionand the exponential term is the transmission co- efficient. For C#J N 4-5 eV, fieldemission is normally Almost all experiments in the past havebeen performed observed at E N 1-5 X lo7 V/CIYL Plots of log (I,/E2) withindividual emitters, both for metals and semi- versus l/E are linear. It' is usually more convenient to conductors. When dealing with the simultaneous emission. plot log I, versus l/Ti~,which is also linear to a good ap- from a multiplicityof emitters where va,riationsin emission proximation. In addition to metals, n-type semiconductors fromindividual emitters may exist, the questionarises generally also yield linear F-N behavior [12], indicating as tjo whether F-N behavior (as in region I in the case that asfor metallic emitters,the supply of electrons within of p-Si) is t,o be expected at all. This problem has been the emitter is sufficient for the emission to be determined treated byTornaschke and Alpert [l6], who, by numerical only by the transparencyof the surface barrier. methods, were able to show that groups of up to 100 Field emission fromp-semiconductors is considera,bly emitterswith randomly selected field enhancement more complex for several reasons. Emission can be stronglyfactors and emitting areas do indeed yield linear E?--N influenced by the state of the surface, field penetration plots. Ample experiment'al confirmation of this result, into the semiconductor, Limit,ed a,vailability of electrons, is provided by our data [17) which indicate conclusively and by the fact tha,t emission can arise from botJh con- thatthe simultaneous emission fromalmost a million duction and valence bands. Deviations from a F-N de- emittershas alinear F-N dependenceon volta.ge. It pendence are usuallyobserved for p-typeemitters [11. would thereforeappea,r that, analyses of individual Theoreticaltreatments of fieldemission from semicon- emitters are appropriate to arrays of many such emitters. ductors have been attempted [la] and numerical solu- tionsdo indeed depict cert'ain deviations from F-N 111. FIELDINTENSIFICATION behavior, but in many important aspects,experimental The electric field, E, appearing in the F--N expression observations are not adequately described by theoretical (1) is that at the tipof the field emitter. E can be written predictions. as The generad features of field emission from p-type semi- E == fjEV (2) conduct'orshave been discussed by others [13],[14:] with andare summarized inFig. 1. The four important' re- EV = .Vv/d (3) gimes of operation are shown by regions I-IV. In region I, there is a sufficient number of electrons in the conduc- where d is the anode-to-emitter spacing, VVis the voltage tion band, and theemission current is determined only by applied between anode and emitter, and Ev is tJhe field the tunneling probability at the surface. For the higher due to Vv if the emitt'er surface were planar. It has gen- voltages in 11, field penet,ration creates a depletion region erally been assumed that /?, the field intensification and causes the current to be limited by t'hesupply of factor, depends on1.y on the geometryof the emitt'er andis electrons and not the transparency of the ba.rrier. At the independent of the applied voltage. This is clearly justified still highervoltages of region 111, the field penetration for metallicemitters, where the log (current)-inverse is generally considered tobe sufficiently strongtha,t voltage relationship is linear. Quite a different situation impact ionization in the space-charge region (scr) causes isfound in p-type semiconductors, where the field can the rapid current increaseshown.

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