US 3820988 A
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United States Patent [191 Tan et al.
[ 1 June 28, 1974 5 METHOD OF SENSITIZING ZINC TELLURIDE  lnventors: Yen T. Tan; Raiinder P. Khosla;
John R. Fischer; Deepak K. Ranadive, all of Rochester, NY.
 Assignee: Eastman Kodak Company,
 Filed: Mar. 13,1972 ] Appl. No.: 234,382
 US. Cl....f. 96/].5, 250/501, 423/508,
. I 7 423/509 [581 Int. Cl. 603g 5/02, G03g 5/04  Field of Search 252/501; 96/] R, I PC, 96/].5; 423/508, 509
 References Cited UNlTED STATES PATENTS 3: $0 w, li filfftfilz 9l 3,104,229 8/!963 Koelmans et al. 252/50! 3,174,823 3/l965 Kopelman 423/509 3,188,594 6/l965 Koller ct al. .252/50l X 3,462,323 8/1969 Groves 423/509 X 3,551,763 12/1970 Hakki 423/508 X OTHER PUBLICATIONS Primary Examiner-Roland E. Martin, Jr. Attorney, Agent, or Firm-Dennis M. DeLeo 5 7] ABSTRACT Zinc telluride can be sensitized by annealing in the presenee of vapors of both zinc metal and a Group IIIA metal dopant. The sensitiz ed zinc telhiride eiz hibits photoconductivity.
. 5 h me N. p iii fl i about 750 to about 900C.
FIELD OF INVENTION This invention relates to semiconductors and to methods for preparation thereof. In particular, this invention relates to means for preparing p-type semiconductors which exhibit photoconductivity.
DESCRIPTION OF PRIOR ART Zinc telluride is a well known compound. Pure zinc telluride is relatively electrically conductive and is not particularly photoconductive. Various attempts have previously been made to alter zinc telluride. For example, Bube and Lind in Phys. Rev. 105, 1711, (1957), first observed photoconductivity in zinc telluride crystals which are doped with indium and aluminum. Further work done by Makaranko and Rybalka reported in Soviet Physics, Semiconductors, 4, 847, 1970) involved infrared quenching in zinc telluride which contained iodine, indium or gallium impurities. Although some photoresponse was noticed in these works, the observed, pho'tocurrent was relatively small. Bube and Lind, for instance, reported a current of only 40 microamps with an applied voltage of 100 volts and an intensity of 1.5 mw/cm Although this prior-work is interesting, there still exists a need for techniques to prepare zinc telluride crystals having relatively high photoconductivity. I
SUMMARY OF THE INVENTION We have found that essentially pure zinc telluride can be rendered photoconductive by an annealing technique. This process results in a p-type'semiconductor having useful photoconductive properties. The resultant p-type materials are useful in electrophotography, in semiconductor junction devices, in photoconductive cells and the like. In addition, the present process provides a means for varying the temperature for peak photosensitivity of the compound by varying the annealing time. 9
DESCRIPTION OF THE PREFERRED EMBODIMENTS The objects of the present invention are accomby the presence of zinc and the addition of electron donors which increase the so-called compensation ratio.
As mentioned above, pure zinc telluride has a relatively high conductivity. This is believed to be the result of native acceptors near the valence band. These native acceptors, which are theorized as being zinc vacancies, allow hole migration in the valence band giving rise to conductivity. In order to reduce this conductivity, we anneal in the presence of zinc and a Group IIIA metal. The presence of the zinc vapor during the annealing process tends to reduce the concentration of native acceptors which aids in the reduction of conductivity. In addition, we introduce an impurity (that is, a Group IIIA metal dopant) within the band gap of the zinc telluride crystal. The introduction of this dopant into the band gap results in compensation of the native acceptor by the ionization of the donor dopant.
After suitable annealing, the zinc telluride is quenched to rapidly cool the annealed compound to about room temperature. The material is quenched while the zinc telluride is maintained in the sealed container with the zinc vapor and Group IIIA metal vapor. This quenching is done in order to maintain the high temperature defect structure of the treated crystal. A typical time for annealing prior to quenching would be a minimum of about 10 hours. The material must be annealed for a period of time sufficiently long so that the zinc and Group IIIA dopant will diffuse beneath the surface of the zinc telluride crystal. For practical purposes, 20 hours is a preferred minimum to insure sufficient diffusion beyond the surface. The material can be annealed for as long as about 150 hours. Further annealing beyond this does not give rise to any significant change in properties. By varying the annealing time, one can vary the temperature for peak photosensitivity. This allows one to essentially obtain a material having a peak photosensitivity at a different temperature.
The starting zinc telluride crystal should be pure material. It may be off-stoichiometric, but should contain no more than about 0.3 mole percent excess of telluriuin. If a greater excess than this is present, the time plished by a process of annealing zinc telluride in the presence of vapors of zinc and vapors of a dopant followed by cooling. The dopant used in this invention is a Group lIlA metal such as gallium and indium, etc.
7 In accordance with this'invention, the zinc telluride to be treated is placed in a sealed container and annealed at elevated temperatures in the presence of vapors of both zinc and a Group IIIA metal. The annealing can be carried out at temperatures between about 600C and the melting point of the zinc telluride. The material can be treated at temperatures below about 600C, but the time required to produce useful results (e.g.-, to reduce the hole concentration sufficiently) becomes excessive. At temperatures above about 950C, the material begins to evaporate and special precautions must be taken to prevent condensation on cooler portions of the container. From the standpoint of efficiency, the preferred annealing temperature range is The exact mechanism of the present process is not fully understood at this time. However, it appears to be a combined effect of a reduction of hole concentration of annealing is changed significantly and difficulties arise in achieving a product useful as a photoconductor or the like. It appears that the presence of a large excess of tellurium essentially negates the effect of the presence of the zinc vapors during annealing and poor results are thus obtained. Zinc telluride crystals useful fortreatment by the present invention can be prepared by standard techniques known in the art. The zinc telluride crystals can conveniently be prepared by the procedures of J. Steininger'and R. E. England as described in Growth of Single Crystals of ZnTe and ZnTe Se, by Temperature Gradient Solution Zoning, Transactions of the Metallurgical Society of AIME, Volume 242, page 444 (1968). Evaporated layers of zinc telluride can also be treated by the process of this invention.
The following embodiments are included for a fur driven through a temperature gradient of 25c/in. The crystals thus formed contain about 0.12 percent excess tellurium. The bulk crystals are cut into Hall spiders about 1mm thick and weighing about 0.1 to about 0.3 grams. The resultant Hall spiders are placed in a quartz vacuum ampoule about 5 to cc. in size with about 0.2 grams of zinc metal of the purity used to make the original crystal and about 0.1 gram of similarly pure gallium metal. The ampoule is evacuated to about 1 X 10" torr and sealed. The ampoule is then annealed at about 835C for about 39 hours and quickly quenched to room temperature in an oil bath. The annealed sam ple is then removed and etched in concentrated solution of hot sodium hydroxide. Lithium contacts are applied by placing a small drop of 10 molar LlNOg solution on the contact area and then heating the sample in a flowing hydrogen atmosphere at about 350C for a minute. This technique is described by Aven and Garwacki in J. Electrochem. Soc, 14, 1063 (1967). Electroless gold from an l-lAuCh solution is deposited on the lithium contacts. The resultant lithium-gold electrodes give good, low resistance ohmic contacts over a range of temperatures. The dark resistance of the sample is measured and found to be greater than 10 ohms. The photoresponse is measured by determining the specific sensitivity S of the sample. Specific sensitivity, as described by R. H. Bube, RCA Eng., 5, 28 (1960), is defined as S AiP/VP where A i is the change in photocurrent, 1 is the distance between electrodes, V is the applied voltage and P is the light absorbed in watts. The specific sensitivity is independent of the applied field and the intensity of the illumination if both vary linearly with photocurrent. The applied field is varied between 2 to v/cm. The specific sensitivity is calculated for samples illuminated with a tungsten source and a monochrometer at 560 nm. The intensity ofillumination is about 1 mw/cm The result of these measurements indicate the specific sensitivity at room temperature to be 0.02. The peak specific sensitivity at 200K is 0.7, which compares with the peak specific sensitivity of 0.1 to 2.0 obtained with highly sensitized cadmium sulfide. A bulk sensitivity may be calculated for the non-current-carrying electrodes of the Hall spider in order to exclude contact resistance. The bulk sensitivity 5,, is defined by the following equation: S A0'(Al l )l' /p where A0- is the bulk photoconductivity and A is the cross-sectional area. The bulk sensitivity of the sample is 1.6 at about 200K and 560 nm. Embodiment 2 Embodiment l is repeated with the exception that the sample is annealed for 64 hours and the peak specific sensitivity is 0.16 and the bulk sensitivity is 2.9. Embodiment 3 Embodiment l is repeated except that the sample is annealed for 99 hours with the resultant peak bulk sensitivity of 2.2 at room temperature and about 560 nm.
' The dark resistance is 10 ohms.
Control No. l
Embodiment 1 is repeated with the exception that the ampoule contains only zinc telluride and gallium metal. The ampoule is annealed for 64 hours and quenched as previously. There is practically no photoconductivity at room temperature of the resultant sample and the peak specific sensitivity is 10" at about l43K and about 560 nm. Control No. 2
Embodiment l is repeated once more except that the ampoule contains only zinc telluride and zinc metal with annealing for about 50 hours. The specific sensitivity at K is about 10 for the resultant material and the dark resistance is about 300 ohms.
Control No. 3
Embodiment l is repeated except that the sample is not subjected to annealing. There is no noticeable photosensitivity and the ratio of light to dark conductivities (signal to noise ratio) is less than 1.01. The dark resistance is about ohms.
Embodiment 4 Embodiment 1 is repeated with the exception that the ampoule contains zinc telluride, zinc metal and indium metal and is annealed for about 63 hours at 850C. The resultant sample shows a peak specific sensitivity of 0.08 and a bulk sensitivity of 2.3 at K and 560 nm.
Control No. 4
Embodiment 4 is repeated with the exception that the ampoule contains zinc telluride and indium metal only and is annealed for 62 hours at 835C. The resultant sample shows practically no photosensitivity at room temperature and a peak specific sensitivity of about 10' at 154K and 560 nm.
Embodiment 5 The annealed zinc telluride formed by the procedure of Embodiment l is ground into a powder and combined with an electrically insulating polymeric binder and coated onto a conductive support. The resultant electrophotographic element is then charged under a corona charger and subjected to illumination. The charge applied to the element is dissipated in the areas of exposure in a manner proportional to the amount of illumination received.
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.
1. A method of preparing photoconductive zinc telluride consisting essentially of the steps of:
1. annealing zinc telluride crystals, having no greater than about 0.3 mole precent excess tellurium, said annealing occurring a. at a temperature in the range of about 600C to the melting point of said zinc telluride, b. for at least about 10 hours and c. in the presence of vapors of zinc and a Group [Ila metal dopant, wherein said vapors of zinc and Group Illa metal dopant are present at the saturation vapor pressure for the temperature of said annealing; and
2. quenching to cool said annealed zinc telluride.
2. The method as described in claim 1 wherein said annealing occurs at a temperature in the range of about 750C to about 900C.
3. The process as described in claim 1 wherein said dopant is selected from the group consisting of gallium and indium.
4. A photoconductive zinc telluride compound consisting essentially of zinc telluride which has been annealed in the presence of a gaseous Group 111A metal dopant by the process of claim 1.
5. The method as described in claim 1 wherein said resultant annealed zinc telluride has a peak specific sensitivity of greater than 10*.