United States Patent [191 [11] Patent Number: 4,828,938 Lichtmann et a1. [45] Date of Patent: May 9, 1989

[54] METHOD FOR DEPOSITING MATERIALS CONTAINING TELLURIUM AND PRODUCT OTHER PUBLICATIONS [75] Inventors: Lawrence S. Lichtmann, Redondo Kuech et a1, “Low Temperature CVD Growth of Epi Beach; James D. Parsons, Newbury taxial HgTe on CdTe”, J. of the Electrochemical Soci Park, both of Calif. ety, vol. 128, No. 5, pp. 1142-1144, May 1981. Hoke et a1, “Metalorganic Growth of CdTe and [73] Assignee: Hughes Aircraft Company, Los HgCdTe Epitaxial Films at a Reduced Substrate Tem Angeles, Calif. perature Using Diisopropyltelluride”, Applied Physics [21] Appl. No.: 851,004 Letters, vol. 46, No. 4, pp. 398-400, Feb. 1985. [22] Filed: Apr. 11, 1986 Primary Examiner-Sadie L. Childs Attorney, Agent, or Firm-Wanda K. Denson-Low; V. [51] Int. Cl.‘ ...... B32B 9/00; C23C 16/30 G. Laslo; A. W. Karambelas [52] US. Cl...... 428/689; 427/255; 427/255.2; 428/697; 428/938; 437/225 [57] ABSTRACT [58] Field of Search ...... 427/255.2, 255, 87, A method for chemical vapor deposition of materials 427/314; 428/689, 697, 938; 437/225 containing tellurium, such as cadmium telluride and ' [56] References Cited mercury cadmium telluride, wherein the reactant U.S. PATENT DOCUMENTS \source of the tellurium is a tellurophene or methyltel lurol. These reactant sources have high vapor pressures, 2,932,592 4/ 1960 Cameron ...... 427/87 and the reactant source vapors emitted from the reac 3,224,912 12/1965 Ruehrwein 427/87 tant sources have decomposition temperatures of less 3,825,439 7/1974 Tick ...... 427/ 87 3,966,512 6/1976 Nonaka ...... 427/87 than about 300° C., so that deposition may be accom 4,058,430 11/1977 Suntola et al...... 427/87 plished at low temperatures of about 250° C. The reac 4,352,834 10/1982 Taketoshi et a1...... 427/87 tant source vapor containing tellurium is mixed with a 4,445,965 5/1984 Milnes ...... 427/87 reactant source vapor containing another substance to 4,509,997 4/ 1985 Cockayne et al...... 427/87 be codeposited, such as dimethylcadmium or dime- » 4,566,918 l/l986 Irvine et a1...... 427/87 thylmercury, and contacted with a substrate maintained 4,568,397 2/1986 Hoke et a1...... 148/175 4,650,539 3/1987 Irvine et a1...... 427/255.2 at the deposition temperature, the deposition being pref erably accomplished in an inverted vertical chemical FOREIGN PATENT DOCUMENTS vapor deposition reactor. 135344 3/1985 European Pat. Off. , ...... 148/175 1001234 2/1983 U.S.S.R...... 427/87 20 Claims, 2 Drawing Sheets US. Patent May 9,1989 Sheet 1 of2 4,828,938

4,828,938 1 2 cadmium telluride, there must further be a reactant METHOD FOR DEPOSITING MATERIALS source of mercury. Reactant sources for all three of CONTAINING TELLURIUM AND PRODUCT these components are known, and successful chemical vapor deposition of both cadmium telluride and mer BACKGROUND OF THE INVENTION 5 cury cadmium telluride has been accomplished, at sub This invention relates generally to chemical vapor strate temperatures of about 350 C. and higher. deposition of semiconducting thin ?lms, and, more spe Chemical vapor deposition of laterally large thin ci?cally, to deposition of ?lms of materials containing ?lms of (II, III, IV-tellurium) materials such as those tellurium. based upon cadmium telluride at a substrate tempera~ Thin, single-crystal ?lms of doped semiconductor ture of less than 300° C., and preferably about 250° C., materials play important roles in many devices such as would be highly desirable, but has heretofore not been electronic components, lasers and detectors for detect possible by unassisted pyrolysis. When conventional ing particular types of radiation. The development of deposition temperatures of 350° C. and higher are used, these devices depends upon the ability to fabricate thin there can be signi?cant diffusion of atoms within the ?lm crystals having the required chemical composition, structure and between adjacent regions of different level of dopants, degree of crystalline perfection, sur composition and dopant levels, thereby altering the face quality, and thickness uniformity over large areas. crystal perfection and reducing intentional internal The thin ?lms are typically on the order of 10 microns composition gradients that are engineered into the thin or less in thickness, and are fabricated by deposition on ?lms to produce speci?c electronic properties, as in the av substrate. 20 preparation of heterojunctions. . Materials formed from elements in groups II, III, and The ability to deposit such thin ?lms at low tempera IV of the periodic table (and their related subgroups tures is currently limited by the unavailability of reac such as 11a and 11b), together with elements in the group tant sources of tellurium having vapors that decompose VI, have been found to have important semiconducting at low temperatures. Currently preferred tellurium properties that can be applied in a variety of devices. sources are the organometallic compounds dimethyltel Among such (II, III, IV-VI) materials are those luride and diethyltelluride, neither of which achieves wherein the group VI component is entirely or partly complete thermal decomposition below 450° C. Addi tellurium, these materials being termed (II, III, IV-tel tionally, these compounds can contain oxygen impuri lurium) materials. Such materials can be compounds ties which can cause polymerization of tellurium if pho with narrowly de?ned composition limits or solid solu tocatalysis is used to provide non-thermal energy to tions with variable composition ranges. reduce the deposition temperature. Even if only a par One class of (II, III, IV-tellurium) materials of partic tial decomposition is accepted, the lowest deposition ular interest is based upon cadmium telluride. Doped cadmium telluride thin ?lms have found particular ap temperatures that can be achieved with known reactant plication in electronic devices. Doped mercury cad sources of tellurium in unassisted pyrolysis is about 350° mium telluride thin ?lms are nearly ideal for detecting C. radiation in the near and far infrared ranges. Focal plane There have been proposed several approaches to arrays for far infrared detectors can be fabricated in achieving reducing decomposition temperatures for such ?lms. The promise of mercury cadmium telluride thin ?lms of materials such as cadmium telluride. Light in this and other applications such as far-infrared detect 40 energy can be supplied to assist in decomposition in a ing diodes and superlattices has been demonstrated, but process termed photocatalysis, but deposition rates and the ability to fabricate the required thin ?lm crystals uniformity of the ?lm being deposited are dif?cult to with suf?ciently large lateral dimensions and uniform control. The ?lms can be grown at low temperatures thicknesses has yet to be established. where decomposition is incomplete, but there may re One approach to the fabrication of (II, III, IV-tel sult excess carbon contamination and incorporation of lurium) materials such as cadmium telluride and mer second phases of unreacted metal alkyls in the ?lm. cury cadmium telluride is chemical vapor deposition, Extremely precise control of the substrate temperature wherein reactant source vapors containing the various is also required, since the film growth rate is tempera constituents of the material are reacted at the surface of ture dependent. Slight variations in substrate tempera the selected substrate. Chemical vapor deposition offers ture across a large substrate can result in ?lm thickness the potential for close control of the material properties variations, which can render the resulting ?lm unusable, of the ?lm such as chemistry, dopant levels and crystal as, for example, in a focal plane array detector. line perfection, and of the physical properties of the ?lm Decomposition of the tellurium reactant source such as surface quality and thickness uniformity. With vapor can also be promoted by heating the walls of the the proper selection of the reactant source vapors and 55 chemical vapor deposition reactor, but this can lead to reaction conditions, such a reaction results in deposition heterogeneous nucleation in the gaseous phase and pre of a thin ?lm of the material of interest onto the surface mature condensation. Yet another approach is to use of the substrate, with the undeposited portions of the elemental tellurium as the source of tellurium, but the reactant source vapors being carried away from the walls of the gas supply system and the reactor must be substrate and out of the system in a flowing gas stream. heated, leading to the same problems as just discussed. The energy to effect the reaction can be supplied by Thus, while it is possible to prepare thin ?lms of (II, heat or other means such as light. The success of this III, IV-tellurium) materials such as cadmium telluride chemical vapor deposition technique for any particular and related materials by chemical vapor deposition material depends directly upon selection of the proper techniques, the known approaches all involve deposi reactant sources for the substances to be codeposited. 65 tion at excessively elevated temperatures or the use of Focusing on one particular example, in order to de modi?cations that introduce unacceptable side effects. posit cadmium telluride, there must be reactant sources There exists a need for a method of preparing such thin of cadmium and tellurium, while to deposit mercury ?lm materials, having controllable chemistry and dop 4,828,938 3 4 . ant levels, and a high degree of crystalline perfection, wherein a substrate is maintained at a temperature suf? and in a physical form having an acceptably smooth cient that the reactant source vapors in the mixture surface and uniform thickness, all of these features being decompose and codeposit onto the substrate a thin ?lm attainable in ?lms of large lateral dimensions for use in of a material containing tellurium and the other sub devices such as detectors. The method should also be strate being codeposited. Thin ?lms produced by this reproducible and economical, with an acceptably high technique using a growth temperature of 250° C. can be growth rate of the ?lm. The present invention ful?lls made with high surface quality and thickness variations this need, and further provides related advantages. that are so small that they cannot be measured using SUMMARY OF THE INVENTION ' apparatus sensitive to variations of 3 percent in thick 10 The present invention resides in a method of deposit ness. ing thin ?lms of (II, III, IV-tellurium) materials by More generally, a method for depositing on a sub chemical vapor deposition. The method allows deposi strate a thin film of a material containing tellurium com tion at substrate temperatures of less than 300° C. Depo prises the steps of providing an organometallic reactant sition of laterally large, uniform thin ?lms of doped and 5 source of tellurium, the source being substance that undoped (II, III, IV-tellurium) materials such as cad produces a molecular reactant source vapor containing mium telluride and mercury cadmium telluride has been tellurium and has a vapor pressure of at least 0.5 milli accomplished using the approach of the invention at a meters of mercury at ambient temperature, the reactant substrate temperature of 250° C. The growth rate in this source vapor having a decomposition, temperature, range of temperatures is independent of temperature whereat elemental tellurium is released, at less than and intensity of the ambient light, leading to excellent about 300° C. and having a molecular half life of less uniformity of ?lm thickness over large areas. Films than about 1 second at a temperature of from about 200° having a thickness on the order of about 1 micrometer C. to about 300° C.; mixing the reactant source vapor and a lateral size of about 4 square centimeters, and containing tellurium with a reactant soutrce vapor con without measurable thickness variation, have been fab 25 taining another substance to be codeposited with the ricated. The maximum growth rate at 250 C is about 5 tellurium, the other substance being selected from the mi crons per hour, twice that achieved by other low group consisting of elements from group II, group III, temperature techniques. The electrical characteristics and group IV of the periodic table, and mixtures of the ?lms are comparable with the best results ob thereof; and contacting the mixture to a substrate main tained by other techniques. 30 In accordance with the invention, a method of depos tained at a temperature of less than about 300° C., iting on a substrate a thin ?lm of a material containing whereupon the reactant source vapors in the mixture tellurium comprises the steps of providing a reactant react to codeposit a material containing tellurium upon source vapor containing tellurium selected from the the substrate. A variety of (II, III, IV-tellurium) materi group consisting of a tellurophene and methyltellurol; 35 als, including cadmium telluride and mercury cadmium mixing the reactant source vapor containing tellurium telluride, can be prepared using this approach. This with a reactant source vapor containing another sub method is preferably practiced using an inverted verti stance to be codeposited with the tellurium; and con cal chemical vapor deposition reactor to achieve uni tacting the mixture to a substrate maintained at a tem form growth over large lateral areas. The reactant perature whereat the reactant source vapors in the mix source of tellurium is preferably a tellurophene or me ture decompose and codeposit a material containing thyltellurol. tellurium upon the substrate. The method is applicable As will now be appreciated, the method of‘ the pres to (II, III, IV-tellurium) compounds. In the most impor ent invention can be used to deposit large thin ?lms of tant embodiment, the substance codeposited is cad materials containing tellurium, having uniform thick mium, mercury, or combinations of cadmium and mer 45 ness and excellent surface smoothness. High quality cury, the resulting thin ?lm being cadmium telluride or doped and undoped cadmium telluride and mercury mercury cadmium telluride. A preferred reactant cadmium telluride thin ?lms can be grown in commer source vapor containing cadmium is dimethylcadmium, cial sizes and at economical growth rates, at growth and a preferred reactant source vapor containing mer cury is dimethylmercury or diethylmercury. Preferred temperatures wherein growth is independent of temper substrate materials are cadmium telluride and indium ature and light intensity, and wherein atomic diffusion antimonide, but there is no limitation to these particular in the ?lm is sufficiently low that layered structure substrates. The temperature of the substrate is prefera having abrupt gradients and transitions are feasible. bly from about 200° C. to about 300° C., most preferably Other features and advantages of the present invention about 250° C. 55 will become apparent from the following more detailed In another embodiment that is particularly useful in description, taken in conjunction with the accompany preparing the ?lms of large lateral extent, a method of ing drawings, which sets forth, by way of example, the depositing on a substrate a thin ?lm of a material con principles of the invention. taining tellurium comprises the steps of providing a BRIEF DESCRIPTION OF THE DRAWINGS reactant source vapor containing tellurium selected 60 from the group consisting of a tellurophene and methyl FIG. 1 is a side elevational view of an inverted verti tellurol; mixing the reactant source vapor containing cal chemical vapor deposition reactor for growth of tellurium with a reactant source vapor containing an thin ?lms; other substance to be codeposited with the tellurium, FIG. 2 is a side sectional view of the reactant gas the other substance being selected from the group con 65 supply system for the reactor of FIG. 1; and sisting of cadmium, mercury, and combinations of cad FIG. 3 is an Auger pro?le of mercurcy cadmium mium and mercury; and introducing the mixture into an telluride thin ?lm grown on a cadmium telluride sub inverted vertical chemical vapor deposition reactor, strate. 4,828,938 5 6 tal base portion having an opening 34 therein. A plural DETAILED DESCRIPTION OF THE ity of holes 36 are symmetrically located in the wall PREFERRED EMBODIMENT portion of the pedestal 26, preferably such that the top Chemical vapor deposition (CVD) is a process most portion of the holes 36 is level with the lower wherein a thin ?lm of a material composed of selected surface of the flange 32. The pedestal 26 is mounted substances or constituents can be codeposited onto a within the reactor tube 12 by placing the outwardly substrate in a precisely controlled manner. The sub ‘extending ?ange 32 over the upper surface of the lip 24 stances to be codeposited are provided in reactant to be supported thereby. An optional spacer 40 may be source vapors, which are typically prepared from solid provided between the supporting lip 24 and the ?ange or liquids termed reactant sources (although in some 32. In this manner, the cylindrical axis of the pedestal 26 cases reactant source vapors can be furnished directly in is maintained coextensive with the midline of the reac gaseous form). A reactant source vapor is produced by tor tube 12. The lower end portion of the pedestal 26 is contacting a carrier gas to the reactant source, whereby maintained in a plane substantially perpendicular to the molecules of the reactant source vapor become mixed vertical midline of the reactor tube 12. The opening 34 into the carrier gas due to their escape from the reactant in the pedestal 26 is also symmetrically centered about source because of its vapor pressure. The carrier gas the midline of the reactor tube 12. carries the reactant source vapors into a reaction tube to The inner base portion surface of the pedestal 26 is contact a substrate upon which the ?lm is deposited. recessed uniformly, concentric with the opening 34, to Energy is supplied at the point where the reactant eceive the substrate 30. The substrate 30 is thereby source vapors contact the substrate, the energy being mounted facing vertically downwardly, and is aligned thermal energy, light energy, or other appropriate type. perpendicular to, and symmetric about, the midline of The energy causes bonds within the molecules of the the reactor tube 12. The susceptor 28, preferably a solid reactant source vapor to break, freeing the constituents cylindrical block of graphite optionally coated with a that are to be codeposited. With the selection of the sealing layer of silicon carbide, rests atop the substrate proper reactant source vapors, the desired material is codeposited as a ?lm on the surface of the substrate, and External to the reactor tube 12, a water cooling the remaining components of the reactant source vapors jacket 46 permits the direct temperature control of the are swept away in the ?owing gas stram and thence out inner reactor tube surfaces 18 and 20. A conventional of the reaction tube. The deposition continues until a radio frequency (RF) coil 48 is positioned opposite and desired thickness of the material is built up on the sub 30 encircling the susceptor 28 to heat the susceptor 28 and, strate. in turn, the substrate 30, by induction heating. Alterna In practicing the preferred embodiment of the inven tively, a radiation heater can be employed to heat the tion, a reactant source vapor containing tellurium and substrate 30. Such a radiation heater is preferably posi reactant source vapors containing sources of other sub tioned in the cavity of the pedestal 26 in close proximity stances to be codeposited are contacted simultaneously 35 to the substrate 30. A thermocouple 50 is inserted to the surface of a substrate in an inverted vertical through an air-tight electrical lead opening 52 in the chemical vapor deposition reactor 10 of the type de reactor tube cap 14 and into the cavity of the pedestal picted in FIG. 1. The reactor 10 comprises a glass reac 26 to measure temperature as chemical vapor deposition tor tube 12 and a glass reactor tube cap 14. A gasket 16 proceeds. is provided between the reactor tube 12 and the cap 16 A mixture of the reactant source vapors is introduced to permit an air tight seal between the tube and the cap. into the reactor tube 12 through the gas inlet 22 and A substantial length of the reactor tube 12 encloses a contacted to the heated substrate 30, whereupon a thin substantially cylindrical volume, preferably having a film layer containing decomposition products of the circular cross section that is de?ned by the inner reactor reaction of the reactant source vapors is deposited. The tube surface 18. The length of this portion of the reactor 45 residual reaction products not deposited onto the sub~ tube 12 is at least twice its diameter and preferably strate 30 are removed from the vicinity of the substrate between three and ?ve times its diameter. The reactor 30 and eventually from the reactor tube 12 through the tube 12 is oriented vertically such that the reactor cap gas outlet 23, by the forced ?ow of additional gas intro 14 is positioned above the remainder of the reactor 10. duced into the reactor tube 12 through the gas inlet 22. The lowest portion of the reactor tube 12 is a generally The reactant source vapors are produced and mixed cylindrical funnel shaped inner reactor tube surface 20 with a carrier gas outside of the reactor tube 12 in a gas terminating at the lowest end in a gas inlet 22. The supply system 54, as illustrated in FIG. 2. A carrier gas mixture of reactor vapors is introduced into the reactor is provided from a supply 56. The carrier gas for the tube 12 through the gas inlet 22, and the residual gases deposition of materials containing tellurium is prefera remaining after the deposition reaction are removed bly hydrogen. Hydrogen from the supply 56 is diffused through a gas outlet 23 at the upper end of the reactor through palladium in a carrier gas cleaner 58, to remove tube 12. The taper of this funnel shaped portion of the impurities that might otherwise later be unintentionally reactor 10 should be less than approximately 50 degrees deposited upon the substrate 30. Portions of the carrier as measured from the vertical mid-line of the reactor gas are passed through subsections of the gas supply tube 12. system 54 to collect fractions of reactant source vapors A supporting lip 24, inwardly protruding from the for delivery to the reactor 10. In the embodiment illus cylindrical inner reactor tube surface 18, provides sup trated in FIG. 2, a portion of the hydrogen carrier gas port for a pedestal 26, susceptor 28, and substrate 30. is bubbled through liquid dimethylcadmium in a bub The pedestal 26 is a generally cup-shaped cylinder hav bler 60 to incorporate a partial pressure of the dimethyl ing a wall portion terminating at the top most portion of 65 cadmium reactant source vapor into that portion of the the pedestal 26 as an outwardly extending flange 32. hydrogen stream. Another portion of the hydrogen The wall portion extends inwardly at right angles at the carrier gas is passed over solid 2,5-dihydrotellurophene bottom most portion of the pedestal 26 to form a pedes in a sublimber 62 to incorporate a fraction or partial 4,828,938 7 8 pressure of 2,5-dihydrotellurophene reactant source Tetrahydrotellurophene, C4HgTe, is fully saturated vapor into that portion of the hydrogen stream. In a with two hydrogen atoms joined to each of the four similar fashion, additional components can be added to carbon atoms: the hydrogen gas stream in a supplementary source, schematically illustrated at reference numeral 64. The H I H H H additional components could include, for example, a mercury-containing reactant source vapor orreactant source vapors for dopants to be included in the depos \c/ \C/ / \Te/ \ ited ?lm. The portions of the hydrogen carrier gas H H stream to which reactant source vapors have been added are then mixed with a diluent portion 66 of the Tetrahydrotellurophene has a lower decomposition hydrogen gas stream that adjusts the overall concentra temperature than C4H4Te, but also a much lower vapor tions of the reactant source vapors to appropriate levels pressure. and dilutes the gas stream. The flows of the gases are 2,5-dihydrotellurophene, C4H6Te, is a partially satu determined and controlled by electronic mass flow rated molecule wherein two of p the carbon atoms are controllers 68 that adjust and maintain precise gas flows joined by a double bond and each has one attached during extended deposition procedures. hydrogen atom, while two of the carbon atoms each To provide for a deposition rate that is suf?cient for have two attached by hydrogen atoms: economical operation of the CVD reactor 10, the re 20 spective reactant source from which each reactant source vapor is produced must have a suf?ciently high vapor pressure that a high flow is introduced into the c: carrier gas. It has been determined that the vapor pres sure of the reactive source of tellurium should be at least 25 about 0.5 millimeters of mercury at an ambient tempera ture of about 22 C to obtain a useful growth rate of the 2,5-dihydrotellurophene is the presently preferred ?lm containing tellurium. tellurophene for use as a reactant source vapor for The obtaining of a sufficient ?ow rate of the reactant chemical vapor deposition of tellurium, as its vapor source vapor containing tellurium must be accompanied 30 pressure and decomposition temperature provide a by a decomposition characteristic of the reactant vapor good compromise between the extremes of the satu molecule wherein suf?cient bonds of the molecule rated and unsaturated molecules. The vapor pressure of break to release elemental tellurium, below the selected 2,5-dihydrotellurophene is about 0.5 millimeters of mer deposition temperature, here about 200° C. to about cury at ambient temperature, and it decomposes fully at 300° C., in a half life of less than about 1 second. That is, temperatures below 250° C. with a half life of less than a high vapor pressure of a very stable gaseous species 1 second. will not successfully deposit the desired ?lm layers, Methyltellurol (CI-I3TeH) also provides the desired since insufficient decomposition of the reactant vapor high vapor pressure and decomposition temperature molecules will occur. Conversely, a low vapor pressure below 300° C. It is less preferred than the tellurophenes, of a fully-dissociated gaseous molecule will produce because of its sensitivity to light and moisture. If these ?lm growth, but at unacceptably low growth rates. potential problems are avoided, methyltellurol becomes Unfortunately, the reactant source vapor pressure is a highly desirable source of tellurium. usually found to be reduced for reactant source vapor The reactant sources for the species to be codeposited species having low decomposition temperatures, and with tellurium must also exhibit a suf?ciently high vice versa. 45 vapor pressure, and the reactant source gases produced Several reactant source vapor species containing therefrom must have low complete decomposition tem tellurium have been identi?ed as having a suf?ciently peratures. Dimethyl cadmium (CH3—Cd-—CI-I3) meets high vapor pressure and also a decomposition charac this requirement as a source of cadmium. Dimethylmer teristic whereby essentially complete dissociation is cury (CH3—Hg—-CH3) and diethylmercury (CZH5—H achieved at temperatures below about 300° C. in a half 50 g—C2H5) are satisfactory sources for mercury. Die life of less than about 1 second. The tellurophenes are a thylmercury, with weaker Hg-C bonding, has a lower group of heterocyclic compounds having four carbon decomposition temperature, but dimethylmercury has a atoms and a tellurium atom bonded in a ring, with vari high vapor pressure. ous numbers of hydrogen atoms bonded to the carbon The use of these reactant source vapors in a CVD atoms. (As used herein, the term “tellurophene” means reactor, with the substrate maintained at a temperature all of the members of this heterocyclic family.) The of at least about 200° C., preferably about 200° C. to basic member of the family, C4H4Te, has one hydrogen about 300° C., and most preferably about 250° C., re atom bonded to each of the four carbon atoms, and has sults in deposition of a ?lm of cadmium telluride on the a high vapor pressure in conjunction with a moderately substrate. high decomposition temperature. Its structure is: 60 In the preferred approach, cadmium telluride epitax~ ial layers were grown on (001) CdTe single crystal substrates and (001) InSb single crystal substrates, at atmospheric pressure, in the reactor 10. The reactant sources were 2,5-dihydrotellurophene (obtained from 65 American Cyanamid Co.) for tellurium and dimethyl / cadmium (obtained from Alfa Ventron) for cadmium. H Palladium-diffused hydrogen gas was used as the diluent as well as the carrier gas for both the 2,5 -dihy 4,828,938 9 10 drotellurophene and the dimethylcadmium. The total TABLE l-continued flow through the reactor was about 2.9 standard liters H1 Flow/Cd Hg Flow/T e Total H3 Flow per minute, with the relative flow rate of each of the Run (sccm) (sccm) (sccm) two reactant source vapors adjusted to yield equal 10 65.0 800 2895 amounts of cadmium and tellurium in the ?lm. 11 75.0 800 2875 Cadmium telluride epitaxial ?lms of about 2.54 centi 12 55.0 800 2905 meter diameter were grown on indium antimonide sub 13 60.0 800 2900 strates at substrate temperatures range from about 250° 14 50.0 800 2900 15 60.0 800 2900 C. to about 330° C., and on cadmium telluride substrates 16 50.0 800 2900 at substrate temperatures ranging from about 250° to 10 about 285° C. In these temperature ranges, it was ob served that the growth rate of the ?lm was independent The results of the 16 growth runs of cadmium tellu of temperature and light level, and varied linearly with ride on cadmium telluride substrates are summarized in the flow rate of the hydrogen gas through the tellurium Table 2. TABLE 2 Carrier Hall Temp Time Thickness Conduc Concen mobility Run (0) (min) (micron) [Cd]:[Te] (“11105) (cm-3) (30010 1 27s 30 1.2 9 1.67 2 X 1017 57 2 285 180 5.8 6 * * * 3 285 120 3.8 6 0.20 2 X 1016 64 4 282 60 3.8 3 0.38 5 >< 1016 48 5 282 60 3.8 3 0.04 7 >< 101-‘‘ 35 6 282 60 3.8 3 0.10 2 x 1019 44 7 282 60 3.8 4 0.12 4 >< 10‘6 19 8 282 60 3.8 5 0.77 1 >< 1017 49 9 282 60 3.8 4 0.10 2 X 1016 3e 10 282 60 3.8 4 0.19 3 X 1016 40 11 282 60 3.8 4 0.29 5 X 1016 38 12 282 50 3.8 3 0.14 1 X 10“5 72 13 282 30 1.9 3 0.50 6 X 1016 47 14 282 30 1.9 3 0.33 4 X 1016 so 15 272 60 3.8 3 0.25 5 X 1019 35 16 250 60 3.8 3 0.09 7 >< 1015 80 '-Not determined source. Having a growth rate independent of tempera 35 ture and ambient light is an important advantage, since Electron channeling contrast patterns were prepared variations in temperature and light intensity across the for all of these materials, and these patterns veri?ed that surface of a growing ?lm do not then cause nonuniform the epitaxial layers were single crystal and of the same ities in the thickness of the film. It is dif?cult to maintain (001) crystallographic orientation of the substrate. precise temperature control over large surface areas, 40 Fifteen samples were prepared of cadmium telluride and a growth rate that is independent of temperature deposited upon a (001) indium antimonide substrate. signi?cantly contributes to the ability to grow laterally The gas flow parameters during deposition are pres large, uniform ?lms ented in Table 3. Electrical conductivity, p-type carrier concentration TABLE 3 45 and ambient temperature Hall mobility were measured H2 Flow/Cd H2 Flow/T e H; Flow/Total on epitaxial layers grown on the cadmium telluride Run (sccm) (sccm) (sccm) substrates by the van der Pauw technique using samples 1 72.0 91 2963 of 5 millimeter by 5 millimeter size with cloverleaf 2 91.0 900 2791 patters. 3 72.0 500 2872 Sixteen runs were made to grow cadmium telluride 4 87.5 500 v.1898 5 87.5 400 2898 ?lms on cadmium telluride substrates. The gas ?ow 6 36.5 400 2897 parameters are stated in Table l. The term “H2 7 54.0 400 2894 Flow/Cd” means the flow rate of hydrogen carrier gas 8 58.5 4-00 2899 through the dimethylcadmium source in standard cubic 9 51.5 400 2892 55 10 12.5 100 2893 centimeters per minute (sccm). The Total HzFlow is the 11 25.0 200 2895 sum of diluent hydrogen gas and the hydrogen gas - 12 50.0 400 2900 carrying the reactant source vapor species. 13 70.0 400 3000 14 75.0 400 2995 TABLE 1 15 65.0 400 2995 . H2 Flow/Cd Hz Flow/T e Total H2 Flow Run (sccm) ' (sccm) (sccm) The substrate temperature and growth results for the 1 87.5 500 2888 2 67.5 400 2998 15 samples of cadmium telluride grown on indium anti 3 70.0 400 3000 monide are reported in Table 4. The ?lm thickness is an 4 70.0 800 3040 average value determined by preparing Auger electron 5 60.0 800 3000 65 spectroscopy pro?les of progressively sputtered sur 6 80.0 800 3000 7 80.0 800 3000 faces. By this technique, two different interfaces mark 8 90.0 800 2900 ing the depth of the deposited layer can be identi?ed, 9 70.0 800 2900 one for tellurium/antimony and the other for cadmium 4,828,938 11 12 /indium. The average of the two measurements is re group consisting of a tellurophene and methyltel ported in Table 4. lurol; TABLE 4 mixing the ?rst reactant source vapor with a second reactant source vapor having therein molecules Temp Time Thickness containing another substance to be codeposited Run (C.) (min) (microns) [II] :[VI] with the tellurium; and 1 250 30 0.75 35.1 contacting the mixture to a substrate maintained at a 2 250 30 0.55 4.5 3 250 30 0.36 6.4 temperature of from about 200° C. to about 300° C. 4 300 30 1.67 9.6 2. The method of claim 1, wherein one substance 5 285 30 1.09 12.0 codeposited with tellurium is cadmium. 6 280 20 0.71 4.1 3. The method of claim 2, wherein the source of the 7 285 30 1.03 6.0 B 285 30 1.05 5.0 cadmium is dimethylcadmium. 9 280 30 1.07 4.4 4. The method of claim 1, wherein the substance 10 270 30 0.32 2.8 codeposited with tellurium is selected from the group l l 285 30 0.49 4.3 15 consisting of cadmium, mercury, and combinations of 12 329 15 0.68 4.3 cadmium and mercury. 13 288 20 0.62 6.0 14 285 20 0.80 6.4 5. The method of claim 4, wherein the substance 15 285 20 0.91 5.5 codeposited includes mercury, and the source of the mercury is selected from the group consisting of dime The uniformity of the cadmium telluride epitaxial 20 thylmercury and diethylmercury. layer on the indium antimonide was measured by sec 6. The method of claim 1, wherein the substrate is ondary ion mass spectrometry (SIMS) to assess thick selected from the group consisting of cadmium telluride and indium antimonide. ness uniformity for a sample chosen at random. This technique is accurate to within about 3 percent change 7. The method of claim 1, wherein the reactant source vapor is methyltellurol. for the thicknesses studied. There was no detectable 8. The method of claim 1, wherein the temperature of thickness variation over an area of about 3 square centi the substrate is maintained at about 250° C. meters on the surface of the sample, indicating that any 9. The method of claim 1, wherein said step of con thickness variation was less than 3 percent. Mercury telluride was deposited on single crystal tacting is conducted in an inverted vertical chemical 0 vapor deposition reactor. cadmium tellurice- using an approach similar to that 10. A combination of a thin ?lm and a substrate pre described above. Diethylmercury was placed in the pared by the method of claim 1. bubbler 60 in place of dimethylcadmium. The hydrogen 11. A method of depositing on a substrate a thin ?lm gas flow rate through the diethylmercury was about 900 of a material containing tellurium comprising the steps sccm, the hydrogen gas flow rate through the 2,5-dihy of: drotellurophene was 800 sccm, and the total hydrogen providing a reactant source vapor containing tellu gas flow rate was 2900 sccm. Deposition temperatures rium selected from the group consisting of a tel of 275° C. and 300° C. were used, with total deposition lurophene and methyltellurol; times of 30 to 60 minutes. Four growth runs were made. mixing the reactant source vapor containing tellu FIG. 3 is an Auger pro?le of mercury, cadmium and rium with a reactant source vapor containing an tellurium as a function of depth below the surface. The other substance to be codeposited with the tellu surface layer of about 0.2 micrometers is a mercury rium, the other substance being selected from the cadmium telluride solid solution alloy ?lm due to diffu gorup consisting of cadmium, mercury, and combi sion of cadmium from the bulk of the substrate. FIG. 3 nations of cadmium and mercury; and demonstrates that mercury, cadmium and tellurium are introducing the mixture into an inverted vertical all present in the ?lm, but the results are not calibrated chemical vapor deposition reactor, wherein a sub with standards to determine the exact amounts of each strate is maintained at a temperature of from about element present. 200° C. to about 300° C. As is now apparent, the method of the present inven 12. The method of claim 11: wherein the face of the tion permits the chemical vapor deposition of epitaxial substrate upon which deposition occurs has a surface thin ?lms of (II, III, IV-tellurium) compounds at area of greater than about 1 square centimeter. growth temperatures well below those previously pos 13. The method of claim 11, wherein the reactant sible. New reactive sources of tellurium have been iden source vapor is methyltellurol. ti?ed, which meet the criteria de?ned herein for suc 14. A combination of a thin ?lm and a substrate pre cessful CVD at temperatures below 300° C. of materials pared by the method of claim 11. containing tellurium. Although particular embodiments 15. A method for depositing on a substrate a thin ?lm of the invention dealing with the deposition of cadmium of a material containing tellurium, comprising the steps telluride, mercury telluride, and mercury cadmium tel of: luride have been described in detail for purposes of providing an organometallic reactant source of tellu illustration, various modi?cations may be made without 60 _ rium, the source being a substance that produces a departing from the spirit and scope of the invention. molecular reactant source vapor containing tellu Accordingly, the invention is not to be limited except as rium and has a vapor pressure of at least 0.5 milli by the appended claims. meters of mercury at ambient temperature, the What is claimed is: reactant source vapor having a decomposition tem 1. A method of depositing on a substrate a thin ?lm of 65 perature, whereat elemental tellurium is released, a material containing tellurium, comprising the steps of: at less than about 300° C. and having a molecular providing a ?rst reactant source vapor having therein half life of less than about 1 second at a temperature molecules containing tellurium selected from the of from about 200° C. to about 300° C.; ’ 4,828,938 13 14 mixing the reactant source vapor containing tellu group consisting of cadmium, mercury, and combina rium with a reactant source vapor containing an tions thereof. other substance to be codeposited with the tellu 17. The method of claim 15, wherein the source of rium, the other substance being selected from the tellurium is selected from the group consisting of a group consisting of elements from group II, group tellurophene and methyltellurol. III, and group IV of the periodic table, and mix 18. The method of claim 15, wherein said step of contacting is accomplished in an inverted vertical tures thereof; and chemical vapor deposition reactor having a substrate contacting the mixture to a substrate maintained at a facing downwardly. temperature of less than about 300° C., whereupon 10 19. The method of claim 15, wherein the substrate is the reactant source vapors in the mixture react to selected from the group consisting of cadmium telluride codeposit a material containing tellurium upon the and indium antimonide. substrate. 20. A combination of a thin film and a substrate pre 16. The method of claim 15, wherein the substance to pared by the method of claim 15. be codeposited with tellurium is selected from the

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