TELLURIDES

Abstract: A method of making pure hydrogen telluride is dis­ closed, involving the reaction of atomic hydrogen (produced by the microwave-induced dissociation of hydrogen gas) on pure metallic tellurium (9). The invention also provides a controlled method of making metal tellurides (especially cadmium mercury telluride for use in photodetector applica­ tions) involving the thermal decomposition of hydrogen tel­ luride (however produced) over a hot substrate (14) in the presence of gaseous source(s) of the appropriate metal(s) (11). The use of hydrogen telluride (which has hitherto been considered to be too explosively unstable to be useful) enables cadmium mercury telluride to be synthesised at temperatures below 250°C.

Description: The present invention relates to the synthesis of hydrogen telluride and the synthesis of metal tellurides from hydrogen telluride. The invention relates particularly but not exclusively to the synthesis of binary and ternary mercury tellurides such as Hgl xCdxTe, Hgl Mn Te and Hgl x (Mnl yCdy)xTe and to photodetectors incorporating such tellurides. Hitherto, such binary and ternary mercury tellurides have commonly been produced by vapour-phase epitaxial methods involving the thermal decomposition of vapourised cadmium and tellurium alkyls in the presence of mercury vapour. However tellurium alkyls only decompose at high temperatures, so that the formation of cadmium mercury telluride by vapour phase epitaxy has hitherto only been possible at temperatures greater than approximately 3500 C. This is frequently disadvantageous. Furthermore, cadmium and tellurium alkyls form adducts in the vapour phase which decompose at a lower temperature than the decomposition temperature of either of the separate alkyls. Consequently the value of x in the formula CdxHgl Te is a complex function of reaction conditions when the cadmium mercury telluride is formed by pyrolysis of mixed cadmium and tellurium alkyls. According to one aspect of the present invention, in a controlled method of producing a metal telluride, gaseous hydrogen telluride is uniformly mixed with a gaseous source of said metal, characterised in that flows of pure gaseous hydrogen telluride and gaseous sources of metal M and mercury ate mixed and the flowing mixture is thermally decompose by a heated substrate (14) to form a layer of metal mercury telluride ( xHgl Te) on said substrate. The gaseous source of the metal may be the metal vapour itself if the metal is sufficiently volatile, Otherwise volatile thermally decomposable organometallic compounds such as metal alkyls may be usedsuitable volatile organometallic compounds include dimethyl cadmium, di-ethyl cadmium and hydrogen-substitute metal alkyls. The process should be carried out in the absence of light (which photochemically decomposes hydrogen telluride) and the hydrogen telluride should be free from traces of water, oxygen and other sources of free radicals which may initiate an explosive chain reaction. The hydrogen telluride may suitably be diluted in a suitable inert diluent such as hydrogen and should according to reports be at a partial pressure below 23 Torr at room temperature, since explosions can occur at higher partial pressures. Reference is made to the following articles (which relate to the combustion properties of hydrogen telluride) in connection with the safety aspects of the process: A S Yushin, RV Pekhomov, Izv. Akad, Nauk. SSSR, Ser. Khim. (12), 2801-3, 1976, (CA 86 100084). V S Mikheev, L I Nikolaev, V Ya. Verkhovskii, A S Yuskin, Izv. Akad. Nauk SSSR, Ser. Khim. (9), 2117-19, 1977 (CA 88: 25066). GP Ustyugov, E N Vigdorovich, Izv. Akad. Nank SSSR, Neorg. Mater., 5(1), 163-5, 1969 (CA 65: 91010). However, we have found that pure hydrogen telluride is unexpectedly kinetically stable even at temperatures as high as 2500C. Cadmium mercury telluride of controlled composition may be made in accordance with the present invention by providing controlled flows of the gaseous hydrogen telluride and at least one of the or each said gaseous sources, allowing said flows to mix and subsequently thermally decomposing substantially all of the flowing mixture over a heated substrate to form a layer of said metal telluride on said substrate. By this means the composition of the metal telluride can be controlled by controlling the relative flow rates of the components of the gas mixture. The composition of the metal telluride layer may be varied over its thickness by altering the relative flow rates of the components of the gas mixture. The relative flow rates may be altered continuously or discontinuously to give corresponding continuous or discontinuous variations in the composition of the layer over its thickness. The hydrogen telluride may be generated in situ under controlled conditions. The gaseous source of mercury is suitably mercury vapour and the value of x in the formula Cdx Hg l-x Te is suitably controlled by controlling the relative flow rates of the gaseous cadmium source (which is suitably dimethyl cadmium) and the hydrogen telluride. The mercury vapour may be in equilibrium with a hot mercury bath. The total partial pressure of the reactants is suitably between 1 and 10 Torr, which may be supplemented by a sufficient partial pressure of hydrogen or other in ert diluent to bring the total pressure up to approximately atmospheric pressure. The substrate may suitably be composed of carbon, in which case it may be locally heated by a microwave or R.F. source. Alternatively a substrate such as cadmium telluride, indium antimonide, silicon, gallium arsenide, spinel (Mg A12 04) or sapphire (Al203) may be used, and may be mounted on a carbon susceptor heated by a microwave or R.F. source. Doped cadmium mercury telluride may be grown by a method in accordance with the invention by providing a controlled flow of a thermally decomposable gaseous source of dopant which is decomposed on the hot substrate and co-deposited with the other reactants. Thus aluminium, gallium, arsenic and phosphorus dopants may be obtained from vapours of their respective trimethyl compounds and silicon and germanium dopants may be obtained from their respective hydrides. Thus photovoltaic as well as photoconductive cadmium mercury telluride layers may be grown in accordance with the invention. The invention includes within its scope infra-red photodetector devices (whether photovoltaic or photo conductive) incorporating cadmium mercury telluride layers grown by any of the novel methods described hereinabove. Suitable apparatus for generating gaseous hydrogen telluride comprises means for passing a flow of molecular hydrogen over a tellurium surface and means for generating microwave radiation at said surface so as to dissociate the flowing hydrogen. Said surface may be the interior surface of a tellurium tube and the tellurium tube may be surrounded by a microwave cavity. The frequency of the microwave radiation is not critical, but may suitably be 2,5OMHz. The above novel apparatus is particularly suitable for generating hydrogen telluride for use in methods of making high-purity tellurides (particularly cadmium mercury telluride) in accordance with the invention because the only reactants are hydrogen and tellurium, which may both be obtained free from hydride-forming impurities. However hydrogen telluride may also be obtained by other methods involving the reaction of atomic hydrogen with tellurium - for example an electric discharge between tellurium electrodes in a hydrogen atmosphere may be used. Methods involving the reaction of mineral acids on metal tellurides (for example aluminium telluride) or the electrolysis of aqueous solutions using a tellurium cathode produce impure hydrogen telluride which cannot easily be purified to a sufficient extent. A method and apparatus in accordance with the invention will now be described by way of example with reference to the accompanying drawing, which shows apparatus for the production of cadmium mercury telluride layers suitable for photodetector applications. The apparatus shown (which is generally of stainless steel) comprises a manifold 1 from which purified hydrogen is fed to mass-flow meters 2a, 2b, 2c, 2d and 2e. Mass-flow meter 2a directly feeds a tubular reactor 10 with hydrogen and meters 2b, 2c and 2d feed alkyl bubblers 6b 6a and hydrogen telluride generator 7 via pneumatic valves 4b b and 4c respectively. Mass-flow meter 2e feeds a bypass line 21. Valves 3a, 3b and 3c may be opened and valves 5a, 5b and Sc may be closed to turn off the associated hydrogen telluride generator 7 and alkyl bubblers 6a and 6b. Alkyl bubbler 6a contains dimethyl cadmium at a controlled temperature and alkyl bubbler 6b (which is similarly temperature-controlled) contains an alkyl of any suitable dopant that is required. Additionally or alternatively, dopants such as phosphorus which are conveniently available as hydrides may be added directly to the system via a mass flow controller 2f. Hydrogen telluride generator 7 comprises a tellurium tube 9 of high purity (typical impurity levels being less than 1 part per million) in a 2450 MHz microwave cavity 8. The intensity of the microwave radiation is controlled so as to control the hydrogen telluride output, the temperature of the tellurium 0 tube 9 remaining at approximately 25 C. Since the temperatures of the alkyl bubblers 6a and 6b are controlled, controlled flows of cadmium alkyl, hydrogen telluride and optionally hydride and alkyl dopants may be fed, highly diluted with hydrogen, into a mixer 22. and thence to reactor 10. The partial pressure of each reactant vapour is of the order of 1 Torr, the total pressure being approximately 1 atmosphere. Reactor 10 is provided with a mercury bath 11 which is heated by a furnace 1; to approximately 150 0C to provide a mercury atmosphere which mixes thoroughly with the reactant gases from mixer 22. A carbon susceptor 15 is locally heated by an R.F. coil 13 and thermally decomposes the hydrogen telluride1 cadmium alkyl and dopant sources (if present) in a smoothly controlled reaction to form an epitaxial layer of cadmium mercury telluride of uniform controlled composition on inclined face 14. A suitable substrate layer such as cadmium telluride may be initially deposited on Surface 14 in the absence of a mercury bath. When the layer has reached a suitable thickness, it may be processed by known methods and fabricated into an infra-red photodetector device. A liquid coolant jacket 23 prevents the reactor from overheating and condenses excess mercury vapour. The excess reactant gases and hydrogen are exhausted by a pump 17 and burnt (for safety reasons) after being passed over an activated charcoal bed 19. A cold trap 16 condenses unreacted reactant gases and the pressure within the system is controlled by valves 18 and 20. In an alternative embodiment of the invention, hydrogen telluride may be generated in situ over a tellurium tube in reactor 10, a suitable microwave cavity being provided around the reactor. Higher temperatures may then be used.

Claims: Claims 1. A controlled method of producing a metal telluride wherein gaseous hydrogen telluride is uniformly mixed with a gaseous source of said metal, characterised in that flows of pure gaseous hydrogen telluride and gaseous sources of metal M and mercury are mixed and the flowing mixture is thermally decomposed by a heated substrate (14) to form a layer of metal mercury telluride ( M Hg Te) on said substrate. 2 A method according to Claim 1 wherein the partial pressure of the hydrogen telluride is between 1 and 10 Torr and the hydrogen telluride and gaseous sources of cadmium and mercury are diluted with an inert diluent gas. 3. A method according to Claim 1 or Claim 2 wherein the temperature of said substrate is below 2500C. 4. A method according to any preceding Claim wherein the relative flow rates of the hydrogen telluride and the gaseous source of metal M are varied so as to vary the composition MxHgl,xTe of the deposited telluride layer over its thickness. 5. A method according to any preceding Claim wherein said gaseous source of mercury is mercury vapour. 6. A method according to any preceding Claim wherein said metal M is cadmium and said gaseous source of the metal M is a volatile organometallic cadmium compound. 7. A method according to Claim 6 wherein said organometallic compound is a cadmium alkyl. 8. A method according to Claim 7 wherein said organometallic compound is dimethyl cadmium or diethyl cadmium. 9. A method according to any preceding Claim wherein said substrate is heated by a microwave or R.F. source. 10. A method according to any preceding Claim wherein an alkyl of aluminium, gallium, arsenic or phosphorus, or a hydride of silicon or germanium is decomposed with the hydrogen telluride at said substrate (14) to form a doped telluride layer. 11. A method according to any preceding Claim wherein the hydrogen telluride is generated by an electric discharge between tellurium electrodes in a hydrogen atmosphere. 12. A photodetector device incorporating a cadmium mercury telluride layer grown by a method according to any preceding Claim. 13. Apparatus for forming a binary or ternary mercury telluride from hydrogen telluride and gaseous sources of mercury and optionally a metal M, said apparatus comprising a reactor vessel(10) housing both a substrate (14) and a tellurium surface (9), means (3a,4c,5a) for flowing hydrogen across said surface, a microwave cavity (8) surrounding at least part of said reactor vessel and arranged to dissociate hydrogen at said surface and thereby generate hydrogen telluride, and means (13) for heating said substrate to decompose the hydrogen telluride and gaseous source mercury and said metal M (if present) and form a telluride layer on said substrate. 14. A controlled method of making mercury telluride wherein gaseous hydrogen telluride is uniformly mixed with a gaseous source of mercury and the mixture is thermally decomposed by a hot substrate to form a layer of mercury telluride on said substrate.

Inventors: BEVAN, MALCOLM JOHN, HYLIANDS, MICHAEL JAMES, WOODHOUSE, KAREN TACE
Applicants: THE MARCONI COMPANY LIMITED
Application year: 1984
Main classification: C01B19
IPC codes: C23C16, C30B25, C01B19