Manufacture of Cadmium Mercury Telluride
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Europâisches Patenta mt ® COÏ) EuropeanEur°Pean Patent Office ® Publication number: 0 040 939 Office européen des brevets B1 ® EUROPEAN PATENT SPECIFICATION ® Date of publication of patent spécification: 02.01 .85 ® '"t. Cl.4: C 30 B 25/02, C 30 B 29/48 ® Application number: 81302178.9 (22) Dateoffiling: 15.05.81 (54) Manufacture of cadmium mercury telluride. (30) Priority: 27.05.80 GB 8017334 ® Proprietor: The Secretary of State for Defence in Her Britannic Majesty's Government of The United Kingdom of Great Britain and (43) Date of publication of application: Northern Ireland Whitehall 02.12.81 Bulletin 81/48 London SW1 A 2HB (GB) (4jj) Publication ofthe grant of the patent: (7?) Inventor: Irvine, Stuart James Curzon 02.01 .85 Bulletin 85/01 14 Pixief ield, Cradley Malvern Worcestershire WR13 5ND (GB) Inventor: Mullin, John Brian (§) Designated Contracting States: The Hoo, Brockhill Road DE FR IT NL Malvern Worcestershire WR14 4DO (GB) (5B) References cited: @ Representative: Edwards, John Brian et al US-A-3 218 203 Procurement Executive Ministry of Defence US-A-3 664 866 Patents 1 A (4), Room 2014 Empress State US-A-3 725 135 Building Lillie Road London SW6 1TR (GB) J. Electrochem. Soc: Solid State Science & Technology, May 1981, pp. 1142-1144 Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall be filed in a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been paid. (Art. 99(1 ) European patent convention). Courier Press, Leamington Spa, England. limitation in respect of composition control i.e. the value of x (in CdxHg1-xTe) cannot be inde- The invention relates to the manufacture of pendently controlled. Thus to produce epitaxial the material cadmium, mercury telluride i.e. layers having different values of x it is neces- CdxHg,_XTe commonly referred to as CMT or sary to use differently composed solutions of MCT. CMT in Te. Such a material in its semiconducting form is A vapour phase epitaxial (VPE) process for used as a detector of infra red radiation in growing CMT has been reported by Vohl Et thermal imaging systems. These detectors com- Wolfe (J. Electronic Materials, 7 (1978) 659). prise small pieces of CMT cut and polished flat This uses an open flow process with inde- with electrical contacts. U.K. Patent Specifica- pendently controlled sources of the elements tion No. 859,588, published 25 January 1961, Cd, Hg, and Te. However this method suffers a describes the production and use of CMT fundamental limitation in the inability to effect detectors. adequate control of the values of x at the low At present CMT appears to be the most deposition temperature that is needed to useful of all infra red detectors and is therefore produce CMT particularly in the important range used in the majority of high performance x=0.2-0.3. Because of the low vapour thermal imager systems. pressure of Cd and Te in the region of 400°C CMT is a difficult material to grow and the input vapours can suffer a capricious reduc- handle, partly because of the volatile nature of tion in composition before they reach the sub- the components. strate. When the substrate is held at a tem- Present methods of manufacture can be perature suitable for epitaxial CMT growth the broadly classified into bulk melt growth and epi- temperature gradient in the deposition chamber taxial methods. is not high enough to prevent condensation of The most important melt growth methods CdTe upstream from the substrate. are: the Bridgman method involving growth in a Epitaxial layers of CMT have also been pro- sealed container carried out in a vertical or hori- duced by subliming sources of HgTe onto a zontal manner; the cast quench anneal method; CdTe substrate in close proximity - so-called a cast recrystalline anneal method; and a so- close-spaced epitaxy - with or without the called slush method. All these methods involve presence of additional Hg. Examples include the batch preparation that is lengthy and expensive work Cohen-Solal and co-workers, and Tufte taking weeks rather than days to complete. A and Stelzer. References to these works can be further disadvantage is that the crystals pro- found in J. Appl. Phys. 40 (1969). duced are roughly cylindrical and need slicing, This technique relies on the production of grinding, lapping, etching and dicing into small CMT by the interdiffusion of Cd and Hg between pieces for use as e.g. detectors. the substrate and the epitaxial layer. It suffers Epitaxial methods of manufacturing semi- from the problem of compositional non- conductors on the other hand are intrinsically uniformity in the direction normal to the plane quicker in so far as they produce thin layers of of the layer. It does not have the advantages of semiconductor material onto a substrate independent control of composition enjoyed by directly often in a matter of hours or minutes. an open flow technique. In the case of materials like GaAs, inP, and GaP Epitaxial layers of GaAs have been grown well developed methods are available for the successfully by VPE using gallium alkyl and growth of homo-epitaxial layers of these com- arsine. pounds onto substrates of the parent semicon- This contrasts with the situation concerning ductor by either liquid or vapour phase pro- CMT where it is common knowledge that the cesses. However no such well developed art is attempted growth of CMT using the three alkyls available in the case of CMT. of the elements Cd, Hg and Te in combination In the case of the epitaxial growth of CMT has not been successful. from the it has been reported by Harman, liquid there has been a need, since 1961, J. Electronic Materials 8 (1979) 191; and by Although for a relatively simple method of growing CMT Schmit and Bowers, Appi. Phys. Letters 35 the existing techniques of bulk growth, and epi, (1979) 457; and by Wang et al, J. Electro- taxial growth have proved inadequate in terms chem. Soc. 127 (1980) 175; and by Bowers et of speed of preparation and in the versatility of al, I.E.E.E. Trans. Electron Devices ED 27 (1980) growing material of the desired composition. 24; and et al, I.E.E.E. Trans. Electron by Wang There is a need therefore to provide an effective Devices ED 27 (1980) 154; that it is possible to method of manufacturing layers of CMT which of CMT from supersaturated solu- grow layers method allows independent control of the com- tions in tellurium or onto sub- excess mercury position CdxHg1-xTe and that preferably allows of cadmium telluride (CdTe). Such strates pro- for independent control of doping. cesses demand considerable skill and a very long development period. The epitaxial layers According to this invention a method of frequently suffer from surface blemishes which growing a layer of the ternary alloy cadmium can render them useless for device fabrica- mercury telluride onto a substrate comprises tions. Such methods also suffer a fundamental the steps of: providing an atmosphere of mercury vapour at a bath of mercury inside the vessel adjacent to required temperature and pressure inside a the substrate. vessel; The vessel heater may be an electrical controlling the temperature of the substrate resistance heater surrounding the vessel to heat mounted in the vessel controlling the tem- both the vessel and mercury bath. perature of the vessel walls independently of The substrate may be mounted on a carbon the substrate temperature and at a lower susceptor and heated by an RF coil surrounding value than that of the substrate; part of the vessel. Alternatively resistance providing a supply of a volatile cadmium alkyl heaters may be used inside the vessel, or an into the vessel and onto the substrate; infra red heater may be used to illuminate the providing a supply of a volatile tellurium alkyl substrate surface. into the vessel and onto the substrate; The Cd and Te-alkyl may be supplied by passing high purity hydrogen through two the arrangement being such that the cadmium bubblers containing the appropriate Cd and Te- and the tellurium alkyls decompose in the alkyls. region of the substrate and the cadmium and The invention will now be described, by way tellurium combine with mercury on the sub- of example only with reference to the accom- strate to form a layer of cadmium mercury panying drawing which is a schematic flow telluride whose cadmium composition is con- diagram. trolled by the supply of the cadmium alkyl and As shown high purity hydrogen is supplied to the overall composition of the cation content is a hydrogen manifold 1 which maintains a controlled by the supply of the tellurium alkyl supply for five mass-flow controllers 2, 3, 4, 5, the mercury vapour pressure being held greater and 23. Mass flow controller 2 supplies than the decomposition pressure of the growing hydrogen via a bypass line 14 to a combustion layer. chamber 31 which burns exhaust vapour in a The grown layer may be a single epitaxial hydrogen flame. Mass flow controllers 3 and 4 layer or multiple layers. Such a layer or layers supply hydrogen to alkyl bubblers 6, and 7, may be graded in composition. The layer or which respectively contain an alkyl or cadmium layers may also be suitably doped. For example such as dimethyl cadmium and an alkyl of tellur- two layers may be grown with two different ium such as diethyl telluride.