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. Hydrogen flow values of x so that a detector, sensitive to both from the controllers 3 and 4 can be diverted via the 3 to 5 and 8 to 14 µm wavebands may be valves 8 and 9 to the bypass line 14 or through made. Also a passivating layer of CdTe may be valves 10, 11 and 12, 13 thus enabling the alkyl grown on the CdxHg1-xTe layer. Suitable II-VI flows to be turned on and off. Hydrogen compounds or mixed alloys may be grown on bubbling through the liquid alkyl will become the layer e.g. CdTe, ZnS, CdTexSe(1-x) which saturated with alkyl vapours at the ambient may be used to make heterojunctions or form temperature of the liquid alkyl, typically 25°C. antireflection coatings, etc. These alkyl plus hydrogen streams are mixed in The substrate may be CdTe, a 11-Vi a mixer 15 with a further dilution flow of compound or mixed II-VI alloy. The substrate hydrogen supplied by the mass flow controller may be silicon (Si), gallium arsenide (GaAs), 5. By control of flows through controllers 3, 4, spinel (MgAl2O4), alumina or sapphire (Al2O3). and 5, the concentrations of cadmium and tel- The volatile cadmium alkyl may be dimethyl lurium alkyls in the mixed stream can be inde- cadmium, diethyl cadmium, or dipropyl pendently determined over a wide range of cadmium. values. The volatile tellurium alkyl may be diethyl tel- The alkyl plus hydrogen mixture is passed luride, dimethyl telluride, dipropyl telluride, or into a reactor vessel 16 which is heated with an dibutyl telluride, or an equivalent hydrogen sub- electrical resistance furnace 17 and R.F. induc- stituted tellurium alkyl, such as, e.g. hydrogen tion coil 18. Inside the reactor vessel is a ethyl telluride [H(C2Hs)Te]. mercury bath 19 and a carbon susceptor 21 Apparatus for growing a layer of cadmium carrying the substrate 20 to be coated with a mercury telluride on a substrate according to layer of CMT. The furnace maintains the the method of this invention, comprises a vessel temperature of the reactor vessel wall from the for containing the substrate, heating means for mercury reservoir 19 to the substrate 20 equal heating the vessel, a substrate heater for inde- to or greater than the mercury reservoir tempera- pendent control of the substrate temperature at ture, the mercury reservoir being heated by a value above the temperature of the vessel thermal conduction through the reactor wall 24. walls, means for supplying a cadmium alkyl into The RF induction coil 18 couples into the carbon the vessel, and means for supplying a tellurium susceptor 21 thereby heating the substrate to a alkyl or hydrogen substituted tellurium alkyl into temperature above that of the reactor wall 24 the vessel, the arrangement being such that the so that the cadmium and the tellurium alkyls cadmium and tellurium alkyls decompose in the will crack and deposite cadmium and tellurium mercury atmosphere to form the compound onto the surface of the substrate 20. The CdxHg1-xTe on the surface of the substrate. temperature of the mercury reservoir 19 is The mercury vapour may be provided by a determined by the requirement of an equili- brium partial pressure of mercury to be main- red detectors may be made. Such a detector tained at the growth interface. The hot reactor may be a layer of CMT on a CdTe substrate with wall 24 ensures that the mercury partial pres- a passivating layer of oxide or CdTe on the CMT sure in the vapour stream is the same at the layer surface. The detector may be in the form substrate 20 as over the mercury reservoir 19. of a strip with electrodes on the surface at each Typical conditions for growth of a layer of end as described in UK Patent Specification CMT on a CdTe substrate are:- alkyl bubbler 1,488,258. Such a detector is photo con- temperature 25°C; vessel wall and mercury ductive and has the image of a thermal scene bath temperature 220-320°C (e.g. around scanned over its surface. 300°C): substrate temperature 400-430°C Another type of IR detector uses a p-n (e.g. around 410°C): Hydrogen dilution flow junction e.g. the junction between two dif- through controller 5 is around 500 ml/min: flow ferently doped, p and n doped, CMT layers to through Cd alkyl bubbler around 2-200 form a photo-voltaic detector. A voltage is ml/min (e.g. 40 ml/min); flow through Te alkyl applied by electrodes across the p-n junction bubbler around 500-1100 ml/min (e.g. 1,000 and changes in current are a measure of the ml/min). The walls of the vessel 16 are suf- infra-red photons that are absorbed by the ficiently hot to prevent condensation of Hg detector. Such a detector may be formed into a without significant decomposition of the alkyls, large array of IR detectors capable of imaging a whilst the temperature of the substrate 20 is thermal scene, without a scanning system, to sufficient to decompose the alkyls at the sub- form a so-called staring array system. strate 20 surface. The substrate may be inclined The materials CdxHg1-xSe and CdxHg1-xS slightly e.g. 4° to give more uniform growth may be grown in a manner analogous to that along the substrate. described above. In one example a layer of Cd0.2 Hg0.8 Te was grown on CdTe at 2° off the [100] orientation and inclined at 4° to the gas flow under the fol- lowing conditions:- alkyl bubbler temperature 1. A method of growing a layer of the ternary 25°C; Hg temperature 290°C; substrate alloy cadmium mercury telluride onto a sub- temperature 410°C; Hz dilution flow 400 strate comprising the steps of: ml/min; Cd alkyl flow 50 ml/min; Te alkyl flow 950 ml/min. providing an atmosphere of mercury vapour A water cooling jacket 22 at one end of the at a required temperature and pressure inside vessel 16 condenses out the unreacted a vessel; mercury and prevents overheating of reactor controlling the temperature of the substrate vessel and plate seals. The exhaust vapour mounted in the vessel; stream is then mixed with the bypass 14 stream controlling the temperature of the vessel of hydrogen and burnt in the combustion walls independently of the substrate temper- chamber 31 for safety reasons. ature and at a lower value than that of the A vacuum pump 30 is connected to the substrate; vessel 16 via a cold trap 29 for initial purging of providing a supply of a volatile cadmium alkyl the vessel 16. into the vessel and onto the substrate; The layer of CMT grown on the substrate providing a supply of a volatile tellurium alkyl may include one or more dopants. Such a into the vessel and onto the substrate; dopant is provided by passing hydrogen from the manifold through a mass flow controller 23 the arrangement being such that the cadmium to a bubbler 25 containing an alkyl of the and the tellurium alkyls decompose in the dopant. Alternatively a volatile hydride of the region of the substrate and the cadmium and dopant in hydrogen may be used. From the tellurium combine with mercury on the sub- bubbler the alkyl passes to the mixer 1 and strate to form a layer of cadmium mercury tel- thence to the vessel 16. Valves 26, 27, 28 luride whose cadmium composition is con- control the flow of hydrogen and alkyl. trolled by the supply of the cadmium alkyl and Examples of dopants and their alkyls are as the overall composition of the cation content is follows:- Al, Ga, As, and P from the respective controlled by the supply of the tellurium alkyl, alkyls (CH3)3AI, (CH3)3Ga, (CH3)3As, (CH3)3P. the mercury vapour pressure being held greater Examples of dopants and their hydrides are than the decomposition pressure of the growing as follows: Si, Ge, As, and P from their respec- layer. tive hydrides SiH4, GeH4, AsH3 and PH3. A 2. The method of claim 1 comprising the supply of the hydrides e.g. SiH4 may be supplied additional step of providing a supply of a volatile direct from gas cylinders. alkyl dopant or a volatile hydride dopant into the In a modification the cadmium and tellurium vessel and onto the substrate. alkyls may be supplied together or inde- 3. The method of claim 1 wherein the pendently direct into the vessel 16 to mix above cadmium alkyl is selected from dimethyl the substrate 20. These supplies may be via cadmium, diethyl cadmium, and dipropyl thermally-insulated tubes. cadmium. Using the above method and apparatus infra 4. The method of claim 1 wherein the tel- lurium alkyl is selected from diethyl telluride, 5. Verfahren nach Anspruch 2, dadurch dimethyl telluride, dipropyl telluride, dibutyl tel- gekennzeichnet, daß der flüchtige Dotierstoff luride, or an equivalent hydrogen substituted aus den Alkylen (CH3)3 Al, (CH3)3 As, (CH313 P telluride alkyl. oder den Hydriden SiH4, GeH4, AsH4, PH3 5. The method of claim 2 wherein the volatile gewählt wird. dopant is selected from the alkyls (CH3)3AI, 6 Verfahren nach Anspruch 1, dadurch (CH3)3Ga, (CH3)3As, (CH3)3P, or the hydrides gekennzeichnet, daß das Substrat auf einer SiH4, GeH4, AsH,, PH3. Temperatur im Bereich von 400 bis 430°C ge- 6. The method of claim 1 wherein the sub- halten wird und die Behälterwände auf einer strate is maintained at a temperature in the Temperatur im Bereich von 220 bis 320°C ge- range 400 to 430°C. and the vessel walls are halten werden. maintained at a temperature within the range of 220 to 320°C.

1. Une méthode pour faire croître une couche de l'alliage terniaire tellurure de cadmium et de mercure sur un substrat, comprenant les étapes 1. Verfahren zum Wachstum einer Schicht consistant à: aus der ternären Cadmium-Quecksilber-Tel- lurid-Legierung auf einem Substrat, gekenn- créer une atmosphère de vapeur de mercure zeichnet durch die Schritte: à une température et un pression requises à l'intérieur d'un récipient' Vorsehen einer Atmosphäre aus Queck- commander la température du substrat silberdampf bei einer erforderlichen Temper- monté dans le récipient; atur und Druck in einem Behälter; commander la température des parois du Steuern der Temperatur des im Behälter récipient indépendamment de la tempéra- montierten Substrats; ture du substrat, à une valeur plus faible que Steuern der Temperatur der Behälterwände celle du substrat; unabhängig von der Substrattemperatur und fournir une alimentation en alcoyle de auf einen niedrigeren Wert als den des Sub- cadmium volatil au récipient et sur le sub- strats; strat; Vorsehen eine Zufuhr eines flüchtigen Cad- fournir une alimentation d'un alcoyl de mium-alkyls in den Behälter und auf das Sub- tellure volatil au récipient et sur le substrat; strat; Vorsehen einer Zufuhr eines flüchtigen l'arrangement étant tel que les alcoyles de Telluralkyls in den Behälter und auf das Sub- cadmium et de tellure se décomposent dans la strat; région du substrat et que le cadmium et le tellure se combinent avec le mercure sur le sub- wobei die Anordnung derart ist, daß sich die strat pour constituer une couche de tellurure de Cadmium- und die Telluralkyle im Bereich des cadmium et de mercure dont la teneur en Substrats zersetzen und sich das Cadmium und cadmium est commandée par l'alimentation en Tellur mit Quecksilber auf dem Substrat unter alcoyle de cadmium et la composition générale Bildung von Cadmium-Quecksilber-Tellurid ver- de la teneur en cation est commandée par l'ali- binden, dessen Cadmiumzusammensetzung mentation en alcoyle de tellure, la pression de durch die Zufuhr des Cadmiumalkyls gesteuert vapeur de mercure étant maintenue plus élevée wird und dessen Kationengehalt der Gesamt- que la pression de décomposition de la couche zusammensetzung durch die Zufuhr des Tellur- en cours de croissance. alkyls gesteuert wird, wobei der Quecksilber- 2. La méthod de la revendication 1, com- dampfdruck höher als der Zersetzungsdruck der prenant l'étape supplémentaire consistant à wachsenden Schicht gehalten wird. fournir une alimentation en un dopant alcoyle 2. Verfahren nach Anspruch 1, gekenn- ou hydrure volatil au récipient et sur le substrat. zeichnet durch den zusätzlichen Schritt des Vor- 3. La méthode de la revendication 1, dans sehens einer Zufuhr eines flüchtigen Alkyl- lequelle l'alcoyle de cadmium est choisi parmi de dotierstoffes oder eines flüchtigen Hydrid- dimethyl cadmium, le diethyl cadmium et la dotierstoffes in den Behälter und auf das Sub- dipropyl cadmium. strat. 4. La méthod de la revendication 1, dans 3. Verfahren nach Anspruch 1, dadurch laquelle l'alcoyle de tellure est choisi parmi le gekennzeichnet, daß das Cadmiumalkyl aus diéthyltellurure, le diméthyltellurure, le Dimethylcadmium, Diäthylcadmium und Di- dipropyltellurure, le dibutyltellurure, ou un propylcadmium gewählt wird. alcoyle de tellure à hydrogène substitué 4. Verfahren nach Anspruch 1, dadurch équivalent. gekennzeichnet, daß das tellurid, Dipropyl- 5. La méthode de la revendication 2, dans tellurid, Dibutyltellurid oder einem äquivalenten laquelle le dopant volatil est choisi parmi les wasserstoffsubstituierten Telluridalkyl gewählt alcoyles (CH3)3AI, (CH3)3Ga, (CH3)3As, (CH3)3P wird. ou les hydrures SiH4, GeH4, AsH3, PH3. 6. La méthode de la revendication 1, dans 430°C tandis que les parois du récipient sont laquelle le substrat est maintenu à une tem- maintenues á une température dans le domaine pérature dans le domaine allant de 400 à allant de 220 à 320°C.