CA1229290A - Manufacture of cadmium mercury telluride - Google Patents

Manufacture of cadmium mercury telluride

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Publication number
CA1229290A
CA1229290A CA000462750A CA462750A CA1229290A CA 1229290 A CA1229290 A CA 1229290A CA 000462750 A CA000462750 A CA 000462750A CA 462750 A CA462750 A CA 462750A CA 1229290 A CA1229290 A CA 1229290A
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Prior art keywords
layer
substrate
compound
vessel
cadmium
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CA000462750A
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French (fr)
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Stuart J.C. Irvine
Jean Giess
John B. Mullin
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Qinetiq Ltd
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UK Secretary of State for Defence
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    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C16/00—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes
    • C23C16/44—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes characterised by the method of coating
    • C23C16/455—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes characterised by the method of coating characterised by the method used for introducing gases into reaction chamber or for modifying gas flows in reaction chamber
    • C23C16/45561—Gas plumbing upstream of the reaction chamber
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C16/00—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes
    • C23C16/22—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes characterised by the deposition of inorganic material, other than metallic material
    • C23C16/30—Deposition of compounds, mixtures or solid solutions, e.g. borides, carbides, nitrides
    • C23C16/305—Sulfides, selenides, or tellurides
    • C23C16/306—AII BVI compounds, where A is Zn, Cd or Hg and B is S, Se or Te
    • H—ELECTRICITY
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    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02367—Substrates
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    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
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    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02367—Substrates
    • H01L21/0237—Materials
    • H01L21/024—Group 12/16 materials
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    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02436—Intermediate layers between substrates and deposited layers
    • H01L21/02439—Materials
    • H01L21/02469—Group 12/16 materials
    • H01L21/0248—Tellurides
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
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    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02436—Intermediate layers between substrates and deposited layers
    • H01L21/02494—Structure
    • H01L21/02496—Layer structure
    • H01L21/02505—Layer structure consisting of more than two layers
    • H01L21/02507—Alternating layers, e.g. superlattice
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    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02518—Deposited layers
    • H01L21/02521—Materials
    • H01L21/02551—Group 12/16 materials
    • H01L21/02562—Tellurides
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    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
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    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02518—Deposited layers
    • H01L21/0257—Doping during depositing
    • H01L21/02573—Conductivity type
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    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02612—Formation types
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    • H01L29/00—Semiconductor devices adapted for rectifying, amplifying, oscillating or switching, or capacitors or resistors with at least one potential-jump barrier or surface barrier, e.g. PN junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof  ; Multistep manufacturing processes therefor
    • H01L29/02—Semiconductor bodies ; Multistep manufacturing processes therefor
    • H01L29/12—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by the materials of which they are formed
    • H01L29/22—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by the materials of which they are formed including, apart from doping materials or other impurities, only AIIBVI compounds
    • H01L29/2203—Cd X compounds being one element of the 6th group of the Periodic System
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    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/0248—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies
    • H01L31/0256—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by the material
    • H01L31/0264—Inorganic materials
    • H01L31/0296—Inorganic materials including, apart from doping material or other impurities, only AIIBVI compounds, e.g. CdS, ZnS, HgCdTe
    • H01L31/02966—Inorganic materials including, apart from doping material or other impurities, only AIIBVI compounds, e.g. CdS, ZnS, HgCdTe including ternary compounds, e.g. HgCdTe
    • H—ELECTRICITY
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    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/18—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof
    • H01L31/1828—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof the active layers comprising only AIIBVI compounds, e.g. CdS, ZnS, CdTe
    • H01L31/1832—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof the active layers comprising only AIIBVI compounds, e.g. CdS, ZnS, CdTe comprising ternary compounds, e.g. Hg Cd Te
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10S148/00—Metal treatment
    • Y10S148/063—Gp II-IV-VI compounds
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10S148/00—Metal treatment
    • Y10S148/064—Gp II-VI compounds
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10S148/00—Metal treatment
    • Y10S148/16—Superlattice

Abstract

ABSTRACT
A layer (37) of Cd Hg1-xTe is grown on a substrate (20) by growing layers of HgTe (35)x t1 thick and CdTe (36) t2 thick alternately.
The thicknesses t1 and t2 combined are less than 0.5 µm so that interdiffusion occurs during growth to give a single layer (37) of Cd Hg1-xTe. The HgTe layers (35) are grown by flowing a Te alkyl (7) into a vessel (16) containing the substrate (20) and filled with an Hg atmosphere by a Hg bath (19). The CdTe layers (36) are grown by flowing a Cd alkyl (6) into the vessel (16) where it combines preferentially with the Te on the substrate (20). Varying the ratio of t to t varies the value of x. Dopants such as alkyls or hydrides of Al, Ga, As and P, or Si, Ge, As and P respectively may be introduced (25) to dope the growing layer.

Description

MANUFACTURE I CADMIUM MERCURY TAILORED

The invention relates to the manufacture of the material cadmium mercury tailored i.e. Cud Hug To commonly referred to as CUT
x 1-x or MET

Such a material in its semi conducting form is used as a detector of infer red radiation in thermal imaging systems. These detectors comprise small pieces of CHIT cut and polished flat to which are attached electrical contacts. US Patent Specification Jo. 859,58~, published 25 January 1~61, describes the production and use of CUT
lo detectors.

At present CUT appears to be the most useful of all infer red detectors and is therefore used in the majority of high performance thermal image systems.
CUT is a difficult material to grow and handle, partly because of the volatile nature of the components.

Present methods of manufacture can be broadly classified into bull melt growth and epitaxial methods.

:~2~3~ 3 The most important melt growth methods are: The Bridgman method involving growth in a sealed container carried out in a vertical or horizontal manner; the cast quench anneal method; a cast recrystallize anneal method; and a so-called slush method. All these methods involve batch preparation that is lengthy and expensive taking weeks rather than days to complete. A further disadvantage is that the crystals produced are roughly cylindrical and need slicing, grinding, lapping, etching and dicing into small pieces for use as e.g. detectors.
Epitaxial methods of manufacturing semiconductors on the other hand are intrinsically quicker in so far as they produce thin layers of semiconductor material directly onto a substrate often in a matter of hours or minutes. In the case of materials like baas, In, and Gap well developed methods are available for the growth of homo-epitaxial layers of these compounds onto substrates of the parent semiconductor by either liquid or vapor phase processes. However no such well developed art is available in the case of CUT

In the case of the epitaxial growth of CUT from the liquid it has been reported by Herman, J. Electronic materials 8 (1979) 191; and by Schmitt and Bowers, Apply Pays. Letters I (1979) 457; and by awing et at, J. Electrochem. Sock 127 (19~30) 175; and by Bowers et at, I.E.E.E. Trans. Electron Devices ED 27 ~1980) I and by Wang et at, I.E.E.E. Trans. Electron Devices ED 27 (1980) 154; that it is possible to grow layers of CUT from supersaturated solutions in excess tellurium or mercury onto substrates of cadmium tailored (Cute). Such processes demand considerable skill and a very long development period. The epitaxial layers frequently suffer from surface blemishes which can render them useless for device fabrications. Such methods also suffer a fundamental limitation in respect of composition control i.e. the value of x (in Cud Hug To) cannot be independently controlled. Thus to produce epitaxial layers having different values of x it is necessary to use differently composed solutions of CUT in To.

A vapor phase epitaxial VIE process for growing Clout. has been reported by Vowel & Wolfe (J. Electronic Material, 7 (1978) 659).
This uses an open flow process with independently controlled sources of the elements Cud, Hug, and To. However this method suffers a fundamental limitation in the inability to effect adequate control of the values of x at the low deposition temperature that is needed to produce CUT particularly in the important range x = 0.2-0.3.
Because of the low vapor pressure of Cud and To in the region of 400C the input vapors can suffer a capricious reduction in lo composition before they reach the substrate. When the substrate is held at a temperature suitable for epitaxial CUT growth the temperature gradient in the deposition chamber is not high enough to prevent condensation of Cute upstream from the substrate.

Epitaxial layers of CUT have also been produced by subliming sources of Hate onto a Cute substrate in close probity - so-called close-spaced epitaxy - with or without the presence of additional Hug.
Examples include the wont; of Cohen-Solal and co-workers, and Tulle and Styler. References to these works can be found in I. Appl.Phys. 40 (1969) 4559.

This technique relies on the production of CUT by the inter diffusion of Cud and Hug between the substrate and the epitaxial layer. It suffers from the problem of compositional non-uniformity in the direction normal to the plane of the layer. It does not have the advantages of independent control of composition enjoyed by an open flow technique.

I

Another epitaxial growth technique it described in J. Electrochem Sock Vol. 128 (1981) p~1171.

In this technique a layer of Cute is deposited onto a mica substrate followed by a layer of Hate. This is later followed by annealing to cause inter diffusion and results in a single layer of Cud Hug To on mica. Only a single layer of one value of x can be grown in this manner. Also for some applications the presence of a mica substrate reduces or prevents good device performance.
Epitaxial layers of Gays have been grown successfully by VIE using gallium alkyd and Arizona. This contrasts with the situation concerning Clout. where it is common knowledge that the attempted growth of CUT using the three alkyds of the elements Cud, Hug and To in combination has not been successful.

One method for overcoming the above problems is described in Go Patent Application 2,078,695 A. This describes growing Cult onto a substrate using alkyds of Cud and To. The substrate is arranged in an atmosphere of Hug inside a chamber and Cud, and To alkyds are simultaneously admitted into the chamber. my controlling the substrate temperature independently from the chamber temperature and the various pressures and flow rates the material Cud Hug To is x 1-x grown.

A limitation with the above method is the precision with which ore can control the lateral uniformity of x. This becomes increasingly important when growing large areas of CUT for use in so-called staring arrays of detectors.

According to this invention the above problem is avoided by sequentially growing very -thin separate layers of Cole and Hate which inter diffuse whilst growing to give the desired Cud Hug To layer.
Each layer may be grown at its optimum growth conditions.
According to this invention a method of growing a layer of the ternary alloy Cud Hug To onto a substrate comprises the steps of providing an atmosphere of mercury vapor at a required temperature and pressure inside a vessel; controlling the temperature of the substrate independently of the vessel temperature; providing separate supplies of a cadmium compound, a tellurium compound, and a dilettante gas into the vessel to grow a layer of Hate t thick and a layer of Cute t thick in either order; switching the supply of cadmium compound to the substrate on and off to grow a layer of Cute and of Here! the combined thickness t + t of the two layers being not greater than 0.5 jut thick, the arrangement being such that the Cud compound decomposes preferentially with the To compound in -the region of the substrate to form Cute as a layer on the substrate, the To compound combines with the Hug vapor to form a Hate layer on the substrate, the thickness of both layers allowing diffusion during growth to give a layer of Cud Hug To where 0 < x < 1 preferably Owl x 0.9.
x ox The flow rate of Cud and To compounds may be greater during growth of Cute than the flow rate of compounds during growth of }lute. The supply of cadmium compound to the vessel may be arranged in the vessel to direct the cadmium compound over or away from the substrate. In another form the substrate may be moved between gas flows containing Cud with To and Hug, and To with Hug, both flows including the dilettante gas.

~22~2~

The grown CUT layer may be a single epitaxial layer or multiple layers. Such a CUT layer or layers may be selectively graded in composition. The CUT layer or layers may also be suitable doped.
For example two CUT layers may be grown with two different values of x so that a detector, sensitive to both the 3 to 5 and 8 to 14 sum wave bands may be made. Also a passivating layer of Cute may be grown on the Cud Hug To layer. Suitable II-VI compounds or mixed alloys may be grown on the layer e.g. Cute, Ins, Cute So which may be used to make hetero-junctions or form anti-reflection coatings, lo etc.

The grown layer may be annealed for a period preferably less than 10 minutes to allow further diffusion of the last grown layer. This annealing occurs under a flow of Hug and dilettante gas vapor of typically 0.05 atmospheric partial pressure of Hug and 500 cumin flow of H .

The value of x is determined by the thickness of the Cute and ilgTe layers according to the formula x = 2 t -I t The substrate may be Cute, a II-VI compound or mixed II-VI alloy, silicon (So), gallium arsenide (Gays), spinet (Meal 0 ), alumina or sapphire (Allah), etc.

I

The volatile cadmium compound may be an alkyd such as dim ethyl cadmium, deathly cadmium, or dipropyl cadmium, etc.

The volatile tellurium compound may be an alkyd such as deathly tailored, deathly tailored, dipropyl tailored, or dibutyl tailored, etc., or equivalent hydrogen substituted tellurium alkyds, such as, e.g. hydrogen ethyl tailored HO H ate.

Apparatus for growing a layer of cadmium mercury tailored according to the method of this invention, comprises a vessel containing a substrate, heating means for heating the vessel, a substrate heater, a means for supplying a mercury vapor inside the vessel, means for supplying a cadmium alkyd into the vessel, and means for supplying a tellurium alkyd or hydrogen substituted tellurium alkyd into the vessel.

The mercury vapor may be provided by a bath of mercury inside the vessel adjacent to the substrate.

The vessel heater may be an electrical resistance heater surrounding the vessel to heat both the vessel and mercury bath.

The substrate may be mounted on a carbon sister and heated by an I coil surrounding part of the vessel. Alternatively resistance heaters may be used inside the vessel, or an infer red heater may be caused to illuminate the substrate and/or substrate holder.

The compounds of Cud and To may be supplied by passing high purity hydrogen through two bubblers containing the appropriate compounds of Cud and Ten aye The invention will IIOW be described by way of example only with reference to the accompanying drawings of which:-Figure 1 is a schematic flow diagram; and Figures pa, b are sectional views of a substrate during growth and after growth of Cud Hug To material.
x ox As shown high purity hydrogen is supplied to a hydrogen manifold 1 which maintains a supply for five mass-flow controllers 29 3, 4, 5, and 23. Issue flow controller 2 supplies hydrogen via a bypass line 14 to a combustion chamber 31 which burns exhaust vapor in a hydrogen flame. assay flow controllers 3 and 4 supply hydrogen to alkyd bubblers 6, and 7, which respectively contain an alkyd of cadmium such as dim ethyl cadmium and an alkyd of tellurium such as deathly tailored. Hydrogen flow from the controllers 3 and 4 can be diverted via valves and 9 to the bypass line 14 or through valves lo 11 and 12, 13 thus enabling the alkyd flows to be turned on and off.
Hydrogen bubbling through the liquid alkyd will become saturated with alkyd vapors at the ambient temperature of the liquid alkyd, typically 25 C. These alkyd plus hydrogen streams are mixed in a mixer 15 with a further dilution flow of hydrogen supplied by the mass flow controller 5. Valves 32, 33 allow flow from the controller 5 to be directed to the mixer 15 and/or the bypass line 14. By control of flows through controllers 3, 4, and 5, the concentrations of cadmium and tellurium alkyds in the mixed stream can be independently determined over a wide range of values.

~l~2~3r~
g The alkyd plus hydrogen mixture is passed into a reactor vessel 15 which is heated with an electrical resistance furnace 17 and OF
induction coil 18. Inside the reactor vessel is a mercury bath 19 and a carbon sister 21 carrying the substrate 20 to be coated with a layer of CUT The furnace maintains the temperature of the reactor vessel wall from the mercury reservoir 19 to the substrate 20 equal to or greater than the mercury reservoir temperature, the mercury reservoir being heated by thermal conduction through the reactor wall 24. The Rough induction coil I couples into the carbon lo sister 21 thereby heating the substrate to a temperature above that of the reactor wall 24 so that the cadmium and tellurium alkyds will crack and deposit cadmium and tellurium onto the surface of the substrate 200 The temperature of the mercury reservoir 19 is determined by the requirement of the equilibrium partial pressure of I mercury to be maintained at the growth interface. The hot reactor wall 24 ensures that the mercury partial pressure in the vapor stream is the same at the substrate 20 as over the mercury reservoir 19.

The walls of the vessel 16 are sufficiently hot to prevent condensation of Hug without significant decomposition of the alkyds, whilst the temperature of the substrate 20 is sufficient to decompose the alkyds at the substrate 20 surface. The substrate may be inclined slightly e.g. 4 to give more uniform growth along the substrate.
A water cooling jacket 22 at one end of the vessel 16 condenses out the unrequited mercury and prevents overheating of reactor vessel and plate seals. The exhaust vapor stream is then mixed with the bypass 14 stream of hydrogen and burnt in the combustion chamber 31 for safety reasons.

A vacuum pump 30 is connected to the vessel 16 via a cold trap 29 for initial purging of the vessel 16.

_ To grow CUT on Cute a cleaned substrate 20 is placed on the sister 21 in the vessel 16. Typical growth conditions are: vessel walls 16 and Hug bath 19 temperatures 200-320 C Peg around 220 to 240 C);
pressure inside the vessel around atmospheric; substrate temperature 5 400-430 Keg around 410 C); alkyd bubbler 6, 7 temperature around 25C.

Valves 12, 13 are opened and 9 is closed to admit the To alkyd together with H into the vessel 16. The To and Hug combine and 10 form a Hate 35 (Figure 2) layer on the substrate. Typically a layer 0.16 sum thick is formed in less than one minute.

Valves 10, 11 are then opened and 8 is closed to allow Cud alkyd into the vessel 16 and valves 33 closed 32 opened to add a dilution flow.
15 The partial pressures of Cud and To are arranged to be about equal.
As a result the Cud alkyd combines preferentially with the To alkyd in the region of the substrate 20 to form a layer 36 of Cute. Typically a layer 0.04 sum thick is formed in less than one minute.

20 The combined thickness of the Hate and Cute layer it < 0.5 Jut thiclc, preferably < 0.25 sum thick. Also the ratio of t to + t ) is arranged so that the value of x lies in the required range for example 0.2 or 0.3 for use in infer red detectors.

Flow rate of gas through the vessel 16 varies with the growing layer.
Typically the flow rate during Cute growth is 4 to 12 times that during Hate growth. Typical flow rates for a 4 cm diameter vessel are 500 cumin during Hate growth and 4,000 cumin during Cute growth.

The valves 10, 11 and 8 are opened and closed Jo that alternate layers of Cute and Hate respectively are grown (figure aye Due to the small thickness of each grown layer diffusion takes-place during growth. The result is a layer 37 of material Cud Hg1 To (Figure 2b). Such diffusion is quite limited, typically to less than gym.
Thus the value of x can be changed during growth of many layers to give a device with a gradually changing composition (varying x) or one with sharp changes in the value of x.

The layer of CUT groom on the substrate may include one or more do pants. Such a Dupont is provided by passing hydrogen from the manifold through a mass flow controller 23 to a bubbler 25 containing an alkyd of the Dupont. Alternatively a volatile hydrides of the Dupont in hydrogen may be used. From the bubbler the alkyd passes to the mixer 15 and thence to the vessel 16. Valves 26, 27, I control the flow of hydrogen and ethyl.

Examples of do pants and their alkyds are as follows:- Al, Gay As, and P from the respective ethyls (OH ) Al, (OH ) Gay (Chihuahuas, 3 3 2 Swahili, (C2H5)3Ga, (C H ) p, Examples of do pants and their hydrides are as follows: Six Cue, At and P from their respective hydrides Six , Get , Ash and PHI, A supply of the hydrides e.g. Six may be supplied direct from gas cylinders.

In another form of the apparatus (not shown) the Cud compound supply 6 is connected via the valve 11 direct into the vessel 16 at a distance from the To compound entrance. A deflector is arranged inside the vessel and is movable to direct Cud either over or away from the lo substrate. This allows Cud, To, and Hug to flow over the substrate, to grow Cute; or for To and Hug to flow over the substrate and grow Hate.
An advantage of this arrangement is that gas flow rates can remain constant whilst the two layers, Cute, and Hate are rowing.

As an alternative to a movable deflector the substrate may be moved between two positions in the vessel. The first position is in a flow of Cud, To and Hug whilst the second position is in a flow of To and Hug gas.

Using the above method and apparatus infer red detectors may be made.
Such a detector may be a layer of Clout. on a Cute substrate with a passivating layer of oxide or Cute on the CUT layer surface. The detector may be in the form of a strip with electrodes on the surface at each end as described in US Patent Specification Jo. 1,488,258.
Such a detector is photo conductive and has the image of a thermal scene scanned over its surface.

Another type of IRE. detector uses a p-n j~mction e.g. the junction between two differently doped, p and n doped, CAM layers to form a photo-voltaic detector. A voltage is applied by electrodes across the p-n junction and changes in current are a measure of the infrared photons that are absorbed by the detector. Such a detector may be formed into a large array of IRE. detectors capable of imaging a thermal scene, without a scanning system, to form a so-called staring array system.

Claims (16)

Claims:-
1. A method of growing a layer of the ternary alloy CdxHg1-xTe onto a substrate comprises the steps of providing an atmosphere of mercury vapour at a required temperature and pressure inside a vessel; controlling the temperature of the substrate independently of the vessel temperature; providing separate supplies of a cadmium compound, a tellurium compound, and a dilutant gas into the vessel to grow a layer of HgTe t1 thick and a layer of CdTe t2 thick in either order; switching the supply of cadmium compound to the substrate on and off to grow a layer of CdTe and of HgTe, the combined thickness t1 + t2 of the two layers being not greater than 0.5 µm thick, the arrangement being such that the Cd compound decomposes preferentially with the Te compound in the region of the substrate to form CdTe as a layer on the substratre, the Te compound combines with the Hg vapour to form a HgTe layer on the substrate, the thickness of both layers allowing diffusion during growth to give a layer of CdxHg1-xTe where 0 < x < 1.
2. The method of claim 1 wherein 0.1? x ? 0.9.
3. The method of claim 1 wherein the combined thickness t1 + t2 is less than 0.25 µm.
4. The method of claim 1 wherein the temperature of the substrate is within 400 to 430°C.
5. The method of claim 1 wherein the ratio of t1 and t2 and hence the value of x is varied during growth of the Cd Hg1-xTe layer.
6. The method of claim 1 and further comprising the steps of stopping the flows of Cd and Te compounds over the substrate, allowing Hg and dilutant gas to flow over the substrate whilst the last grown layer diffuses with the previous layer for a period less than 10 minutes.
7. The method of claim 1 and further comprising the step of admitting a dopant compound into the vessel and over the substrate.
8. The method of claim 1 wherein a further layer is grown on the CdxHg1-xTe, said further layer being of CdTe, ZnS, or CdTexSe1-x.
9. The method of claim 1 wherein the Cd compound is an alkyl selected from dimethyl cadmium, diethyl cadmium, or dipropyl cadmium.
10. The method of claim 1 wherein the Te compound is on the alkyl selected from diethyl telluride, dimethyl telluride, dipropyl telluride, or dibutyl telluride, etc., or equivalent hydrogen substituted tellurium alkyls, or equivalent hydrogen substituted tellurium alkyls.
11. The method of claim 1 wherein the dopant compound is an alkyl selected from the group (CH3)3Al, (CH3)3 Ga, (CH3)3As, (CH3)3P, (C2H5)3Al, (C2H5)3Ga, (C2H5)P or a hydride selected from the group SiH4, GeH4, AsH3, PH3.
12. The method of claim 1 wherein the substrate is CdTe, Si, GaAs, MgAl204, or Al203.
13. The method of claim 1 wherein the supply of cadmium compound to the vessel is switched on and off whilst growing CdTe and HgTe layers.
14. A substrate having a layer of CdXHgl-xTe grown by the method of claim 1.
15. The substrate of claim 14 wherein the value of x varies within the layer.
16. The substrate of claim 14 having a passivating layer of CdTe grown on top of the CdxHgx-lTe layer.
CA000462750A 1983-09-13 1984-09-10 Manufacture of cadmium mercury telluride Expired CA1229290A (en)

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GB838324531A GB8324531D0 (en) 1983-09-13 1983-09-13 Cadmium mercury telluride

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GB2146663B (en) 1986-11-26
GB8422817D0 (en) 1984-10-17
JPS6077431A (en) 1985-05-02
EP0135344A1 (en) 1985-03-27
JPH0650744B2 (en) 1994-06-29
US4566918A (en) 1986-01-28
IL72790A (en) 1988-01-31
GB8324531D0 (en) 1983-10-12
GB2146663A (en) 1985-04-24
DE3466898D1 (en) 1987-11-26
EP0135344B1 (en) 1987-10-21

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