WO1992009719A1

WO1992009719A1 - Method for the deposition of group 15 and/or group 16 elements

Info

Publication number: WO1992009719A1
Application number: PCT/AU1991/000533
Authority: WO
Inventors: Geoffrey Norman Pain
Original assignee: The Commonwealth Industrial Gases Limited; Monash University; Australian And Overseas Telecommunications Corporation Limited
Priority date: 1990-11-23
Filing date: 1991-11-19
Publication date: 1992-06-11

Abstract

The present invention relates to a method for the metal organic chemical vapour deposition of a Group 15 and/or a Group 16 element on a substrate, characterized in that the method comprises employing as a feedstock at least one compound of the formula R2EER2, RE'E'R, R2EE'R, R2EE'ER2, RE(E'R)2 or E(E'R)3, wherein E is a Group 15 element, E' is a Group 16 element and R is an organic ligand. The present invention also provides methods for p-type doping of a II-VI semiconductor and N-type doping of a III-V semiconductor involving the method described above. A novel compound, ethyltellurodiethylstibine (Et2SbTeEt) is also described.

Description

METHOD FOR THE DEPOSITION OF GROUP 15 AND/OR GROUP 16 ELEMENTS

The present invention relates to a method for the deposition of Group 15 and/or Group 16 elements on a substrate.

Group 15 elements are also known as Group VA elements:specifically nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb) and bismuth (Bi). Group 16 elements are also known as the Group VIA elements: specifically oxygen (0), sulphur (S), selenium (Se), tellurium (Te) and polonium (Po).

Production of thin films on surfaces, such as semiconductor substrates, polycrystalline and amorphous powders and bulk crystals by the process of chemical vapour deposition continues to be the subject of considerable research and development. Semiconductor materials of group II-VI elements, such as ZnSe, find most useful application as thin films for devices such as electroluminescent sources, infrared detectors and solar cells. The pseudobinary alloy- mercury cadmium telluride, Hg*L__xCd_χTe (hereinafter referred to as MCT), is particularly valuable for fabrication of many such devices.

The process of metal organic chemical vapour deposition (hereinafter referred to as MOCVD), also known as metal organic vapour phase epitaxy (hereinafter referred to as MOVPE) is a most convenient and practical method for the production of high quality epitaxial MCT and related films. There is a requirement to achieve crystal growth of MCT and related materials by MOCVD at low temperatures and this can be achieved by a variety of known methods. However, the characteristics of the chemical feedstocks can limit the range of process parameters available to the crystal grower, as discussed in G.N. Pain et al, "Selection of organometallics for MOCVD of Hg*L__xCd_xTe and doped semiconductors", Polyhedron, 1990, 9(7), 921-929.

In the growth of MCT and related materials particularly desirable aims are (a) the realization of regions with controlled electrical characteristics through the depth of the layer and (b) the creation, through doping, of p-n electrical junctions where p-type and n-type materials have holes and electrons respectively, as the excess carriers. The range of devices which can be made incorporating p-n junctions include detectors, light emitting diodes, lasers, solar cells and high speed transistors. One approach to n-on-p junctions in MCT is to grow the MCT at high temperatures where the equilibrium cation vacancy defect level is high, resulting in as-grown p- type behaviour. The layer is then subjected to ion implantation and subsequent annealing to produce the junction. In this case the n-type MCT arises through electrically active damage in the layer and is largely independent of the chemical nature of the implanted element. This approach is laborious, expensive and it can be difficult to reproduce due to the commonly observed n-type skin on the p-type material. In addition, more sophisticated devices require highly perfect crystals for optimum performance and could not be made from implant-damaged material.

Another approach is to make p-on-n junctions by shallow implantation of arsenic ions into MOCVD grown MCT which has a weak unintentional n-type character followed by an activation anneal, as described in L.O. Bubulac, D. D. Edwall, D. McConnell, R.E. DeWames, E.R. Blasejewski and E.R. Gertner, "P-on-n arsenic -activated junctions in MOCVD LWIR HgCdTe/GaAs", Semicond. Sci. Technol., 1990, 5, S45-S48. This approach however inevitably leads to dopant concentration grading dependent on the implantation and interdiffusion profiles.

It is clearly desirable to be able to grow p-on-n and n-on-p junctions at will and to avoid implantation and annealing. It is also desirable to grow doping superlattices which are unobtainable by implantation processes.

MCT grown at low temperatures by MOCVD is generally n-type without deliberate doping as reported for example in G.N. Pain et al., "Large-area HgTe-CdTe superlattices and Hg*L__xCd_xTe multilayers on GaAs and sapphire substrates grown by low-temperature metalorganic chemical vapour deposition", J. Vacuum Sci. Technol., 1990, A 8(2), 1067-1077. Thus it is of prime importance to develop p-type doping capability at low growth temperatures in order to fully exploit the MOCVD process.

P-type doping has been achieved with Group 1 elements by molecular beam epitaxy as reported in P. S. Wijewarnasuriya, I.K. Sou, Y.J. Kim, K.K. Mahavadi, S. Sivananthan, M. Boukerche and J.P. Faurie, "Electrical properties of Li-doped Hg*L__χCd_χTe (100) by molecular beam epitaxy", Appl. Phys. Lett., 1987, 51(24), 2025-2027. However lithium and other elements of Group 1 are fast diffusers in MCT, and p-type doping of MCT by MOCVD has not been reported with these elements.

Group 15 elements are preferred because they are slow solid-state diffusers and hence should form stable device structures. Deliberate extrinsic p-type doping of CdTe by MOCVD was first reported by S.K. Ghandhi, N.R. Taskar and I.B. Bhat in "Arsenic-doped CdTe layers grown by organometallic vapour phase epitaxy", Appl. Phys. Lett., 1987, 50 (14) 900-902; N.R. Taskar, V. Natarajan, I.B. Bhat and S.K. Ghandhi "Extrinsic doped n- and p-type CdTe layers grown by organometallic vapour phase epitaxy", J. Crystal Growth, 1988, 86, 228-232.

Doping levels up to 2 X 10¹⁷ cm~***^t were obtained using arsine (ASH3) diluted in hydrogen. The application of this material to p-n junction solar cells was discussed in H.G. Bhimnathwala, N.R. Taskar, W.I. Lee, I. Bhat, S.K. Ghandhi and J.M. Borrego, "Photovoltaic properties of CdTe layers grown by OMVPE", Proceedings of the 19th IEEE Photovoltaic Specialists Conference, 1987, 1476- 1481. The p-type layers were grown at 350°C. Theoretical calculations showed that n-p CdTe solar cells could have an open circuit voltage of 0.90V, a short circuit current of 22.2 mA cm~2 and an efficiency of 21% under AMI.5 illumination. Subsequently it was shown that p-type MCT could be grown by MOCVD with arsine at 370"C in S.K. Ghandhi, N.R. Taskar, K.K. Parat, D. Terry and I.B. Bhat, "Extrinsic p-type doping of HgCdTe grown by organometallic epitaxy", Appl. Phys. Lett., 1988, 53(17), 1641-1643 and in N.R. Taskar, I.B. Bhat, K.K. Parat, D. Terry , H. Ehsani and S.K. Ghandhi, "The organometallic epitaxy of extrinsic p-doped HgCdTe", J. Vacuum Sci. Technol., 1989, A7(2) 281-284.

Under high arsine flow the cadmium fraction of the MCT was observed to decrease and this was attributed to undesirable prereaction of arsine with dimethylcadmium. Others have also reported p-doping MCT with arsine and phosphine in MOCVD at the higher temperature of 410°C, P. Capper, P.A.C. Whiffin, B.C. Easton, CD. Maxey and I. Kenworthy, "Group V acceptor doping of Cd_xHgι__xTe layers grown by metal-organic vapour phase epitaxy", Materials Lett., 1988, 6(10) 365-368; P. Capper, CD. Maxey, P.A.C. Whiffin and B.C. Easton, "Incorporation and activation of Group V elements in MOVPE-grown Cd_xHg*L__xTe", J. Crystal Growth, 1989, 97, 833-844; CD. Maxey, P. Capper, P.A.C Whiffin, B.C. Easton, I. Gale, J.B. Clegg, A. Harker and C.L. Jones, "Extrinsic doping at low concentrations for Cd_jjHg-_j^._jjTe layers grown by MOVPE", J. Crystal Growth, 1990, 101, 300-304.

It was found that arsenic was only incorporated in the CdTe layers during the Interdiffused Multilayer Process (IMP) used to grow the MCT. This implies that dimethylcadmium reacts with arsine below the growth temperature, possibly via a Lewis acid - Lewis base adduct. The failure to incorporate arsenic in the HgTe layers is to be expected due to the thermal stability of arsine, which does not decompose below 500^βC and its lack of reactivity toward the organometallic tellurium source and mercury.

In a subsequent study, CD. Maxey, P. Capper, P.A.C. Whiffin, B.C. Easton, I. Gale, J.B. Clegg and A. Harker, "Arsenic diffusion effects in Cd_xHg₁__xTe layers grown by metal-organic vapour phase epitaxy", Materials Lett., 1989, 8(5), 190-193, arsenic interdiffusion profiles in interdiffused multilayer process (hereinafter referred to as IMP) grown MCT were reported showing that extensive post-growth annealing at high temperatures would be required to evenly distribute the arsenic throughout the epilayer. It would be desirable therefore to find a p- type dopant which could be incorporated in both the CdTe and HgTe layers of IMP grown MCT. The low interdiffusion rate found does however indicate that junctions once formed by arsenic doping would be stable.

The extreme toxicity of arsine and phosphine, their high thermal stability, tendency to prereaction with dimethylcadmium and the need to use high pressure gas cylinders with the active gases diluted in order to give controllable low-level dopant delivery, make these gases undesirable feedstocks for MOCVD doping of MCT.

Attempts at p-doping MCT using organometallics of the Group 15 elements have been reported, J.S. Whiteley, P. Koppel, V.L. Conger and K.E. Owens, "Annealing and electrical properties of organometallic vapour phase epitaxy-interdiffused multilayer process grown HgCdTe", J. Vacuum Sci. Technol., 1988, A6(4), 2804-2807. Using trimethylarsenic (hereinafter referred to as TMAs) p-type material with carrier concentration of up to 10**-⁷ cm"^*--' was obtained at a growth temperature of 410°C and an injected Cd:Te:As ratio of 182:182:30. This indicates that less than 0.01% of the available arsenic was incorporated or electrically active, consistent with the high thermal stability of TMAs. When trimethylantimony (hereinafter referred to as TMSb) was used, p-type material was obtained but the cadmium fraction of the layers was reduced and poor surface morphologies were observed. Only a small fraction of available antimony was incorporated or electrically active. The pyrolysis of TMSb in hydrogen has been studied in D.A. Jackson, Jr., "Influence of carrier gases on pyrolysis of organometallics", J. Crystal Growth, 1989,94, 459-468, with the onset of decomposition recorded at 382°C TMAs is known to be more stable than TMSb. Thus both TMAs and TMSb are unsatisfactory p-type dopants for low temperature MOCVD of MCT due to their thermal stability and high vapour pressures. It is likely that both compounds would lead to carbon contamination of epilayers due to the strong element- carbon bonds.

The triethyl derivatives Et E (E = As, Sb or Bi) have been evaluated for MOCVD applications in V.A.

Yablokov, I.A. Zelyeav, E.I. Makarov and N.S. Lokhov, "Kinetic study of the thermal decomposition of ethyl derivates of arsenic, antimony and bismuth", Zhurnal Obschei Khimii, 1987, 57(9), 2034-2037 with the decomposition temperature decreasing down the group. As expected, they have considerably lower vapour pressures (6.2 and 2.9 mm Hg for the arsenic and antimony compounds, respectively at 20°C) than the methyl compounds. These vapour pressures are still relatively high for the purpose of introducing dopant levels of the elements.

There is thus clearly a need for an improved method of MOCVD for depositing specific element(s) on a substrate, and one object of the present invention is to provide such a method. A further object of this invention is to provide an improved method of MOCVD which is also capable of being used for p-type or n-type doping of semiconductors.

According to one aspect of the present invention there is provided a method for the metal organic chemical vapour deposition of a Group 15 and/or a Group 16 element on a substrate, which comprises employing as a feedstock at least one compound of the formula R₂EER2, RE'E'R, R₂EE'R, R₂EE'ER₂, RE(E'R)₂ or E(E'R)_3^ wherein E is a Group 15 element, E' is a Group 16 element and R is an organic ligand.

According to another aspect of the present invention there is provided a method for the metal organic chemical vapour deposition of a material having the stoichiometry EE' on a substrate, wherein E is a Group 15 element and E' is a Group 16 element, which comprises employing as a feedstock at least one compound of the formula R EE'R, R EE'ER₂, RE(E'R)₂ or E(E'R)₃ wherein E and E' are as defined above and R is an organic ligand.

In a specific application, the methods of the present invention may be used in p-type or n-type doping of semiconductors. Compounds suitable for use as a feedstock in the p- type doping method of the present invention generally possess the following characteristics:

(i) a low decomposition temperature;

(ii) compatibility with the other reagents, i.e. no appreciable prereaction; (iii)decomposition without retention of carbon; (iv) usually a liquid so that the conventional "bubbler" source container can be used; (v) a sufficiently low vapour pressure between 0°C and ambient temperatures that it can be used with standard mass flow controllers to give a wide range of dopant atom concentrations; and

(vi) the dopant atom should have a low interdiffusion coefficient at the growth temperature.

Thus, according to a further aspect the present inventi n there is provided a method for p-type doping of a II-VI semiconductor, which comprises depositing one or more dopants while growing the semiconductor by employing as the feedstock to a metal organic chemical vapour deposition process at least one low vapour pressure compound of the formula R₂EER₂, RE'E'R, R₂EE'R, R₂EE'ER₂, RE(E'R)₂ or E(E'R) , wherein E is a Group 15 element, E' is a Group 16 element and R is an organic ligand.

Examples of compounds suitable for use as a feedstock in the p-doping method of the invention and which possess the aforementioned characteristics include tetraethyldiarsine (Et₂AsAsEt₂)and tetraethyldistibine (Et₂SbSbEt₂). Low vapour pressure refers to pressures less than lmm Hg at room temperature.

The p-type doping method of the present invention may be used to produce the commercially significant p- type MCT.

According to a still further aspect of the present invention there is provided a method for n-type doping of a III-V semiconductor, which comprises depositing one or more dopants while growing the semiconductor by employing as the feedstock to a metal organic chemical vapour deposition process at least one low vapour pressure compound of the formula RE'E'R, R₂EE'R, R₂EE'ER₂,

RE(E'R)₂ or E(E'R)₃, wherein E is a Group 15 element, E' is a Group 16 element and R is an organic ligand.

The present invention also provides a method for the metal organic chemical vapour deposition of a material having the stoichemistry EE'X on a substrate, wherein E is a Group 15 element, E' is a Group 16 element and X is a halogen which comprises employing as a feedstock at least one compound of the formula R₂EE'R, R₂EE'ER₂, RE(E'R)₂ or E(E'R) , wherein E and E' are as defined above and R is an organic ligand, together with a separate volatile source of halogen.

Preferably, the Group 15 elements are selected from phosphorus, arsenic, antimony and bismuth and the Group 16 elements are selected from sulphur, selenium and tellurium. The organic ligands are suitably of the type which will be non-deleterious when used in the methods of the invention. Examples include alkyl, cycloalkyl, vinyl, alkoxy and aryl, each of which may be optionally substituted. A suitable optional substituent is alkyl. Preferably the organic ligand is a C₂_₂o alkyl group, more preferably ethyl, tert-butyl, iso-propyl, 1-ethylpropyl or 2-ethylbutyl.

Suitable materials having a stoichiometry of EE' include AsSe and BiTe. An example of a material having a stoichiometry of EE'X is SbTel.

The grown film or the substrate may be of any suitable known type for example, metals, alloys, glasses, oxides, chalcogenides, pnictides, superconductors, semiconductors, polycrystalline powders, amorphous powders and bulk crystals.

Examples of II-VI semiconductors include ZnSe, CdTe and HgTe. Examples of III-V semiconductors include InSb and InP.

The compound ethyltellurodiethylstibine (Et₂SbTeEt), which is suitable for use in the methods of the invention, is novel and accordingly contributes a further aspect of the present invention.

A most convenient and preferred route for the synthesis of ethyltellurodiethylstibine (EtTeSbEt ) and related compounds is the direct reaction of the appropriate ditelluride or diselenide with the diarsine, distibine or dibismuthine. The reaction can be conducted in the absence of solvent, in which case distillation of the low boiling product is unnecessary, as taught in H. Breunig, W.W. du Mont and T. Severengir "Dimethyl( -tolyltelluro)stibine", Organometallic Syntheses (R.B.King and J.J. Eisch eds.). Volume 4, Elsevier, Amsterdam, 1988 p.587-8. Other synthetic routes include treatment of e.g. the salt NaTeR with the dialkylhalide of As, Sb or Bi.

The methods of the invention may be performed in any suitable known MOCVD reactor, for example, the reactor described in G. N. Pain et al., "Large area HgTe-CdTe superlattices and Hg*_j___χCd_χTe multilayers on GaAs and sapphire substrates grown by low-temperature metal organic chemical vapour deposition", J. Vac. Sci. Technol., 1990, A8(2), 1067-77.

Compounds of the type RE'E'R (E' = S, Se or Te) and R EER₂ (E =■ P, As, Sb or Bi) (R = e.g. methyl, ethyl, benzyl, CF ) may be synthesized via a number of routes, and undergo useful exchange reactions, as shown in A.J. Ashe and E.G. Ludwig "The exchange reaction of tetramethyldipnictogens with dimethyldichalcogenides", J. Organometallic Chem. , 1986, 308(3), 289-96; "The exchange reaction of tetramethyldiphosphine, -diarsine, -distibine and -dibismuthine" J. Organometallic Chem., 1986, 303(2), 197-204 and references therein. Alkali metal reduction of the element in liquid ammonia followed by treatment of the salts with alkyl halides is an effective route to many compounds of interest, as shown in G.M. Bogoyubov, Yu.N. Shlyk and A.A. Petrov, "Organic derivatives of Group V and Group VI elements VII. Ammoniacal synthesis of disulfides, diselenides, ditellurides and distibines" Zhurnal Obshchei Khimii, 1969, 39 (8), 1804-1808 and references therein. This paper also discloses the boiling point of tetraethyldistibine as 94°C at 1 mm Hg pressure. Tetraethyldistibine is also prepared in good yield by reduction of diethylantimonybromide with magnesium as reported in H.J. Breunig, V. Breunig-Lyriti and T.P. Knobloch "Einfache Synthesen von Tetramethyl- und Tetraethyldistiban" Chemiker Zeitung, 1977, 101, 399- 400, where the boiling point was 55°C at 0.02 mm Hg. Tetraethyldiarsine has a low vapour pressure of 1 mm Hg at 20°C compared to trimethylarsine which has vapour pressure of 227.3 mm Hg at the same temperature. Thus, the binuclear derivatives have considerably lower vapour pressures than the mononuclear species and fulfil a prime requirement for p-type dopants.

A study of binuclear ethyl derivatives of antimony, selenium and tellurium by mass spectrometry (G.M. Bogolyubov, N.N. Grishin and A.A. Petrov "Organic derivatives of Group V and Group VI elements X. Mass spectra of tetraethyldistibine, diethyldiselenide, diethylditelluride and the corresponding monoderivatives. Interpretation of mass-spectral intensities" Zhurnal

Obshchei Khimii, 1969, 39(10) 2244-2252) indicated that intact ethyl groups leave the molecules, so that carbon contamination is unlikely if these compounds are used as feedstocks for MOCVD. The element to ethyl bond strength is considerably weaker than the element to methyl bond strength for all elements studied (see e.g. G.B. Stringfellow "Organometallic Vapour-phase Epitaxy - Theory and Practice" Academic Press 1989, Chapter 2. )

Tetraethyldibismuthine, the propyl, isopropyl and butyl derivatives have been reported by H.J. Breunig and D. Mueller "Et4Bi₂: a binuclear bismuth compound" Angew. Chem. 1982, 94(6), 448; "R₄Bi₂; tetraalkyldibismuthines" Z. Naturforsch. 1983, B38(2), 125-129. The methyl derivative disproportionates to trimethylbismuth and bismuth at 25°C as reported in A.J. Ashe and E.G. Ludwig, Jr. "A reinvestigation of Paneth's violet compound. The synthesis of Me₄Bi₂" Organometallics, 1982, 1(10), 1048- 1410. In contrast, tetrakis(trimethylsilyl)dibismuthine and some related dibismuthines are thermally stable as reported in G. Becker and M. Rossler "Trimethylsilyl derivatives of VB elements. 3.", Z. Naturforsch., 1982, B37(l) 91-96 and A.J. Ashe, E.G. Ludwig and J. Olksyszyn, "Preparation and properties of dibismuthines", Organometallics, 1983, 2(12) 1859.

Dibismuthines react with RE'E'R (E' = S, Se or Te) to give R' BiE'R in good yields according to M. Wieber and I. Sauer, "Dimethyl(phenylchalcogeno)bismuthines- Molecules with Bi-S, Bi-Se & Bi-Te bonds", Z. Naturforsch., 1984, 39B(12), 1668. Similarly distibines react with diselenides and ditellurides to give R₂SbE'R' e.g. Et₂SbTeR', H.J. Breunig and H. Jawad, "Syntheses of distiba-selenanes, -telluranes and a tellurostibane", J. Organometal. Chem. 1984, 277(2), 257-60; H.J. Breunig and S.Sabahittin, "Preparation of chalcogenostibines of the type R₂SbER^r (R = Me, Et; E = S, Se, Te; R' = Me, Ph) by exchange reactions of distibines and dichalcogenides", Z. Naturforschung, 1986, 41B, 1387-90 and references therein.

Other compounds of low volatility are of the type RE(E'R)₂, E(E'R)₃ or R₂EE'ER as reported in H.J. Breunig and H. Hussain, "Syntheses of distibinoselenides and tellurides and a tellurostibine", J. Organometallic Chem., 1984, 277(2), 257-60; H.J. Breunig and D. Ditmar "Reactions of tetrapropyldibismuthine with chalcogens and tetramethyldistibane", Z. Naturforschung, 1986, 41B(9), 1129-32; M. Wieber and I. Sauer "Synthesis of etra-p- tolyldibismuthine and its cleavage with diphenyldichalcogenides, elemental sulfur, selenium and p-benzoquinone", Z. Naturforschung 1987, 42B(6), 695-8; H.J. Breunig and S. Geulec "Preparation of dichalcogenostibines or the type RSb(ER' )₂ (E = S or Se)", Z. Naturforschung., 1988, 43B(8), 998-1002; J.J.I. Arsenault and P.A.W. Dean, "A preparative and multinuclear magnetic resonance spectroscopic study of As(SPh)_x(SePh)₃__x(x =0-3), Sb(SPh)_x(SePh)₃__x(x = 0-3), Bi(SPh)₃, Bi(SePh)₃, [Sn(SPh)_x(SePh)₃__x]"(x = 0-3), Pb(SePh) ~, and Pb(SPh)₃~ and some related thiolatoρlumbates(II)", Can. J. Chem. 1983, 61(3), 1516- 23; R.A. Pyles, K.J. Irgolic and G.C Pappalardo "Study of arsenic-chalcogen bond: conformational and molecular properties of tris(phenylchalcogen) arsines, As(XPh)₃(X = S, Se, Te) and related compounds", Congr. Naz. Chim. Inorg. [Atti] , 12th, pl41-2. These and similar compounds are suitable as feedstocks and dopants for MOCVD.

The invention is further described in and illustrated by the following examples. These examples are not to be construed as limiting the invention in any way.

The following abbreviations are used:

MOCVD - metal organic chemical vapour deposition

MCT - mercury cadmium telluride (Hg*L__χCd_χTe).

V - voltage

R - resistance

Rs - sheet resistance Rho - resistivity

B - magnetic field

Rh - Hall coefficient

n - carrier concentration

EXAMPLE 1 - Preparation of ethyltellurodiethylstibine

A stoichiometric quantity of diethylditelluride was added to a sample of tetraethyldistibine in a stainless steel bubbler, held at - 78°C, and the vessel was slowly warmed to room temperature. The bubbler was carefully evacuated at room temperature in order to remove traces of starting materials, leaving the product EtTeSb(Et)₂, as a low vapour pressure liquid, in near quantitative yield. The compound was characterized by nuclear magnetic resonance spectroscopy, *--H, delta 2.48 (quartet), 1.5 (multiplet), 1.48 (triplet), 1.23 (triplet) ²J(¹H, ¹²⁵Te) 25.0 Hz, 7.6 HZ (-TeEt), ³J(¹H, --H) 7.7 Hz (-SbEt₂); ¹³C{¹H}, delta 21.2, 11.8, 5.9, -10.9; mass spectrometry 338.1 (P⁺), 309.1 (P-Et), 281.0 (P-Et-C₂H ), 250.9 (SbTe).

EXAMPLE 2 - Preparation of p-type mercury cadmium telluride

Mixed cool vapours of dimethylcadmium, diethyltellurium and tetraethyldistibine were introduced into a horizontal MOCVD reactor using palladium alloy diffused hydrogen as the carrier gas. A description of the apparatus may be found in G.N. Pain et al, "Large area HgTe-CdTe superlattices and Hgι__xCd_xTe multilayers on GaAs and sapphire substrates grown by low-temperature metalorganic chemical vapour deposition", J. Vac. Sci. Technol., 1990, A8(2), 1067-77. A thin layer of doped CdTe was grown followed by a thin layer of undoped HgTe and the process was repeated until the desired thickness of material was deposited. The multilayer film was annealed and solid-state interdiffusion yielded p-type MCT. Room temperature Hall effect measurements using gold metallization yielded calculated hole mobilities of 40-100 cm²/Vsec and hole concentrations of 9 x lO¹^ to 10¹⁷ cm^-3.

EXAMPLE 3 - Resistivity and Hall Coefficient measurements on antimony doped MCT

The following resistivity and Hall Coefficient measurements were carried out on antimony doped MCT.

Trial A3

Sample current = .001 mA Sample thickness = 1 μm

Magnetic field = 1.9 kG UP

RESISTIVITY

Rs = 107.29E+3 Ohm/sq Rho = 10.73 Ohm.cm HALL COEFFICIENT

V:B=0 (mV) V:B on (mV) Hall Voltage .(mV)

-3.043 -2.866 .178

-3.009 -3.004 .005

1.453 1.228 -.225

2.494 2.298 -.195

Rh = 313.22 Mobility = 29.19 cπT2/(V.s) n = 1.993E+16 cm"-3

Trial BI

Sample current .001 mA

Sample thickness = 1 μm Magnetic field 1.9 kG UP

RESISTIVITY

Rs = 111.5E+3 Ohm/sq Rho = 11.15 Ohm.cm

HALL COEFFICIENT

V:B=0 (mV) V:B on (mV) Hall Voltage (mV)

1.203 1.197 -.005

1.215 1.031 -.184

-1.743 -1.661 .082

-1.790 -1.901 -.111

Rh = 286.84 Mobility = 25.73 cm~2/(V.s) n = 2.176E+16 cm"-3

Trial D3 Sample current = .001 mA Sample thickness = 1 μm Magnetic field = 1.9 kG UP RESISTIVITY

HALL COEFFICIENT

V: B=0 (mV ) V:B on (mV) Hall Voltage (mV)

3.133 3.125 -.008

2.993 2.914 -.079

-3.400 -3.550 -.150

-3. 657 -3.821 -.164

Rh = 528.16 Mobility = 49.45 cm"2/(V.s) n = 2.182E+16 cm"-3

Similar voltage and resistance values were obtained for the remaining trials at identical sample current, sample thickness and magnetic field values. For brevity, only a summary of the results of the remaining trials is given.

Trial A2

Rs = 89.58E+3 Ohm/sq Rho = 8.96 Ohm.cm

Rh 449.54 Mobility 50.18 cm"2/(V.s) n = 1.389E+16 cπT-3

Trial B2

Rs = 88.36E+3 Ohm/sq Rho - 8.84 Ohm.cm

Rh 470.79

Mobility 53.28 cm"2/(V.s) n 1.326E+16 cm"-3 Trial C2

Rs = 112.67E+3 Ohm/sq Rho = 11.27 Ohm.cm

Rh - l.E+3 Mobility = 89.11 cm"2/(V.s) n = 6.218E+15 cm"-3

Trial D2

Rs = 90.51E+3 Ohm/sq Rho = 9.05 Ohm.cm

Rh = 490.13 Mobility = 54.15 cm'2/(V.s) n = 1.274E+16 cm"-3

Trial Al

Rs = 81.43E+3 Ohm/sq Rho = 8.14 Ohm.cm

Rh = 403.55 Mobility = 49.56 cm"2/(V.s) n = 1.547E+16 cm"-3

Trial Cl

Rs = 82.95E+3 Ohm/sq Rho = 8.29 Ohm.cm

Rh = 555.26 Mobility = 66.94 cm"2/(V.s) n = 1.124E+16 cm"-3

Trial Dl

Rs = 117.66E+3 Ohm/sq Rho =^* 11.77 Ohm.cm

Rh = 1.14E+3 Mobility = 96.95 cm"2/(V.s) n = 5.472E+15 cm"-3 Trial A4 Rs = 138.1E+3 Ohm/sq

Rho = 13.81 Ohm.cm

Rh = 74.74

Mobility = 5.41 cm"2/(V.s) n = 8.352E+16 cm"-3

EXAMPLE 4 - Electrical Characterization of InSb/MCT/GaAs

The electrical data from Hall effect devices fabricated from a layer of InSb 0.2 to 0.3 microns thick on a 3 year old buffer layer of MCT was obtained.

The sample had high carrier (2 x lO-^***-** electrons cm" ) concentration at room temperature and at 77K which is believed to be due to tellurium doping. It is also possible that a high concentration sheet of carriers is present.

The carrier mobility was calculated ignoring effects of the MCT layer and ranged from 1.3 to 3.4 x 10³ cm²/Vsec (average over 8 devices 2.23 x 10³) at room temperature, with a slight increase at 77K.

These figures are consistent with antimony-rich material and compare favourably with results from R.M. Biefeld and G.A. Hebner, Appl. Phys. Lett. 1990, 57, 1563 which reported MOCVD grown antimony rich InSb on GaAs with mobility of 1830 cm²/Vsec at 300K for samples greater than 2 microns thick. R.M. Biefeld and G.A. Hebner, have shown that it is crucial to adjust the stoichiometry of growth and under these conditions obtained mobilities up to 60,900 at room temperature reducing to 27,000 at 77K. InSb films grown by flash evaporation have electron mobility of 1 cm²/Vsec. Flash evaporation is described in S. Kaur and R.K. Bedi, Mat. Res. Bull. 1990, 25, 1421.

Claims

1. A method for the metal organic chemical vapour deposition of a Group 15 and/or a Group 16 element on a substrate, characterized in that the method comprises employing as a feedstock at least one compound of the formula R₂EER₂, RE'E'R, R₂EE'R, R₂EE'ER₂, RE(E'R)₂ or E(E'R)₃ wherein E is a Group 15 element, E' is a Group 16 element and R is an organic ligand.

2. A method as claimed in Claim 1 for the metal organic chemical vapour deposition of a material having the stoichiometry EE' on a substrate, wherein E is a Group 15 element and E' is a Group 16 element, characterized in that the method comprises employing as a feedstock at least one compound of the formula R₂EE'R, R₂EE'ER₂, RE(E'R)₂ or E(E'R)₃ wherein R is an organic ligand.

3. A method as claimed in Claim 1 for p-type doping of a II-VI semiconductor, characterized in that the method comprises depositing one or more dopants while growing the semiconductor by employing as the feedstock to a metal organic chemical vapour deposition process at least one low vapour pressure compound of the formula R₂EER₂, RE'E'R, R₂EE'R, R₂EE'ER₂, RE(E'R)₂ or E(E'R)₃, wherein E is a Group 15 element, E* is a Group 16 element and R is an organic ligand.

4. A method as claimed in Claim 1 for n-type doping of a III-V semiconductor, characterized in that the method comprises depositing one or more dopants while growing the semiconductor by employing as the feedstock to a metal organic chemical vapour deposition process at least one low vapour pressure compound of the formula

RE'E'R, R₂EE'R, R₂EE'ER₂, RE(E'R)₂ or E(E'R)₃, wherein E is a Group 15 element, E' is a Group 16 element and R is an organic ligand.

5. A method as claimed in Claim 1 for the metal organic chemical vapour deposition of a material having the stoichemistry EE'X on a substrate, wherein E is a Group 15 element, E' is a Group 16 element and X is a halogen, characterized in that the method comprises employing as a feedstock at least one compound of the formula R₂EE'R, R₂EE'ER₂, RE(E'R)₂ or E(E'R)₃, wherein E and E' are as defined above and R is an organic ligand, together with a separate volatile source of halogen.

6. A method as claimed in any one of the preceding claims, characterized in that E is selected from phosphorus, arsenic, antimony and bismuth.

7. A method as claimed in any one of the preceding claims, characterized in that E' is selected from sulphur, selenium and tellurium.

8. A method as claimed in any one of the preceding claims, characterized in that R is selected from alkyl, cycloalkyl, vinyl, alkoxy and aryl, each of which may be optionally substituted.

9. A method as claimed in any one of the preceding claims, characterized in that R is a C₂__2Q alkyl group.

10. A method as claimed in any one of the preceding claims, characterized in that R is selected from ethyl, tert-butyl, iso-propyl, 1-ethylpropyl and 2-ethylbutyl.

11. A method as claimed in any one of Claims 1, 3, 6 and 8 to 10, characterized in that the compound of the formula R₂EER₂ is tetraethyldiarsine (Et₂AsAsEt₂) or tetraethyldistibine (Et SbSbEt₂).

12. A method as claimed in any one of Claims 1 to 10, characterized in that the compound of the formula R₂EE'R is ethyltellurodistibine (Et₂SbTeEt).

13. A method as claimed in any one of the preceding claims, characterized in that the substrate is selected from metal, alloy, glass, oxide, chalcogenide, pnictide, superconductor, semiconductor, polycrystalline powder, amorphous powder and bulk crystal.

14. A p-type doped II-VI semiconductor, whenever produced by a method as claimed in Claim 3.

15. An n-type doped III-V semiconductor, whenever produced by a method as claimed in Claim 4.

16. Ethyltellurodiethylstibine (Et₂SbTeEt)