GB2221924A - Epitaxial films of zinc selenide and zinc sulphide - Google Patents

Abstract

A method of producing epitaxial films of ZnSe and ZnS on a substrate using metalorganic chemical vapour deposition (MOCVD) in a reaction chamber comprises heating a substrate in a reaction chamber and passing H2S or H2Se gas, an adduct and a diluent gas, e.g. H2, over the heated substrate. The adduct is dimethylzinc-trialkylamine, (CH3)2Zn(N(Ra)3) (N(Rb)3), or diethylzinc-trialkylamine, (C2H5)2Zn(N(Ra)3(N(Rb)3, wherein Ra and Rb are alkyl groups taken from the list of methyl, ethyl, propyl and butyl. When dimethylzinc-triethylamine is used as an adduct, the substrate is heated within the range 300 DEG C and 450 DEG C for ZnS growth, and within the range 275 DEG C and 500 DEG C for ZnSe growth. The adduct flow may be within the range 25-100 cc min<-1> and the H2S or H2Se gas flow may also be within the range 25-100 cc min<-1>.

Description

GROWTH OF ZnSe AND ZnS LAYERS This invention relates to a method of growth of inorganic thin films of ZnSe and ZnS using metalorganic chemical vapour deposition (MOCVD) techniques.

The use of low temperature epitaxial growth methods such as MOCVD for obtaining layers of wide-band gap II-VI compounds, including ZnSe and ZnS, is well established (eg P J Wright and B Cockayne, Journal of Crystal Growth 59 (1982) p148). However, the potential of this materials technology to provide short wavelength optoelectronic devices has, thus far, been limited by two major factors. These are, firstly, the prereaction between constituent reactants in MOCVD, which limits the uniformity in growth and, secondly, the presence of deep centres within the band gaps of these II-VI compounds which restricts the near-band edge luminescence and can inhibit attempts to produce p-type material.

The normal method of producing zinc-chalcogenide layers by MOCVD is to react dimethylzinc ((CH3)2Zn) with the appropriate group VI hydride (eg H2S for the growth of ZnS or H2Se for the growth of ZnSe). It is this combination which is very susceptible to prereaction.

Most efforts to try to alleviate this problem have concentrated on the group VI constituent, Se or S, combined with either heterocyclic or alkyl groups (eg B Cockayne and P J Wright, Journal of Crystal Growth 68 (1984) p26, P J Wright, R J M Griffiths and B Cockayne, Journal of Crystal Growth 66 (1984) p26 and T Yokogawa, M Ogura and T Kajiwara, Applied Physics Letters 50 (1987) p1065).

These routes certainly either eliminate or limit prereaction but generally invoke the penalty of higher deposition temperatures and lead to increased concentrations of deep centres, although methyl selenol has been shown to reduce prereaction for the growth of ZnSe whilst maintaining a low temperature growth regime (S Fujita, T Sakamoto, M Isemura and S Fujita, Journal of Crystal Growth 87 (1988) p581).

An alternative approach has concentrated on modifying the zinc source with the use of adducts. An adduct is defined as a compound formed between an electron acceptor molecule (Lewis Acid) and an electron donor moleucle (Lewis Base). Adducts such as (CH3)2Zn(1,4-dioxan) and (C2H5)2Zn(1,4-dioxan) have been used as a source of zinc (P J Wright, B Cockayne, A J Williams, A C Jones and E D Orrell, Journal of Crystal Growth 84 (1987) p552, and B Cockayne, P J Wright, A J Armstrong, A C Jones and E D Orrell, Journal of Crystal Growth - 91 (1988) p57). These reactants, when used in conjunction with the relevant group VI hydrides, reduce but do not entirely eliminate prereaction, whilst still retaining the advantage of low temperature growth.

According to this investigation the above problems are solved by the use of dimethylzinc-trialkylamine - (CH3)2Zn(N(Ra)3)(N(Rb)3) adduct or diethylzinc-trialkylamine adduct (C2H5)2Zn(Ra)3)(N(Rb)3) adduct, where Ra and Rb are alkyl groups taken from the list of methyl, ethyl, propyl and butyl.

According to this invention a method of growing an epitaxial layer of ZnS or ZnSe on the substrate comprises the steps of: heating a substrate in a reaction chamber, flowing H2S or H2Se gas, an adduct and a dilutant gas into the reaction chamber and over the heated substrate, cracking of reactants leading to deposiution on the heated substrate to grow the layer of ZnSe or ZnS, characterised by the step of using dimethylzinc-trialkylamine (CH3)2Zn(N(Ra)3)(N(Rb)3) adduct or diethylzinc-trialkylamine (C2H5)2Zn(N(Ra)3)(N(Rb)3) adduct where Ra and Rb are alkyl groups taken from the list of methyl, ethyl, propyl and butyl.

The preferred adduct for growth, without prereaction, of ZnS and ZnS layers is dimethylzinc-triethylamine - (CH3)2Zn(N(C2H5)3)2.

This may be purchased from Epichem Ltd., Power Road, Bromborough, Wirral, Merseyside, L62 3QF.

Dimethylzinc-triethylamine may be prepared by a modification of the method described by K H THiel (Z. Anorg. Allg. Chem. 325 (1963) p156) as follows: dimethylzinc (24.0g, 0.25mol) is added dropwise, with stirring, to triethylamine (50.5g, 0.5mol), the pure liquid adduct is obtained by vacuum distillation at a temperature of 600C into a receiver cooled by liquid nitrogen to a temperature of approximately -1960C.

This procedure yields 70.5g of dimethylzinc-triethylamine adduct.

The purity of the adduct dimethylzinc-triethylamine prepared by the method described above is such that concentrations of contaminants listed in Table 1 below are less than the detection limits of inductively coupled plasma emission spectroscopy which are given as ppm by weight.

TABLE 1 Ag < 0.4 Cr < 0.4 Mn ( 0.03 Se < 1.0 Al < 0.5 Cu < 0.05 Mo < 0.5 Si < 0.03 As < 0.5 Fe < 0.1 Na < 0.5 Sn < 1.0 Au < 0.5 Ga < 0.5 Nb < 0.5 Sr < 0.1 B < 0.4 Ge < 0.5 Ni < 0.5 Tb < 0.5 Ba < 0.1 Hg < 0.5 P < 0.5 Ti < 0.2 Be < 0.02 In < 0.4 Pb < 1.0 U < 1.5 Bi < 0.5 K < 1.0 Pd < 0.5 V < 0.5 Ca < 0.02 La < 0.4 Pt < 0.5 W < 0.5 Cd < 0.02 Li < 0.4 Rh < 0.5 Y < 0.02 Co < 0.4 Mg < 0.02 Sb < 1.0 The invention will now be described by example only with reference to the accompanying example and figures in which: Figure 1 illustrates MOCVD equipment for growing a layer of ZnS or ZnSe on a substrate.

Figure 2 is a cross-section of a substrate coated with an epitaxial layer of ZnS or ZnSe.

Referring to figure 1, high purity hydrogen is supplied to a hydrogen manifold 1, which provides a supply to four mass flow controllers 2,3,4 and 5. Mass flow controller 4 supplies hydrogen to reaction chamber 23. Mass flow controller 5 supplies hydrogen (when valves 7 and 9 are open and valve 8 is closed) to stainless steel bubbler 6 held at 17.50C containing dimethylzinctriethylamine adduct. This adduct vapour flow can be directed to the reaction chamber 23 or to bypass 34 by use of values 10 and 11.

A H2S/H2 gas mixture is supplied in gas cylinder 12 and H2Se/H2 gas mixture in gas cylinder 15. Both mixtures are 5% H2S (or H2Se) in hydrogen. Mass flow controller 3 is fed with the H2S/H2 gas mixture when valve 13 is open and valve 14 is closed. The H2S/H2 gas flow can be directed into the reaction chamber 23 with valve 18 open and valve 19 closed. Likewise, the H2Se/H2 gas mixture flow can be directed at the reaction chamber 23 with valve 20 open and valve 21 closed. If either gas mixture flow needs to go to bypass 34, then valve 18 is set to closed with valve 19 open for H2S/H2 and valve 20 is closed and valve 21 is open for H2Se/H2 gas mixture flow.

Reactants are set flowing to bypass and growth is then initiated by switching the appropriate reactant gas flows to reaction chamber 23. The flows are mixed at reaction chamber entrance 22. The reaction chamber is heated by RF induction coil 24 powered by RF supply 25. Inside the reaction chamber is a susceptor block 26 and undoped (100) orientated GaAs substrate 27. The RF induction coil 24 couples with susceptor block 26 in order to heat substrate 27 to a suitable process temperature. This enables the adduct and hydride (H2S or H2Se) to crack and deposit (znS or ZnSe) on the substrate 27.

The exhaust vapour flow then passes through filter 33 and mixes with bypass 34. This flow then passes through valve 31 and for safety reasons through absorber 35 and is then exhausted. Vacuum pump 29 is connected to reaction chamber 23 through cold trap 28 with valves 30 and 32 open and 31 closed for purging the reaction chamber.

Typical growth conditions for both ZnS and ZnSe may be seen in Table 2.

TABLE 2 ZnSe ZnS Growth Temperature, Tg/ C 275 - 500 300 - 450 H2 carrier gas flow, 1 mien 1 4.7 4.7 Adduct flow, ccmin1 25 - 100 25 - 100 (Bubbler Temperature = 17.50C) Hydride flow, ccmin1 25 - 100 25 - 100 (5% mixture in H2) Growth ratec*mhr-l 1.25 - 5 1.25 - 5 VI:II ratio 1.5:1 - 24:1 1.5:1 - 24:1 Figure 2 is a cross-section of the substrate 27 coated with an epitaxial layer of ZnSe or ZnS 40.

Van der Pauw measurements and C-V profiling of ZnSe layers grown as described above give carrier concentration values of about 5x1014 to 3.4x1016 cm 3 at room temperature (variation within this range is dependent upon growth temperature). These values are similar to those for high quality epitaxial ZnSe (eg T Yao, M Oura, S Matsuoko and T Morisita, Japanese Journal of Applied Physics 22 (1983) p453).

The ratio of intensities of near-band gap to deep centre emission (R) is a useful guide to material quality (eg P J Dean, A D Pitt, M S Skolnick, P J Wright and B Cockayne, Journal of Crystal Growth).

R for material as described above is 102 < R < 104 and thus defines high quality epitaxial ZnSe and ZnS.

Claims (13)

1. A method of growing an epitaxial layer of ZnS or ZnSe on a substrate comprising the steps: heating a substrate in a reaction chamber, flowing H2S or H2Se gas, an adduct and a dilutant gas into the reaction chamber and over the heated substrate, cracking of reactants leading to deposition on the heated substrate to grow the layer of ZnS or ZnSe, characterised by the step of using dimethylzinc-trialkylamine - (CH3)2Zn(N(Ra)3) (N(Rb)3 adduct or diethylzinc-trialkylamine - (C2H5)2Zn(N(Ra)3)(N(Rb)3) adduct where Ra and Rb are alkyl groups taken from the list of methyl, ethyl, propyl and butyl.

2. The method of Claim 1 where the adduct used is dimethylzinc triethylamine - (CH3)2Zn(N(C2H5)3)2.

3. The method of Claim 2 where H2S gas is used and the substrate is heated within the range 3000C and 450aC.

4. The method of Claim 2 where H2Se gas is used and the substrate is heated withiin the range 2750C and 500"C.

5. The method of Claims 3 or 4 where the adduct flow is within the range 25 - 100 ccmin

6. The method of Claim 5 where the H2S or H2Se gas flow is within the range 25 - 100 ccmin1.

7. The method of Claim 6 where the Se:Zn and S:Zn ratios are within the range 1.5:1 to 24:1.

8. A substrate coated by the method of Claim 1 to give a layer of ZnS.

9. A substrate coated by the method of Claim 1 to give a layer of ZnSe.

10. A substrate coated by the method of Claim 2 to give a layer of ZnS.

11. A substrate coated by the method of Claim 2 to give a layer of ZnSe.

12. A substrate coated by the method of any of claims 3, 5, 6 or 7 to give a layer of ZnS.

13. A substrate coated by the method of any of claims 4, 5, 6 or 7 to give a layer of ZnSe.