Preparation of metal alkyls

Details Preparation of metal alkyls Preparation of metal alkyls

1985-11-25

1988-01-19

Sneed, Helen M. S.

Organic compounds -- part of the class 532-570 series

Organic compounds

Heavy metal containing

556 18, 556174, C07F 500, C07F 506

Patent

active

047205610

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to the preparation of metal alkyls useful in the preparation of compound semiconductor materials.
Compound semiconductor materials, eg materials such as gallium arsenide, indium phosphide, gallium phosphide and cadmium mercury telluride, are well known materials having uses in the electronics industry in such applications as microwave oscillators, semiconductor light emitting diodes and lasers, and infrared detectors.
Such materials have been made in the past by forming, usually on a substrate crystal, one or more active layers, by the method of vapour phase epitaxy (VPE).
It has been known for some time that compound semiconductors of the form MQ where M is a Group III element and Q is a Group V element may be produced by VPE by reacting a trialkyl of the element M with a gaseous compound, eg a hydride, of the Group V element Q. This method is a suitable method of preparing gallium arsenide from Ga(CH.sub.3).sub.3 and AsH.sub.3 for example.
Consequently, metal alkyls, in particular Group III trialkyls, such as trimethyl gallium and trimethyl indium have become important in the production of semiconductor materials.
However, metal alkyls such as trimethyl gallium and trimethyl indium are exceedingly hazardous by virtue of their volatile pyrophoric nature, and explosive hydrolytic reactivity.
Special containers are required for trimethyl gallium and trimethyl indium and these are very expensive to provide.
In the article by H Renz and J Weidlein in Electronics Letters, Mar. 13, 1980, Vol 16 No 6, there is described the formation of an adduct of trimethyl indium with the Lewis base P(CH.sub.3).sub.3. The adduct so produced is a solid at room temperature, which is not hazardous and may be purified by zone refining. However, it is difficult to separate the trimethyl indium from the adduct described in that reference.
The purpose of the present invention is to provide a method of forming a metal alkyl from an adduct thereof by providing an adduct which is stable at room temperature and, when required is easily separable into its components above a relatively low thermal decomposition (dissociation) temperature.
According to the first aspect of the present invention there is provided a method of preparing a Group III metal alkyl which comprises comprises (a) forming an adduct of the aalkyl, the adduct having the formula (R.sub.3 M).sub.y.L wherein R.sub.3 represents three alkyl groups R which may be the same or different, M represents a Group III metallic element, L represents an aryl-containing Group V donor ligand provided by an organic Lewis base which is stable at 20.degree. C. and wherein y is an integer equal to the number of Group V donor atoms present in the ligand, and (b) heating the adduct to provide thermal dissociation thereof thereby releasing the metal alkyl as a gaseous product. L is preferably an aryl-containing phosphorus donor ligand.
The present method is especially suitable for the preparation of lower alkyls of Group III metals, particularly hexamethyl dialuminium, trimethyl gallium and trimethyl indium.
Suitable Lewis bases for providing the ligand L are aryl-containing Group V Lewis bases, especially those in which the or each Group V donor atom present in the base is trivalent and is bonded directly to at least one adjacent aryl groups. The main advantages of employing an aryl rather than an alkyl Lewis base are that
i. the basicity of the Lewis base is reduced, so that the adduct produced from it has a lower dissociation temperature, and
ii. the volatility of the Lewis base is generally lower.
As a result of these two advantages, the adducts formed by the method of the present invention are easily dissociated at relatively low temperatures producing a gaseous alkyl product which contains only very low concentrations of Lewis base contaminant.
Aryl-containing Group V Lewis bases which have been found to have acceptably low volatilities for the purpose of the present invention are those having a melting point in the range 50.degree. C. to 200.degree. C. Furth

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Durkin et al, Inorg. Chem. vol. 11 (5) pp. 1054-1059 (1972).
Chemical Abstracts 77 74383p (1972).
Chemical Abstracts 77 19712v (1972).

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