The present invention relates to the preparation of trialkyl gallium compounds. These organometallic compounds find increasing use in the semiconductor industry. In this industry a gallium compound is deposited onto suitable substrates, generally together with one or more compounds of a Group 5 element, such as arsenic or phosphorus. The deposition of such compounds can be carried out via the decomposition of organometallic compounds from the vapour phase. Such decomposition is known as Metal Organic Chemical Vapour Deposition (MOCVD) . When epitaxial layers are grown from such decomposition the technique is better known as Metal Organic Vapour Phase Epitaxy (MOVPE) .

A convenient route for the preparation of trialkyl gallium compounds is via the reaction of gallium (III) chloride with either a Grignard reagent, viz. an alkyl magnesium halide, or an alkyl lithium compound. A disadvantage of these methods resides in the use of gallium (III) chloride which is difficult to obtain in the purity that is required for further use in the semiconductor industry. High purity gallium is available commercially and is therefore a suitable starting material for the preparation of trialkyl gallium compounds.

It is known to prepare trialkyl gallium compounds from alloys or mixtures of gallium and magnesium by reaction with alkyl halides according to the following reaction:

2Ga + 3Mg + 6RX > 2R.Ga + 3MgX-, in which R represents an alkyl group and X means halide. It will be evident that if an excess of magnesium is used the reaction product may contain unreacted magnesium. Further the magnesium may react with the alkyl halide to form a Grignard reagent, i.e. RMgX. Such reactions not only require an excess of magnesium, but also a super-stoichiometric amount of alkyl halide. All this adds to the cost of the preparation.

When a sub-stoichiometric amount of magnesium is used one would expect that the yield of trialkyl gallium compounds, based on original gallium, would decrease. Unreacted gallium would be present in the reaction product which would represent a considerable disadvantage because high purity gallium is expensive. In UK patent specification No. 2,123,423 a process for the preparation of trimethylgallium or triethylgallium is described in which an alloy Ga_Mg,_ is reacted with methyl iodide in the presence of an ether. The ether may be a relatively volatile ether, such as diethyl ether, or an ether with a relatively high boiling point, e.g. di-isopentyl ether or diphenyl ether.

The Soviet Union author's certificate No. 325,847 trialkyl gallium compounds are prepared from stoichiometric mixtures or alloys of gallium and magnesium by reaction with an alkyl halide. T e yields of trialkyl gallium compounds amounted up to 65%.

It has now surprisingly been found that the yield of trialkyl gallium is substantially increased if a Ga-Mg alloy is used which contains a considered excess of magnesium.

Accordingly, the present invention provides a process for the preparation of trialkyl gallium compounds, in which an alloy of gallium and magnesium is contacted with an alkyl halide, in which alloy the atomic ratio of gallium to magnesium ranges from 1:1.6 to 1:2.4.

The advantage of the invention vis-__-vis the above Soviet reference is an increased yield of trialkyl gallium compounds. The advantages thereof vis-a-vis the above UK specification are not only an increased yield, but also a reduction of by-products, like magnesium and Grignard reagent, and a reduced requirement for alkyl halide. The alloy generally contains intermetallic compounds of gallium and magnesium. Such intermetallic compounds are Ga-Mg.., GaMg„ and GaMg. The alloy suitably contains a mixture of such intermetallic compounds. Preferably the alloy contains a combination of intermetallic compounds such that the atomic ratio of gallium to magnesium ranges from 1:1.9 to 1:2.3. Preferably, the

alloy contains from 40 to 100 %wt of GaMg- , provided the overall atomic ratio is within the above range. If the alloy substantially consists of GaMg,. the yield of trialkyl gallium is excellent and the amount of by-products is small. The halogen moiety of the alkyl halide can be selected from chlorine, bromine or iodine. Especially alkyl bromides and alkyl iodides are advantageously used in the present process.

The alkyl groups in the trialkyl gallium compounds may be normal or branched. Although the present process can be carried out with a wide variety of alkyl halides, including those having long chain alkyl groups, the preparation of trialkyl gallium compounds containing alkyl groups with more than 6 carbon atoms is not practical, since these trialkyl compounds have a decreasing thermal stability and increasing volatility and are, therefore, of limited use in MOCVD or MOVPE. Therefore, the alkyl group in the alkyl halide has preferably from 1 to 5 carbon atoms. More preferably, the alkyl moieties are methyl or ethyl groups or mixtures thereof.

The reaction may be carried out under very mild conditions. The pressure may be atmospheric, but also subatmospheric or superatmospheric pressures are feasible. Generally, the pressure is from 0.1 to 10 bar. Since it is most convenient to operate at atmospheric pressure the process is preferably carried out at such pressure. The trialkyl compound is prepared under an inert atmosphere, e.g. under nitrogen, argon or helium. The reaction temperature may vary between wide ranges and will be below the decomposition temperature of the desired compound. For convenience sake the temperature is suitably from ambient to about 200 °C. Preferably, the process is carried out at a temperature from 50 to 160 °C. Since the reaction is exothermic, it is advantageous if the process is carried out in the presence of a solvent. Not only will the solvent ensure a homogeneous distribution of the reactants, but it also provides a convenient means for controlling the transfer of the heat evolved. A wide variety of solvents may be used in the present process. Such solvents include aliphatic or aromatic hydrocarbons, such as pentane, hexane, heptane, benzene, toluene or

xylene. Preferably the solvents contain at least one moiety with electron donating properties. Examples of such moieties are nitrogen and oxygen atoms. Therefore, suitably amides, such as dimethyl formamide, and, more preferably, ethers are used as solvents. The ethers may be cyclic or non-cyclic. They preferably contain from 3 to 18 carbon atoms. Suitable ethers include dioxane or tetrahydrofuran and diethyl ether, diphenyl ether, di-isopropyl ether, di-isopentyl ether and mixtures thereof.

The alkyl halide is preferably used in an amount sufficient to convert all gallium. On the other hand, use of a large excess of the alkyl halide is generally avoided since this excess would only add to the costs and hinder an easy recovery of the desired product. Therefore, the amount of alkyl halide suitably ranges from 3.0 to 5.0, preferably from 4.0 to 4.5 mole per gramatom gallium. Preferably the molar amount of alkyl halide is twice the amount of magnesium in gramatom. This ensures a good conversion of the metals into the trialkyl gallium compound and magnesium halide. Any excess of alkyl halide will react with any excess magnesium present to form a Grignard reagent. After completion of the reaction the reaction mixture will contain the trialkyl gallium compound, magnesium halide and some Grignard reagent. The trialkyl compound therefore needs to be separated from the magnesium halide. All conventional techniques may be applied to obtain such separation. These techniques include filtration, decanta ion etc. Conveniently, the trialkyl compound is recovered by distillation. After a first distillation a second fractional distillation may be applied. In the isolation of the trialkyl gallium compound from the reaction mixture by distillation it may be advantageous to recover the first 1 to 10 percent by volume of the product separately. In such case the main fraction which is then recovered as the desired product has an enhanced purity. The first fraction of the distilled product may be recycled to the original reaction mixture, be used in a subsequent batch of the same reaction, or be discarded. In order to avoid any possible thermal decomposition of the trialkyl compound, the distillation

may be carried out under subatmospheric pressure, thereby lowering the boiling point of the trialkyl compounds. The value of the distillation pressure depends to a large extent on the number of carbon atoms in the alkyl groups because such numbers influence the decomposition temperature and boiling point of the trialkyl compound. For distillation of trimethylgallium the pressure can be atmospheric. For trialkyl compounds with higher alkyl groups the decomposition temperature may be higher than the distillation temperature and thus the distillation pressure is preferably lower than 1 bar. In view hereof the distillation pressures can suitably be selected up to 1000 mbar, and is preferably from < 1 to 500 mbar.

The invention is further illustrated by means of the following examples. EXAMPLE 1

In a number of experiments about 75 g of an alloy was added to 200 ml of di-isopentyl ether in an inert atmosphere. Methyl iodide (Mel) was added at such a rate that the temperature was about 140 °C. After addition of the methyl iodide the reaction mixture w s kept at 70 °C overnight to allow the reaction to go to completion. Subsequently, the suspension obtained containing Mgl„ and an adduct of the ether and trimethylgallium (TMG) was heated to about 110 "C at which temperature the adduct dissociated and distillation of TMG started. The distillation was stopped when the bottom temperature of the suspension reached 195 °C. Water was added to the remaining suspension and insoluble gallium was isolated and weighed. The yield of TMG was calculated as the difference of the amount of gallium in the starting alloy and the amount of unreacted gallium in the suspension. The purity of the TMG was checked with H -NMR. The results of the experiments and some reaction conditions are indicated in the table below. '

In all experiments the Mel: Mg molar ratios were 2.

From the above table it is apparent that the atomic ratio of Ga to Mg has a significant effect on the yield of converted gallium. It is further shown that the amount of methyl iodide required in the reaction is reduced if the process is carried out in accordance with the invention. From experiment No. 4 it is apparent that the mere presence of GaMg- is not sufficient to obtain advantageous results. COMPARATIVE EXPERIMENT

In an analogous manner a physical mixture of pure magnesium and pure gallium (in an atomic ration of 2:1) was reacted with 4 molar equivalent of methyl iodide. Instead of the expected 84% yield only 15% of converted gallium was found.

This indicates that the use of an alloy, preferably containing GaMg-, is required to obtain the advantageous results of the present invention. EXAMPLE 2

GaMg- alloy comprising about 100% of the intermetallic compound GaMg-, was reacted with 3.5 equivalents of ethyl bromide in a mixture of diethyl and diphenyl ether as solvent. The reaction was exothermic and the rate of ethyl bromide addition was such that the temperature did not exceed 140 ^β C. After all ethyl bromide was

added the reaction mixture was stirred for 16 hours at 100 °C to complete the formation of an adduct of triethylgallium (TEG) and diethyl ether. Subsequently the TEG.diethyl ether adduct was distilled at a bottom temperature of 100-140 ^β C. Throughout the distillation the pressure was decreased to maintain the desired distillation rate, starting from 100 mbar to 25 mbar. The yield of TEG.diethyl ether was 95% based on the gallium originally present in the alloy. EXAMPLE 3 GaMg. _q alloy, comprising about 90% of the intermetallic compound GaMg-, was reacted with 3.8 equivalents of methyl iodide in di-isopentyl ether as solvent. The reaction was exothermic and the rate of the methyl iodide addition was such that the temperature of mixture increased to 140 °C. After all methyl iodide was added the reaction mixture was stirred for 16 hours at 75 ^β C. Trimethyl gallium was distilled from the reaction mixture at atmospheric pressure at a bottom temperature of 110-195 °C. The yield of the desired product was 77%, based on gallium originally present in the alloy.