GROUP 2B METALS

The present invention relates to a process for the preparation of di-alkyl compounds of Group 2b metals, and to such compounds, in particular to di-alkyl zinc. In this specification by Group 2b metals are understood zinc, cadmium and mercury. Di-alkyl compounds of Group 2b metals find increasing use in the electronics industry. Compounds and alloys containing elements such as zinc and cadmium may be deposited on substrates from volatile precursor compounds, such as their respective di-alkyl compounds, by thermal decomposition from the vapour phase to give a thin (semiconductor) layer. This technique is known in the industry as Metal Organic Chemical Vapour Deposition (MOCVD) . When an epitaxial layer is grown the technique is better known as Metal Organic Vapour Phase Epitaxy (MOVPE) . A process for the deposition of zinc sulphide films on a semiconductor substrate in which di-alkyl zinc is used in combination with hydrogen sulphide is described in European patent application No. 405,875. A method for the preparation of epitaxial layers of zinc sulphide and zinc selenide is described in UK patent application No. 2,221,924.

The presence of impurities in such semiconductor layers may have a tremendous effect on both their electrical and their optical properties. It is therefore desired that the precursor compounds, such as the di-alkyl compounds of the Group 2b metals, are very pure. For the production of p-type zinc-selenide layers for use in opto-electronic devices, the iodine content in the zinc precursor is of the utmost importance. Iodine is an n-type dopant and hence, controlled p-type doping can only be achieved if the iodine content in the epitaxial layer, and therefore in the zinc and selenium precursors, is very low, preferably below 1 ppm (by weight). Hence, it is not surprising that strenuous efforts have been made to purify these precursors such as the di-alkyl compounds.

In US patent specification No. 4,812,586 the purification of impure dime hylcadmium and dimethylzinc is described. According to this specification impure di-alkyl compounds are prepared by reacting methyl halide with magnesium to yield a Grignard reagent. This reagent is subsequently reacted with cadmium halide or zinc halide to yield the respective impure di-alkyl metal compound. This compound was purified by adduct formation with specific amino compounds followed by removal of impurities from the adduct. The purified adduct was subsequently dissociated and subjected to distillation to yield the purified di-alkyl compound. It is evident that this process requires several steps.

In UK patent specification No. 1,242,789 a process is described in which metallic zinc is alkylated using alkyl halides in the presence of other metals, i.e. alkali metals. According to the examples of this specification the yields obtained in these examples range from 53 to 85%. However, it is apparent that in the example giving the highest yield 0.220 mole zinc yielded 5.8 g, i.e. only 47 mmole diethylzinc. Hence, the yield based on total zinc consumption amounted to only 21%. This process requires an excess of zinc in addition to that which is contained in the di-alkyl compound, because zinc is used as halogen acceptor. So if the use of very pure zinc is required, as would be for MOCVD or MOVPE purposes, this process would involve the waste of considerable amounts of pure zinc. The present process relates to a simple preparation of di-alkyl metal compounds with an efficient use of the Group 2b metal. Whereas in the prior art it is reported that the use of magnesium yields an impure product it has now been found surprisingly that the application of magnesium in the present process results not only in excellent yields but also in high purities.

The present process therefore provides a process for the preparation of di-alkyl compounds of a Group 2b metal, comprising reacting a Group 2b metal with an alkyl halide in the presence of

magnesium to obtain the di-alkyl compound of the Group 2b metal and magnesium halide.

The metals that can be used in the process according to the present invention include zinc, cadmium and mercury. Preferably, zinc or cadmium are used. In the most preferred embodiments zinc is used because zinc is most frequently used in MOCVD and MOVPE processes, and moreover use of zinc gives the highest yields and the highest purities.

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. Use of alkyl iodides tend to result in a product with the highest purity so their use is especially preferred.

The alkyl groups in the alkyl halide 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 di-alkyl Group 2b metal compounds containing alkyl groups with more than 6 carbon atoms is not practical, because these di-alkyl compounds have a decreasing volatility and often a decreasing thermal stability. Therefore, the alkyl group in the alkyl halide has preferably from 1 to 4 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 super- atmospheric pressures are feasible. Generally, the pressure is from 0.1 to 10 bar. For reasons of convenience it is preferred to carry out the process at atmospheric pressure. The di-alkyl compound is prepared under an inert atmosphere, e.g. nitrogen, argon or helium. The reaction temperature may vary between wide ranges. Care is taken that the temperature does not exceed the decomposition temperature of the di-alkyl compound involved. Such decomposition is different for each di-alkyl compound. Taking the foregoing into account, the process is suitably carried out at a temperature from 20 to 170 °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 t e dissipation 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, and amides, such as dimethyl forma ide. Preferably, the solvent contains at least one moiety with electron donating properties. Examples of such a moieties contain a nitrogen or, particularly, an oxygen atom. Therefore, the solvent is preferably an ether. The ether may be cyclic, like tetrahydrofuran or dioxane, or non-cyclic, such as diethyl ether, di-(iso)propyl ether, diphenyl ether and the like. Preferably, an ether with a relatively high boiling point is selected. Such ethers include diphenyl ether and in particular di-isopentyl ether. Other suitable ethers are polyglycol ethers containing up to eight glycol moieties. Suitable examples of such polyglycol ethers include diglyme, triglyme and tetraglyme.

The process according to UK patent No. 1,242,789 employs a minor, almost solely catalytic, amount of alkali metal. Although it is possible to use a minor amount of magnesium in the present process, it is advantageous to use a substantially stoichiometric molar ratio of magnesium with respect to the Group 2b metal used. Therefore, the amount of magnesium preferably ranges from 0.8 to 2.0 equivalent magnesium per equivalent of Group 2b metal.

Surprisingly, it has been found that the purity of the product obtained is further enhanced if a relatively small excess of magnesium is employed. Therefore the amount of magnesium is more preferably between 1.0 and 1.15 equivalent magnesium per equivalent Group 2b metal. The form in which the Group 2b metal and magnesium are present in the reaction mixture is not critical. It is-possible to use a physical mixture of magnesium and the Group 2b metal involved. It is also feasible to employ an alloy of the metals. The relative amounts in the alloy or the mixture are suitably selected such that they correspond with the above molar ratios.

In the process of UK patent No. 1,242,789 an excess of alkyl halide is used. In the present process suitably a substantially stoichiometric amount of alkyl halide is used. Variations are, of course, possible. The yields of the desired di-alkyl compound are

5 enhanced if an excess of alkyl halide is used. However, in such i cases the amount of unreacted reactant or by-products may contaminate the desired di-alkyl compound, thereby preventing the desired purity from being attained. Hence, it is feasible to have the process carried out at a molar ratio of alkyl halide to Group

10 2b metal of 0.8 to 4.0. However, if purity is paramount the process is preferably carried out at stoichiometric ratios. Therefore, the molar amount of alkyl halide is preferably substantially twice that of the Group 2b metal (in gramatom) so that no excess alkyl halide needs to be removed from the reaction mixture.

15 After completion of the reaction, the reaction mixture will contain the di-alkyl compound of the Group 2b metal, magnesium halide, and, optionally, the employed solvent. When an excess of magnesium is used, the reaction mixture will also contain unreacted magnesium. The di-alkyl compound therefore needs to be isolated

20 from the reaction mixture. All conventional techniques may be applied to achieve such separation. These techniques include filtration, decantation etc. Conveniently, the di-alkyl compound is recovered by distillation. After a first distillation a second fractional distillation may be employed. In the isolation of the

25 di-alkyl compound of the Group 2b metal from the reaction mixture by distillation it may be advantageous to recover the first 1 to 10 per cent 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

30 recycled by adding it to the original reaction mixture or to a subsequent batch of the same reaction, or may be discarded. In

_* order to avoid any possible thermal decomposition of the di-alkyl compound, the distillation is advantageously carried out at a temperature below the decomposition temperature of the di-alkyl

35 compound involved. For certain di-alkyl compounds it may thus be

desirable to perform the distillation under subatmospheric pressure. Suitable pressures can, therefore, be selected from 1 bar to-as low as less than 1 mbar.

The present invention enables the preparation of a di-alkyl compound of a Group 2b metal with excellent purity. Those skilled in the art may now for the first time so simply prepare a product with an extremely low halogen content. Therefore, the present invention further provides a product containing at least 99.999 %wt of a di-alkyl compound of a Group 2b metal and less than 1 ppm halogen, less than 1 ppm silicon and less than 1 ppm of a Group 3a metal. By ppm is understood parts per million by weight. The prior art processes all employ at least one halogen compound, be it the group 2b metal compound or the alkyl compound. The residual content of halogen is therefore considerable, e.g. up to about 1000 ppm. For many applications, e.g. the use of a di-alkyl zinc compound as an alternative alkylating agent instead of the well-known magnesium Grignard reagent, this content does not represent any problem. However, for the application in Metal Organic Chemical Vapour Deposition or Metal Organic Vapour Phase Epitaxy of layers for the electronics industry, the purity of the di-alkyl compound is of the utmost importance; particularly with respect to such halogen species. Therefore, the provision of compounds with the purity as obtained in the present invention is extremely advantageous for this industry. Since a purity of from 0.05 to 0.5 ppm halogen is obtainable, the invention also relates to products containing di-alkyl compounds with such amounts of halogen. The halogen most frequently used is iodine. The Group 2b metal is suitably zinc. The most preferred compounds are dimethylzinc and diethylzinc.

The silicon content in the di-alkyl compounds suitably ranges from 0.01 to 1 ppm. The Group 3a metal, e.g. gallium, aluminium or indium, is preferably absent, and suitably less than 0.5 ppm.

Since the compounds according to the present invention are very suitable for use in MOCVD or MOVPE-applications, the present invention, moreover, relates to the use of the compounds of the present invention in MOCVD or MOVPE.

The invention is further illustrated by means of the following examples. In the examples the silicon content was determined by ICP-OES (inductively coupled plasma-optical emission spectroscopy) , using tetramethyl silane as standard, and the iodine content was determined by ICP-MS (inductively coupled plasma-mass spectrometry) , using methyl or ethyl iodide as standard.

EXAMPLE 1

Preparation of dimethylzinc

To a suspension of 73.74 g (1.128 mole) of zinc and 28.80 g (1.184 mole) of magnesium in 480 ml of di-isopentyl ether was added 320 g (2.256 mole) of methyl iodide at such a rate that the temperature rose to 110 °C. After completion of the methyl iodide addition, the reaction was allowed to go to completion under agitation at 70 °C. Subsequently, the reaction mixture was cooled to 40 ^β C and any unreacted methyl iodide was removed in vacuo. Subsequently, the reaction mixture was heated to about 99 ^β C whereupon distillation of dimethylzinc (DMZ) started. As the distillation proceeded, the temperature was increased to about 170 °C. During the distillation a forerun of about 10% of the expected yield was taken. Thereafter the main fraction was obtained, which represented a yield of about 73%, based on zinc. The purity of the main fraction is illustrated by the silicon content (< 0.3 ppm) determined by ICP-OES, and lead content (< 0.2 ppm), determined by ICP-MS. The iodine content was 0.4 ppm as determined by ICP-MS, using methyl iodide as standard. EXAMPLE 2 Preparation of diethylzinc

In a similar way as described in Example 1, diethylzinc (DEZ) was prepared from zinc (1.056 mole), magnesium (1.111 mole) and ethyl iodide (2.112 mole) in 450 ml of di-isopentyl ether. The reaction temperature was maintained at a temperature below 100 °C to avoid decomposition of DEZ. Therefore, the distillation was carried out at a decreasing pressure of 100 to 16 mbar. The bottom temperature did not exceed 98 °C. The first few percent of distilled DEZ were discarded and the main fraction was recovered as

product. The yield of DEZ was about 86% based on zinc. The silicon content amounted to < 0.9 ppm. No other impurities were detected by ICP-OES. Further purification by distillation was accomplished to yield a DEZ fraction with a silicon content < 0.1 ppm and a iodine content of < 0.5 ppm, as determined by ICP-MS, using ethyl iodide as standard. EXAMPLE 3 Preparation of dimethylcadmium

A series of experiments was carried out in which dimethyl- cadmium (DMC) was prepared from cadmium, magnesium, methyl iodide in di-isopentyl ether. The procedure was similar to the one described for the DMZ preparation. The relative amounts of the reactants and the yields obtained are indicated in the table below.

TABLE

Molar equivalents of reactants Yield DMC

Cd Mg Methyl iodide % on Cd

1.0 1.11 2.24 64

1.0 1.05 2.04 30

1.0 1.11 2.23 69 1.0 1.11 2.64 42