The present invention relates to a process for the puri¬ fication of tri-alkyl compounds of Group 3a metals. By Group 3a metals are understood aluminium, gallium, indium and thallium.

Tri-alkyl compounds of Group 3a metals find increasing use in the electronics industry. In this industry the metals are deposited, alone or in combination with other elements, onto suitable substrates. Frequently such metals are deposited as a compound with at least one Group 5a element, such as phosphorus and/or arsenic. The deposition of these compounds can be carried _ou 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 the technique is better known as the Metal Organic Vapour Phase Epitaxy (MOVPE) . The organometallic compounds that are employed in the above techniques are usually alkyl compounds of the metals involved. The organometallic compounds used in this industry need to be very pure because small amounts of impurities may have a tremendous effect on both the electrical and optical performance of the semiconductor layer deposited therefrom. A method for the purification of Group 3a organometallic compounds is described in European patent application No. 372,138. According to this method an intermediate product of a tri-alkyl compound of a Group 3a metal with a compound of another metal is formed. Since the intermediate product is essentially non-volatile, all relatively volatile impurities can be removed. Subsequently the desired Group 3a metal tri-alkyl compound is released from the intermediate product by adding 0.33 equivalents of a halide of the Group 3a metal. A drawback of the method is the introduction of another metal compound, thereby creating the risk of introducing other and/or more impurities into the system. Because it is difficult to obtain the halides of Group 3a metals in a

sufficiently pure form, the mandatory addition of these halides forms another possible source for impurities. Another major drawback is the fact that the method requires a considerable amount of handling of solids part of which must be performed with stringent exclusion of oxygen and moisture.

Another method for purifying Group 3a organometallic compounds is disclosed in US patent specification No. 4,720,561, in which method an adduct of a tri-alkyl compound is prepared using an aryl-containing Group 5 ligand. A preferred species of such ligands is l 2-bis(diphenylphosphino) ethane (Diphos). Use of these types of compounds may involve the problem that significant amounts of Group 5 element are found in the final product, which may cause a problem in the growth of layers other than in III-V semiconductor layers, i.e. layers comprising elements from Groups 3 and 5 of the Periodic Table of the Elements. Major drawbacks of this method are constituted by the considerable amount of solids handling and the high price of Diphos.

In UK patent specification No. 2,123,423 the purification of tri-alkyl gallium is described. In this purification a relatively involatile adduct of tri-alkyl gallium with a high-boiling ether is formed and subsequently volatile impurities are removed therefrom. The adduct is then thermally dissociated to yield the ether and the purified tri-alkyl gallium. As suitable ethers di-isopentyl ether and diphenyl ether are mentioned. In one example the preparation of trimethyl gallium is described using di-isopentyl ether. The result is a relatively pure trimethylgallium product.

The present invention provides a method for the purification of tri-alkyl compounds of Group 3a metals in which no excessive solids handling needs to take place and wherein a product with an excellent purity is obtained.

Accordingly, the present invention provides a process for the purification of a tri-alkyl compound of a Group 3a metal in which process a polyether of the formula:

1 2 wherein each of R and R represents an alkyl group with 1 to 4

3 carbon atoms; each R is independently selected from the group consisting of a hydrogen atom, a methyl and an ethyl group; m represents an integer from 1 to 6; and n represents an integer from 1 to 12, is added to a composition comprising a tri-alkyl compound of a Group 3a metal to obtain an adduct of the tri-alkyl compound and the polyether; the adduct is heated to dissociate the adduct thermally into a mixture containing dissociated tri-alkyl compound of the Group 3a metal; and the mixture is subjected to distillation to recover the dissociated tri-alkyl compound of the Group 3a metal.

The use of the polyether enables the obtaining of a very pure product. Although it is not intended to be bound by any theory, it is believed that the polyether having n+1 oxygen atoms per molecule forms an adduct with a number, generally up to n, of tri-alkyl metal compounds. Upon heating the adduct thermally dissociates, releasing the pure tri-alkyl compound. In some cases only a portion of the tri-alkyl compound in the adduct is released. Upon distillation the pure tri-alkyl compound is distilled over the top.

1 2 To facilitate the separation during distillation the R , R

3 and R groups and m and n are suitably selected such that there is a substantial difference in boiling points between the polyether and the tri-alkyl compound to be purified. Such a substantial difference is suitably from 50 to 150 °C. Such a difference may be irrelevant as to the facilitation of the distillation if the adduct between tri-alkyl compound and polyether only partly dissociates during heating. For practical reasons the number n of oxyalkylene groups in the polyether is preferably from 2 to 8, more preferably

1 2 from 2 to 4. R and R can be selected from alkyl groups having 1 to 4 carbon atoms. Preferably they are methyl or ethyl groups. The integer m may range from 1 to 6. Suitably, m is from 2 to 4. The

3 groups R at each carbon atom may be independently selected from hydrogen, methyl and ethyl. When m>l, preferably not more than one

3 R represents a moiety different from a hydrogen atom- The oxyalkylene group in the polyether is preferably oxyethylene;

3 hence, m represents 2 and each R represents a hydrogen atom. The above preferences concerning the polyether lead combined to the selection of using diglyme (diethylene glycol dimethyl ether) , triglyme (triethylene glycol dimethyl ether) or tetraglyme

(tetraethylene glycol dimethyl ether) as the most preferred polyethers.

The metals of Group 3a of the Periodic Table of the Elements that are usually employed in MOCVD or MOVPE are aluminium, gallium and indium. Therefore these metals are especially preferred in the present process. The alkyl groups in the tri-alkyl compounds may be normal or branched. Although the present process can be carried out with a wide variety of tri-alkyl compounds, including those having long chain alkyl groups, the purification of tri-alkyl compounds containing alkyl groups with more than 6 carbon atoms is not practical, since these tri-alkyl compounds possess decreasing thermal stability and volatility. Therefore, the alkyl group in the tri-alkyl compound has preferably from 1 to 4 carbon atoms. More preferably, the alkyl moieties are methyl or ethyl groups or mixtures thereof.

The composition containing the tri-alkyl compound of the Group

3a metal evidently contains other constituents, such as impurities.

It is preferred to employ the instant invention for compositions having a relatively small amount of impurities. The upper limit is governed by what is practical. This limit may be several percent, say up to 5 %wt. Also, impurities of only a few, say 10, ppm can still be removed by the process of the present invention.

The adduct of the tri-alkyl compound with the polyether according to the invention may be obtained by mixing relatively impure tri-alkyl compound with the polyether and allowing the adduct to form. The formation of the adduct starts instantaneously after addition. The formation can be carried out at virtually any temperature. Practical limits would suggest a temperature range from -20 to 250 ^β C. Suitably the composition containing the

tri-alkyl compound contains an intermediate adduct of the tri-alkyl compound in question and ^' a mono-ether. The adduct with the polyether is then prepared by displacing the mono-ether from the intermediate adduct. When the mono-ether in the intermediate adduct is more volatile than the polyether, the substitution can easily be accomplished by distilling off the released mono-ether, thereby disturbing the equilibrium of the displacement reaction in favour of the formation of the adduct with the polyether. The used mono-ether may be selected form a wide variety of mono-ethers. Suitable mono-ethers include ethers of the formula R'-O-R", in which R' and R" indepently represent hydrocarbyl such as alkyl groups with 1 to 8, preferably 2 to 6 carbon atoms. Preferred mono-ethers are diethyl ether, di-isopropyl ether and di-isopentyl ether. The relative amount of polyether vis-a-vis the tri-alkyl compound may vary within wide ranges. It is preferred that the amount of polyether is selected such that all tri-alkyl compound present may form an adduct with the polyether. However, that does not mean that for every mole of tri-alkyl compound one mole of polyether is needed. As explained above, it is possible to form adducts comprising one molecule of polyether with more than one molecule of tri-alkyl compound. For reasons of convenience, an excess of polyether with respect to the number of oxygen atoms is usually employed. Generally the amount of polyether added to the composition comprising the tri-alkyl compound ranges is less than

10 moles per mole tri-alkyl compound. More specifically, the amount of polyether is 0.1 to 10 moles per mole tri-alkyl compound.

By means of distillation of the mixture of polyether and tri-alkyl compound after dissociation the pure tri-alkyl compound is recovered. The dissociation may be carried out before the distillation. In a preferred embodiment, however, the adduct is heated thereby accomplishing the dissociation, and the dissociated tri-alkyl compound is simultaneously distilled off. In order to avoid the distillation temperature of the mixture to rise above the decomposition temperature of the tri-alkyl compound, the

distillation is preferably carried out at reduced pressure. Such pressure is suitably below 250 mbar. The pressure may be as low as less than 0.1 mbar.

In the distillation of the tri-alkyl compound of the Group 3a metal from the 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 that 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 to be used in a subsequent batch of the same reaction, or may be discarded. Also the residue may be recycled. This will be especially desirable when the dissociation of the adduct of tri-alkyl compound and polyether is only partial and the residue therefore still contains a significant amount of tri-alkyl compound.

The invention will be further elucidated by the following examples. EXAMPLE 1

The purification of trimethyl aluminium In a flask 751 g of distilled trimethyl aluminium (TMA) was added to 549 g diglyme at such a rate that the temperature did not exceed 100 "C. The mixture was stirred overnight at 100 ^β C. The pressure was lowered to 150 mbar and the bottom temperature increased to 150 ^β C. Pure TMA„.diglyme adduct was recovered as the top fraction at a distillation temperature of 158 ^β C (10 mbar) .

A 2 litre flask was charged with 1016 •of TMA-.diglyme adduct and 265 g of distilled TMA was added at such a rate that the temperature rose to 60 ^β C. After all TMA was added, the contents in the flask solidified. Subsequently the flask was equipped with a fractionating column, the pressure was lowered to 100 mbar and the flask temperature was increased to 90 "C. At a distillation head temperature of 58 ^β C TMA was distilled over and collected in fractions. The yield of pure TMA product was 72% based on the TMA added to the TMA,..diglyme adduct. The residue consisted quantitatively of the TMA„.diglyme adduct.

The purity of the TMA recovered is illustrated by its silicon content. The silicon content in the recovered TMA was 0.1 ppm, relative to a volatile TMS (tetramethyl silane) standard, whereas the silicon content in the distilled TMA added to the TMA„.diglyme adduct amounted to 0.4 ppm. The silicon impurities were determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES) . No other elemental impurities were detected. EXAMPLE 2

The purification of triethylgallium A triethylgallium (TEG) adduct with diethyl ether was prepared by adding ethyl bromide to a gallium-magnesium alloy (GaMg_) in a mixture of diphenyl ether and diethyl ether. The reaction was exothermic and the rate of ethyl bromide addition was such that the reaction temperature did not exceed 140 "C. After all ethyl bromide was added the reaction mixture was stirred for 16 hours at 100 °C to yield a TEG.diethyl ether adduct. Subsequently this adduct was distilled at a bottom temperature of 100-140 °C and at decreasing pressure (from 1000 mbar to 25 mbar) to maintain the distillation rate. Then 1171 g of tetraglyme was added to the distilled

TEG.diethyl ether adduct. The molar ratio of TEG to tetraglyme was 1.6:1. Subsequently the diethyl ether was removed at 1 mbar and room temperature for 16 hours, yielding the crude TEG- -..tetraglyme adduct. The silicon content of this adduct was 0.5 ppm, as determined by ICP-OES, using TMS as standard.

This crude adduct was heated to 121-150 ^β C at 30 mbar, thereby dissociating the TEG from the adduct and simultaneously distilling the dissociated TEG. The dissociated TEG was completely free of diethyl ether as determined by H-NMR. The silicon content in the dissociated TEG was < 0.1 ppm, as determined by ICP-OES, using TMS as standard. No other impurities were detected. EXAMPLE 3 The purification of trimethylindium

An adduct of trimethylindium (TMI) and diethyl ether was prepared by adding a solution of methyl magnesium iodide in diethyl

ether to a solution of indium (III) chloride in diethyl ether. After addition, the mixture was stirred for 3 hours at 100 °C and for 16 hours at room temperature. The TMI.diethyl ether adduct was obtained by distillation, collecting a fraction boiling between 133 and 135 °C. The yield was 51%, based on indium chloride.

To TMI.diethyl ether adduct such an amount of tetraglyme was added that the molar ratio of TMI to tetraglyme was 2.5:1. Diethyl ether was distilled off and the distillation was stopped at a bottom temperature of 100 °C at ambient pressure. Last traces of diethyl ether were removed in vacuo, thereby yielding the

TMI_ -..tetraglyme adduct. The obtained adduct was transferred to a 500 ml flask and was dissociated to obtain the free TMI at a temperature of 160 ^β C and reduced pressure. The residue, containing TMI and tetraglyme, was recycled to the next batch. The TMI obtained was further purified by sublimation.

The purity is illustrated by the silicon and magnesium contents. Whereas the TMI.diethyl ether adduct contained 2 ppm silicon and 7.7 ppm magnesium, the purified TMI only contained < 0.05 ppm silicon and 0.01 ppm magnesium. No other impurities were detected by ICP-OES. EXAMPLE 4 Purification of trimethylindium

Similar to the procedure of Example 3 TMI.diethyl adduct was treated with triglyme. After separation of diethyl ether, a TMI„.triglyme adduct was obtained. The adduct was dissociated by distillation as described in Example 3 and thereby TMI was obtained. EXAMPLE 5

Purification of triethylindium

Similar to the procedure given in Example 3 the triethyl indiu (TEI).diethyl ether adduct was prepared from indium (III) chloride and ethyl magnesium iodide. To the TEI.diethyl ether adduct tetraglyme was added such that the molar ratio of TEI to tetraglyme was 3:1. Diethyl ether was removed in vacuo at a bottom temperature of 74 ^β C. Subsequently pure free TEI was obtained via

dissociation of the adduct at 74-115 °C and a pressure of < 1 mbar. The obtained TEI was subsequently subjected to fractional distillation at a pressure of < 1 mbar. No metal impurities could be detected by ICP-OES in the distilled TEI. COMPARATIVE EXPERIMENT 1 Use of dibenzyl ether

30 g of an InMg-, alloy was reacted with 120 g methyl iodide under an atmosphere of purified argon, using a mixture of dibenzyl ether and diethyl ether as solvent. After stirring overnight the reaction mixture was grey-white. After settling of the solids, the liquid was decanted. The bottom temperature of the TMI-containing phase was gradually increased and at a temperature of 200 ^C C decomposition of the contents of the flask occurred. No TMI was distilled. This experiment clearly shows that the use of high-boiling dibenzyl ether for the isolation of TMI is not suitable. COMPARATIVE EXPERIMENT 2 Use of di-isopentyl ether

Similar to the procedure in Comparative Experiment 1 InMg_ alloy was reacted with methyl iodide, but now using a mixture of diethyl ether and di-isopentyl ether (DIPE) as solvent. After addition of the methyl iodide the mixture was stirred overnight at a temperature of 95 ^β C. Subsequently, the diethyl ether was distilled off at atmospheric pressure and a bottom temperature of 190 ^β C. Then the pressure was lowered to < 1 mbar and a TMI-containing product was collected.

The product consisted of TMI.DIPE adduct (molar ratio 5:1). This Experiment clearly shows that DIPE is not suitable for the purification of TMI.