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 is carried out by the decomposition of organometallic compounds from the vapor phase. Such decomposition is known as Metal Organic Chemical Vapor Deposition (MOCVD). When epitaxial layers are grown the technique is better known as the Metal Organic Vapor 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 large 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 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 liberated from the intermediate product by adding a halide of the Group 3a metal. A disadvantage of this method is the introduction of another metal compound and possibly 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 use of these halides necessarily introduces impurities.
Another method for purifying Group 3a organometallic compounds is disclosed in U.S. Pat. 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 1,2-bis(diphenylphosphino)ethane. Use of these types of compounds may result in significant amounts of Group 5 element being found in the final product.
In U.K. 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-oiling 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.
It would be of advantage, however, to provide a method for the purification of tri-alkyl compounds of Group 3a metals wherein a product of excellent purity is obtained.