Organic compounds -- part of the class 532-570 series – Organic compounds – Heavy metal containing
Patent
1987-01-27
1989-03-14
Shippen, Michael L.
Organic compounds -- part of the class 532-570 series
Organic compounds
Heavy metal containing
544225, 546 2, 546 12, 556128, C07F 306, C07F 308
Patent
active
048125865
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to the preparation of Group II metal alkyls, especially in a high purity form.
Epitaxial layers of metals either in elemental form or contained in semiconductor compounds are being increasingly used in the electronics industry. Elements such as cadmium, zinc, gallium and indium may for example be deposited on substrate from their respective di- or tri-alkyls by thermal decomposition of the alkyls in the vapour phase to give the required epitaxial layers.
It is well-known that the presence of impurities in epitaxial layers can have a profound effect on the electrical properties of such layers. Generally speaking it is therefore very desirable to use in the epitaxial deposition process metal alkyls of the highest purity (although controlled impurities such as those provided by p- or n- type dopants may be intentionally added).
A method for the purification of dimethylcadmium has been disclosed by G. B. Coates et al, J Chem Soc, 1962, 3340. In this method an adduct of the alkyl is formed with 2, 2'-bipyridyl. This adduct can be purified by recrystallisation and dimethylcadmium can be liberated fairly rapidly from it in vacuo at about 70.degree. C. One disadantage of this method is that the dimethylcadmium so produce is sometimes contaminated with significant amounts of 2, 2'-bipyridyl. This is particularly the case if higher temperatures are used for recovering the dimethylcadmium. since 2, 2'-bipyridyl contains the group V atom nitrogen, contamination of the dimethylcadmium in this way may have particularly serious consequences for semiconductors grown from this material since group V elements can act as p- type dopants.
The present invention seeks to overcome the above disadvantage by providing in a first aspect, a method of preparing a metal alkyl of geeeral formula MR.sup.1 R.sup.2 in which M is selected from cadimum and zinc and R.sup.1 and R.sup.2 are independently selected from C.sub.1 -C.sub.8 alkyl, which comprises the steps of:
(a) forming an adduct of the metal with a Lewis base comprising a tertiary amine aving at least two tertiary amino groups per molecule whose nitrogen atoms are capable of co-ordinating with M, and
(b) dissociating the adduct to liberate the metal alkyl at a pressure P and at a temperature below the sublimation temperature or boiling point of the tertiary amine at pressure P as the case may be, wherein the tertiary amine has a molecular structure whose nitrogen atoms, for geometric reasons, co-ordinate with different atoms M such that the adduct formed has a vapour pressure of less than 1.33 Pa at its dissociation temperature at that pressure.
Preferably the vapour pressure of the adduct is less than 1.33 Pa at 20.degree. C., most preferably at 3.degree. C., above the dissociation temperature of the adduct at that pressure.
R.sub.1 and R.sub.2 are preferably independently selected from ethyl and methyl, and are most preferably methyl since dimethyl cadmium and dimethyl zinc are generally considered to be the most useful alkyls of these metals in subsequent processes for the deposition of roup II-VI epitaxial layers.
The dissociation temperature of the adduct may be measured by conventional thermogram techniques at a pressure of 1.33 Pa (10.sup.-2 torr), by increasing the temperature of the adduct at a steady rate (e.g. 10.degree. C. per minute) and recording the temperature at which a sharp increase in the heat generated by the adduct begins to take place, signalling the beginning of dissociation. The term "vapour pressure of the adduct" refers to the adduct itself and not its products of dissociation, and its volatility may be determined for the purpose of this invention by increasing the temperature of the adduct to its dissociation temperature at 1.33 Pa and observing whether it sublimes (if it is a sold at that temperature) or boiis (if it is a liquid at that temperature). If neither boiling nor sublimation is observed, then the adduct will in accordance with the method of the present invention have a vapour pressure of less than 1.33 Pa (10.s
REFERENCES:
Coates, J. Chem. Soc., pp. 3340-3348 (1962).
Hamilton John C.
Jacobs Philip R.
Mullin John B.
Orrell Elisabeth D.
Shenai-Khatkhate Deodattta V.
Shippen Michael L.
The Secretary of State for Defence in Her Britannic Majesty's Go
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