Precursors for II–VI semiconductors: requirements and developments

Abstract

Recent advances in the growth of II–VI compound semiconductors by MOVPE have been greatly facilitated by progress in the synthesis, purification and design of precursor materials. The wide band gap alloys ZnSe and ZnS have traditionally been grown using Me 2Zn or Et 2Zn together with H 2Se or H 2S. However, this precursor combination leads to a serious problem of premature reaction which can result in II–VI layers with low uniformity and poor optical and electrical properties. Although the use of alkyl or heterocyclic S and Se compounds eliminates pre-reaction, these alternative precursors invoke the penalty of higher growth temperatures ( T g400°C) with a resulting degradation in II–VI layer quality. Our approach has been to modify the group II precursor so that it is no longer susceptible to premature reaction with the group VI hydride. In this paper we discuss how the use of adducts between Me 2Zn and nitrogen donors successfully prevents pre-reaction whilst retaining the advantage of low temperature growth. The adduct Me 2ZnNEt 3 fulfils many of the requirements of an ideal MOVPE precursor and represents a marked improvement over zinc precursors used previously. An extension of the use of metalorganic adducts to the growth of Cd-based chalcogenides together with a new, in situ method of controlling pre-reaction during the MOVPE growth of CdSe and CdS is also discussed. Advances made in the chemistry of both the Te and Cd precursors which have facilitated lower temperature growth of high quality Cd x Hg 1- x Te are also reviewed briefly.