Growth of II–VI compounds by metal-organic chemical vapour deposition. Structural characterisation of dimethylcadmium-N,N,N′,N′-tetramethylethane-1,2-diamine by gas-phase electron diffraction and its use in the growth of epitaxial layers of CdS and CdSe upon GaAs

Abstract

Aspects of the use of Lewis acid–base adducts of dimethylcadmium in the growth of CdY (Y = S or Se) by metal-organic chemical vapour deposition (MOCVD) have been investigated. The adduct dimethylcadmium-N,N,N′,N′-tetramethylethane-1,2-diamine, Me₂Cd·Me₂NCH₂CH₂NMe₂(1), was made and its structure determined by gas-phase electron diffraction. In 1 the cadmium atom is four-co-ordinate, being bound to two methyl groups [r_α(Cd–C)= 2.11(2)Å] and two nitrogen atoms [r_α(Cd—N)= 2.47(5)Å] with ∠C—Cd—C and ∠N—Cd—N being 132(11) and 84(3)°, respectively. The electron diffraction study provides the first unequivocal evidence for the existence of an adduct of dimethylcadmium being transported in the gas phase. Further evidence for the existence of 1 in the gas phase is provided by mass spectrometry and gas-phase infrared spectroscopy. Compound 1 was subsequently employed in the MOCVD process to grow visibly good-quality layers of CdY (Y = S or Se) on gallium arsenide substrates, but without overcoming the problem of pre-reaction experienced in the growth of CdY (Y = S or Se) by MOCVD.