Stoichiometry driven impurity configurations in compound semiconductors*

As is well known, crystal growth of defect-free compound semiconductors, in contrast to elemental, is inherently limited by non-stoichiometry. High resolution infrared spectroscopy of localized vibrational modes can display unique signatures which reveal the structure of stoichiometry related defect-impurity complexes. The talk will focus on II-VI semiconductors in which group II cations are replaced with a group IIA or a 3d-transition metal ion as an impurity, on the one hand, and a group VI anion replaced with a group VIA impurity, on the other. Incorporation of O replacing Te with a full complement of nearest neighbor Cd's, i.e. $\mathrm{O_{Te}}$, as well as $\mathrm{O_{Te}}$ in association with a Cd vacancy ($\mathrm{V_{Cd}}$) in the zincblende CdTe result in defect centers with unique i.r. signatures. The occurrence of $\mathrm{O_{Te}}$ with $T_d$ symmetry and ($\mathrm{O_{Te}-V_{Cd}}$) with $C_{3v}$ symmetry can be controlled by favoring or suppressing Cd vacancies. In CdSe, with its wurtzite structure, oxygen incorporation occurs in two ways: in one, it is an \textquotedblleft anti-site\textquotedblright ~defect, $\mathrm{O_{Cd}}$, as revealed in its host isotope related fine structure; in the other, oxygen enters in association with Cd vacancies as ($\mathrm{O_{Se}-V_{Cd}}$). The talk will discuss the number of i.r. signatures specific to each center; their polarization characteristics (in CdSe); the striking temperature behavior of the i.r. signatures of ($\mathrm{O_{Te}-V_{Cd}}$) and ($\mathrm{O_{Se}-V_{Cd}}$); and the occurrence of overtones/combinations of the LVMs in CdTe. These investigations provide a wealth of microscopic insights into orientational degeneracy, host isotope effects and acquisition of the temperature averaged higher symmetries by the switchings of the dangling bond of $\mathrm{V_{Cd}}$.