Theory of defect states in glassy selenium

A realistic approach to the electronic theory of bond-coordination defects in chalcogenides, based on self-consistent pseudopotential calculations, is used to study glassy Se. The results of the pseudopotential calculations are interpreted in terms of simpler tight-binding models. The onefold and threefold coordination defects are both found to give rise to nondegenerate, nonhydrogenic gap states, whose properties are unique to the chalcogenides in several respects. The existence of $\pi$ interactions between nonbonding orbitals at defect sites is found to be crucial to an understanding of the electronic structure. These interactions are responsible for large charge transfers between atoms and consequently large energy shifts of atomic valence orbitals, which make these defects quite unlike those in other semiconductors. The origin, character, energy location, and localization of the defect states associated with bond-coordination defects, and with defect pairs and certain relaxed defects, are discussed.