Calculation of Defect States in Amorphous Selenium

Self-consistent pseudopotential and tight-binding techniques are used to study glassy Se. Both onefold and threefold corrdinated defects give rise to nondegenerate, nonhydrogenic gap states. The former state arises from an interaction, unique to the chalcogenides, between a dangling bond and a neighboring lone pair. The latter state is surprisingly delocalized, involving interactions between many orbitals. It is shown that defect electronic configurations, and repulsive interatomic terms, are crucial to the calculation of total energies.