Theory of defect states in glassyAs2Se3

Structural defects in glassy ${\mathrm{As}}_2$ ${\mathrm{Se}}_3$ are classified and labeled according to the constituent like-atom bonds and malcoordinated atoms. A selection rule is formulated to reduce the number of allowed Fermi-level pinning reactions. An elementary Bethe-lattice model is introduced as a starting point for a discussion of the electronic structure of simple defects. The defect states are found to be very different from those in Se; deep gap states arise in ${\mathrm{As}}_2$ ${\mathrm{Se}}_3$ because of unique bond orbitals, whereas they occur in Se due to unique $\pi$ interactions between orbitals. Malcoordinated Se atoms are expected to give rise to hydrogenic levels in ${\mathrm{As}}_2$ ${\mathrm{Se}}_3$, in contrast to Se. Surprisingly, the undercoordinated pnictide defects are positively charged in this model. Finally, defect creation energies and densities at $T_g$ are discussed.