Imperfections in amorphous chalcogenides. III. Interacting-lone-pair model for localized gap states based on a tight-binding energy-band calculation for ${\mathrm{As}}_2$${\mathrm{S}}_3$

Lone-pair electrons on S or Se interacting with each other were proposed as a new type of electrically neutral gap states in amorphous chalcogenides based on energy-band calculations using a simple tight-binding method. Much attention was focused on the ‘‘molecular’’ systems such as amorphous S, Se, and ${\mathrm{As}}_2$${\mathrm{S}}_3$. The electronic structure of amorphous ${\mathrm{As}}_2$${\mathrm{S}}_3$ was estimated by calculations on model crystals in which the structural fluctuation was introduced as the deviation of interlayer configurations from those in crystalline ${\mathrm{As}}_2$${\mathrm{S}}_3$. The energy levels at the top of valence band (VB) were changed with the relative configurations of two layers and went almost up to the midgap of crystalline ${\mathrm{As}}_2$${\mathrm{S}}_3$. The analysis of the linear combination of atomic orbitals coefficient revealed that the top of the VB was found to consist of some sulfur atoms having more than three nearest-neighbor atoms. Considering the above results and random configurations of macromolecules in amorphous ${\mathrm{As}}_2$${\mathrm{S}}_3$, it was suggested that (1) intermolecular configuration and interactions in crystalline chalcogenides are the essential factors affecting electronic structures, and (2) structural fluctuations, especially random configurations of macromolecules, may form some localized states in the midgap region. These states are electrically neutral and ESR inactive and thought to be intrinsic localized states to lone-pair amorphous materials.