Electronic Structure of Crystalline and Amorphous ${\mathrm{As}}_2$${\mathrm{S}}_3$ and ${\mathrm{As}}_2$${\mathrm{Se}}_3$

The optical properties of crystalline and amorphous ${\mathrm{As}}_2$ ${\mathrm{S}}_3$ and ${\mathrm{As}}_2$ ${\mathrm{Se}}_3$ have been investigated from 1 to 14 eV. A simple rigid-band relationship connects the band structures of the two crystals, and the lowest-energy features observed near the interband thresholds are assigned to direct transitions. The effect of disorder on the electronic spectra, aside from the disappearance of critical-point structure, is remarkably slight; the principal density-of-states thresholds are preserved, essentially unshifted, in the glasses.