Optical absorption measurements near the absorption edge are presented for three bulk semiconductor glasses: ${\mathrm{As}}_2$${\mathrm{S}}_3$, ${\mathrm{Ge}}_{33}$${\mathrm{As}}_{12}$${\mathrm{Se}}_{55}$, and ${\mathrm{Ge}}_{28}$${\mathrm{Sb}}_{12}$${\mathrm{Se}}_{60}$. The weak absorption tails observed below the exponential part of the edge also follow an exponential law, and they are not due to a light-scattering artifact. We associate them with localized states in the band gap. The results are interpreted in terms of a model in which optical transitions occur between localized states below the mobility edge and extended states of the opposite band.

• Received 14 October 1971

DOI:https://doi.org/10.1103/PhysRevB.5.3144