Density-functional based tight-binding simulations were made of a series of amorphous arsenic sulfide models. In addition to the charged coordination defects previously proposed to exist in chalcogenide glasses, a novel defect pair, [As4]--[S3]+, consisting of a fourfold coordinated As site in a seesaw configuration, and a threefold coordinated S site in a near-planar trigonal configuration, were found in several models. The valence-alternation pairs [S3]+-S1- were converted into [As4]--[S3]+ pairs under HOMO-to-LUMO electronic excitation. This structural transformation was accompanied by a decrease in the size of the HOMO-LUMO band-gap. This suggested that such transformations could contribute to photo-darkening in these materials.

New Type of Charged Defect in Amorphous Chalcogenides. S.I.Simdyankin, T.A.Niehaus, G.Natarajan, T.Frauenheim, S.R.Elliott: Physical Review Letters, 2005, 94[8], 086401 (3pp)