We report on density-functional-based tight-binding (DFTB) simulations 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 four-fold coordinated arsenic site in a seesaw configuration and a three-fold coordinated sulfur site in a planar trigonal configuration, was found in several models. The valence-alternation pairs S3+-S1- are converted into [As4]--[S3]+ pairs under HOMO-to-LUMO electronic excitation. This structural transformation is accompanied by a decrease in the size of the HOMO-LUMO band gap, which suggests that such transformations could contribute to photo-darkening in these materials.
@article{arxiv.cond-mat/0409441,
title = {A new type of charged defect in amorphous chalcogenides},
author = {S. I. Simdyankin and T. A. Niehaus and G. Natarajan and Th. Frauenheim and S. R. Elliott},
journal= {arXiv preprint arXiv:cond-mat/0409441},
year = {2007}
}