Ab initio theory calculations of the electronic structure of nc-As2S3 and GeS2: an intrinsic mechanism for reversible photo-darkening

Abstract Since photo-darkening in chalcogenide thin films was first reported 30 years ago, several different mechanisms have been proposed for the reversible changes in band edge absorption following cycles of illumination and thermal annealing. This article identifies the optical absorption transitions that initiate these band edge shifts through ab initio calculations of the electronic structure of small clusters that include the short range order as obtained from diffraction studies. The calculations identify an intrinsic mechanism for the initiation of photostructural changes in As 2 S 3 that does not require intra-molecular bond scission-induced defect formation. This bond scission mechanism is limited to alloy compositions such as As 2 S 3 and GeS 4 in which self-organization at the glass transition temperature minimizes bond strain in the annealed or relaxed state. For alloy compositions outside of a narrow compositional window in which self-organization occurs, defect formation via intra-molecular bond scission contributes to the reported photostructural changes as in GeSe 2 and elemental Se.