Ab initio calculations for photo‐darkening and photo‐induced structural changes in As ₂ S ₃ and GeS ₂

Abstract

Using ab initio calculations of the electronic structure, this article identifies the optical absorption transitions that initiate photo-darkening. This approach includes an intrinsic network relaxation mechanism for the initiation of photo-induced structural changes in As 2 S 3 that does not require intra-molecular bond scission-induced defect formation, and is limited to alloy compositions such as As 2 S 3 and GeS 4 in which selforganization 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 this type of self-organization occurs, defect formation via intra-molecular bond scission contributes to the reported photo-induced structural changes, as in GeSe 2 and elemental Se. In the context of constraint theory, As 2 S 3 an ideal optimallyconstrained glass in which the number of bonding constraints per atom exactly equals the network dimensionality, whilst GeSe 2 is over-constrained, and Si is floppy, or under-constrained.

Photoinduced Changes in As2S3 Thin Films

“…Photodarkening in arsenic chalcogenides, or the shift of the optical absorption edge to smaller energies with illumination has both scientific and technological importance [12][13][14]. Over the years many models have been proposed to explain this effect [15][16][17][18] and most explanations involve structural changes which in turn affect the electronic states. However, none has proved definite for all the materials in which it has been observed.…”