Mechanisms of the photodarkening in chalcogenide glasses: Strains or defects?

The photodarkening is observed in many chalcogenide glasses, while its mechanism remains controversial. There are few reports which discuss the whole process. We consider the mechanism through ab initio molecular-orbital calculations for the simplest systems, elemental S and Se clusters. The results manifest that valence-alternation pair (VAP) defects are unstable in the dark, thus it cannot be an origin. On the other hand, intimate VAPs can be quasi-stable, while those affect only band-edge states, which is contradictory with observed spectral modifica-tions. By contrast, strains in van-der-Waals (VDW) distances are likely to broaden the lone-pair electron band. A refined bond-twisting model is able to trace such a process, including annealing stage. Quantitatively, the maximal red-shift of-0.1 eV can be related with quasi-stable strains containable in a medium-range volume of-1 nm3, which can be thermally released through atomic twisting motions over angular barriers with a height of-1 eV.

Automated summary

This study analyzes the mechanism of photodarkening in chalcogenide glasses through ab initio molecular-orbital calculations for elemental S and Se clusters. The results suggest that the strains in van-der-Waals distances are likely to cause the observed spectral modifications in photodarkening.