Modelling of photo-induced changes in chalcogenide glasses: a-Se and a-AsSe

The microscopic processes in amorphous selenium caused by light-illumination have been investigated using molecular dynamics simulation. After photon absorption we treated the excited electron and hole independently. We found covalent bond breaking in amorphous networks with photo-induced excited electrons, whereas excited holes contribute to the formation of inter-chain bonds. We also observed a correlated macroscopic volume change of the amorphous sample. The interplay between photo-induced bond breaking and inter-chain bond formation leads to either volume expansion or shrinkage. Our results provide a new and universal description, which can explain simultaneously the photo-induced volume expansion and shrinkage. In the non-ideal case, a part of the microscopic processes is metastable and the total expansion includes the transient and metastable changes. We can describe this macroscopic effect, too. Our rate-equation models for a-Se and a-AsSe are supported by the results of recent measurements carried out by Y. Ikeda and K. Shimakawa.