Modelling of photoinduced changes in chalcogenide glasses:: a-Se and a-AS2Se3

The microscopic processes in chalcogenide glasses caused by illumination have been investigated using tight-binding molecular dynamics computer simulation. After the photon absorption we have 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. The model is supported by the results of recent measurements carried out by Y. Ikeda and K. Shimakawa.