Microscopic mechanisms for photoinduced metastability in amorphous As2S3 -: art. no. 174204

Using clusters of atoms that model the local structure of amorphous As2S3 (a-As2S3), we here present a model of photoinduced structural changes in a-As2S3. We have performed quantum-chemical calculations on the model clusters and have obtained their equilibrium configurations, charge distributions, molecular-orbital structures, and excitation energies. It has been found that there exist at least two types of metastable structural defects. One has a fivefold-coordinated As unit, and the other comprises a fourfold-coordinated As atom and a nonbridging S atom. Each type of defect results from the breaking of an As-S bond and subsequent structural rearrangement. From the calculations of the excitation energies, we suggest that the fivefold As defect is responsible for a parallel redshift of the optical absorption edge upon light exposure, called photodarkening. On the other hand, the fourfold As defect is likely to contribute to the photoinduced midgap absorption below similar to2 eV induced by low-temperature light exposure. It has also been demonstrated that the formation of these metastable defects is closely associated with interlayer atomic reconfigurations resulting in homopolar As-As linkages.