A model for the photostructural changes in amorphous chalcogenides

Prolonged photoirradiation induces a volume expansion and causes a decrease in the optical bandgap (photodarkening) in well annealed amorphous chalcogenides. A microscopic mechanism for the origin of these changes is proposed in terms of the repulsive Coulomb force between layered clusters which can be negatively charged by electron accumulation in the conduction-band tails. It is also proposed that the photoinduced interlayer Coulomb repulsion increases the interlayer separation, which is responsible for the volume expansion, and it also induces an in-plane slip motion which causes the reduction in the bandgap.