Amorphous chalcogenides, of which selenium is the simplest representative, exhibit a number of unique properties such as the ability to undergo various transformations under the action of the bandgap light. On illumination, the absorption edge shifts to lower energies, and subsequent annealing near the glass-transition temperature leads to a recovery of the initial parameters as demonstrated in Figure 1. Such a photo-induced change could not be observed either in amorphous group IV semiconductors or a-As or in crystalline chalcogenides. A good review of the initial stage of these studies was made by de Neufville.
Reversible photodarkening can also be observed in pure chalcogens, but this process can be achieved only at lower temperatures, which is understandable if one takes into account that the glass-transition temperature of selenium is just above room temperature. This result indicates that there is a correlation between the temperature at which photodarkening is annealed out and the flexibility of the glassy network.
Reversible changes in the optical absorption are accompanied by (reversible) changes in electrical and photoelectric properties, volume, microhardness, glass-transition temperature, and dissolution rate in various solvents, to name a few. The totality of these changes has led investigators to the conclusion that the photoinduced changes in optical absorption are caused by changes in structure.