REVERSIBLE PHOTODARKENING OF AMORPHOUS ARSENIC CHALCOGENS

Illumination with bandgap light induces changes in physical properties of many chalcogenide semiconductors. Fundamental aspects of strike reversible photodarkening (the light-induced red-shift in the optical absorption edge) in arsenic-chalcogen glasses are critically reviewed. For understanding photodarkening at the microscopic level, details of the changes in the atomic structure that accompany the shift in the absorption edge are of particular importance. Study of the structural changes by a variety of techniques has revealed a phenomenon rich in basis physics but has not led to a coherent picture of the underlying microscopic mechanism. Application of advanced experimental probes providing more detailed structural information has clarified some of the fundamental changes in the atomic structure and their relation to changes in the electronic and mechanical properties. Modifications in short-range and intermediate-range order accompany photodarkening. The changes in short-range order in the form of increased As-As bonding are very small and probably do not play a predominant role in the changes in the electronic structure. Evidence suggests that the primary effect of the light-induced changes is the modification of intermediate-range correlations.