Comparative study of the photodarkening relaxation-kinetics in amorphous selenium for above-bandgap and sub-bandgap illumination

The relaxation of photodarkening (PD) brought about by above-bandgap and sub-bandgap excitations is studied in a wide temperature range in amorphous selenium (a-Se) films designed for avalanche photodetectors. The experimental results suggest that in contrast to sub-bandgap excitation, above-bandgap excitation does not cause the formation of self-trapped excitons, viz., photoinduced transformation of the a-Se ground state configuration into a metastable higher-energy configuration. For above-bandgap excitation only transient PD is observed, and its subsequent relaxation can be explained by thermalization and energy relaxation of uncorrelated carriers through the band-tails in order to restore the ground state configuration. In contrast, sub-bandgap excitation causes both transient and reversible PD, with the latter controlled by the formation of self-trapped excitons, whose relaxation requires overcoming an energy barrier of 0.78 ± 0.02 eV either by thermal activation (at elevated temperatures) or by configurational tunneling (for temperatures below room temperature).