The role of defect states and band tails in the increase of capacitance (imaginary part of ac conductivity) with illumination at 20 K is discussed for $a-{\mathrm{As}}_2{\mathrm{Se}}_3$ thin films. The time increase of capacitance with illumination has two components, fast and slow. The initial abrupt increase is attributed to the hopping of holes in the band tails. The slow increase is related to the creation of new defects and the subsequent electron hopping between them. When illumination is switched off, the contribution of hopping of holes in the tail states ceases, while the contribution of newly created defects persists. This persistent change is annealed out at room temperature. The illumination and annealing behaviors of the slow process can be related to those of the light-induced electron-spin resonance.

• Received 7 August 1998

DOI:https://doi.org/10.1103/PhysRevB.59.14856