Photoinduced changes of ac transport in a-As2Se3 films: Role of defects and band tails

The role of defect stares and band tails in the increase of capacitance (im aginary part of ac conductivity) with illumination at 20 K is discussed for a-As2Se3 thin films. The time increase of capacitance with illumination ha s 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 the m. 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 i llumination and annealing behaviors of the slow process can be related to t hose of the light-induced electron-spin resonance.