Persistent photoconductance in a-Si: H/a-SiO_x:H multilayers

Abstract

We report on the existence of a large persistent photoconductance (PPC) effect in multilayers made of alternating layers of glow-discharge deposited hydrogenated amorphous silicon (a-Si: H) and silicon-oxide (a-SiO_x: H). The effect consists of a metastable inerease in dark conductivity caused by a brief light exposure. We find that the PPC effect inereases with inereasing oxide sublayer thickness d₀, reaching saturation at d₀≈ 10nm. The efficiency for the creation of the PPC depends on the oxygen concentration of the oxide used in the multilayers. The magnitude of PPC, measured at room temperature, is independent of the exposure temperature T_e between 4 and 100 K, and then inereases with T_e up to about T_e≈350K. The annealing temperature of the PPC is found to be T_Es≈480-485 K independent of T_e and the doping of the silicon layers. Using photothermal deflection spectroscopy, we observe that, for a given light exposure, the density of light induced defects (Staebler-Wronski effect) is much larger in a-SiO_x: H than in a-Si: H films. Our results suggest that the PPC effect in a-Si: H/a-SiO_x: H multilayers is caused by metastable defects created in the defect structure of the oxide layers. These defects are mainly created by light absorbed in the oxide layers. Defects can also be created by hopping injection into the insulating layers of carriers photoexcited in the adjacent silicon layers. Our results indicate that the mechanism proposed for PPC in a-Si:H/a-SiO_x:H multilayers is universally valid to explain light-induced conductance changes in multilayers made of alternating layers of a-Si: H and a variety of insulating a-Si: H alloys.