The light-induced metastable lattice expansion in hydrogenated amorphous silicon

Abstract A new insight of the light induced metastable lattice volume change is presented for a-Si:H prepared by plasma enhanced chemical vapor deposition (PECVD). The laser optical-lever bending method is demonstrated for detecting a volume change of film semiconductors. The observed light-induced expansion is persistent after light soaking and is recovered by thermal annealing at 200°C for 2 h. The light exposure time dependence of the lattice expansion was the same dependence of the photoconductivity and light-induced defect density. A correlation between the light-induced expansion and the photolitic changes is obtained. The experimentally obtained light-induced expansions normalized by the initial volume, ΔV/V, are the order of 10−6–10−5, which is two orders smaller than for chalcogenide glasses such as a-As2S3. The ΔV/V has a maximum at deposition temperatures between 200°C and 300°C. The ΔV/V is a maximum at hydrogen dilution ratio with silane gas, r=∼12, but the microcrystalline silicon at r=39 has a negligible small ΔV/V. It is demonstrated that the light-induced expansion is from the structure of a-Si:H.