Photoinduced structural change in hydrogenated amorphous silicon(a-Si:H)has been studied by a sensitive bending detection method using an optical lever. We observed thata-Si:Hfilms show not only thermal expansion due to a photothermal effect but also residual and persistent expansion after light soaking. The volume change is recovered by thermal annealing at 200 °C. A dehydrogenated sample annealed at 550 °C and a microcrystalline sample, in which photoinduced defects are not created, show little photoinduced expansion. The photoinduced expansion and photoinduced defect density show identical time evolution. These results suggest that the photoinduced expansion is related to the photoinduced defect creation. A quantitative evaluation of the photoinduced expansion indicates that the photoinduced structural change is spread over several molecular volumes around a photocreated defect. ©1998 American Institute of Physics.