On the thermoelastic properties of hydrogenated amorphous silicon

Abstract

Direct mechanical tests were used to estimate both the coefficient of thermal expansion and the elastic modulus of hydrogenated amorphous silicon (a-Si:H) thin films. Films varying in thickness from approximately 2 to 10 μm were deposited on thin foils of aluminum, Invar and copper by conventional capacitive glow discharge decomposition of silane. The coefficient of thermal expansion was subsequently calculated from the temperature dependence of the radius of curvature of the coated foils, and the elastic modulus was then estimated from the thermal expansion coefficient and the film-to-substrate thickness ratio. The mean values obtained for the expansion coefficient and the elastic modulus of a-Si:H are 4.4 × 10 −6 ° C −1 and 44 000 MPa respectively. These values differ significantly from those reported for crystalline silicon but are in qualitative agreement with observations reported for the thermoelastic properties of other amorphous materials when compared with their crystalline counterparts. This knowledge of the thermal expansion coefficient of a-Si:H can be expected to be of aid in the preparation of thick films by helping to improve the match between film and substrate expansion coefficients.