Nonuniform H distribution in thin-film hydrogenated amorphous Si by small-angle neutron scattering

Small-angle neutron scattering (SANS) is used to search for nonuniform H distributions in hydrogenated amorphous silicon, a-Si:H. Thin films about 2-μm-thick grown with and without D substitution for H by plasma-enhanced chemical vapor deposition show both large (>20 nm) and small (<8 nm) heterogeneous features. The absolute SANS intensities, however, are small and place stringent upper limits on the degree of H heterogeneity present. These results do not support a recently proposed two-domain, amorphous/paracrystalline model. The presence of a small amount of microcrystallinity yields much stronger SANS intensities, consistent with H accumulation in grain boundary regions and/or enhanced microvoid formation. A clear correlation of the larger scattering features with film surface roughness is found