Real Time Spectroscopic Ellipsometry of Amorphous Silicon Grown at High Deposition Rates by Hot-Wire CVD

We use real-time spectroscopic ellipsometry (RTSE) for in-situ characterization of the optical properties and surface roughness (R_s) of hydrogenated amorphous silicon (a-Si:H) grown by hot-wire chemical vapor deposition (HWCVD) with varying deposition rates (5 to 120 Å/s). Early time evolution of the R_s during growth is remarkably similar for all deposition rates. During the first few Ås of growth, there is a sharp increase in R_s as the a-Si:H nucleates in separate islands. This is followed by a reduction of R_s as these areas coalesce into a bulk film, which occurs at an average thickness of 100 Å. After coalescence the R_s rises to a stable value that is dependent upon growth conditions with a general tendency for the R_s to increase with growth rate. However, neither the R_s nor the material electronic properties are unique for a given deposition rate. Films grown under high silane flow and low pressure have a better photo-response and a lower R_s than films grown at the same deposition rate but with low silane flow and high pressure. We observe a stronger correlation of film properties with R_s than with deposition rate; namely a monotonic decrease in photo-response, and increase in optical gap, with increasing R_s.