Real-time spectroscopic ellipsometry (RTSE) has been used to characterize the optical and structural properties of hot-wire CVD (HWCVD) deposited amorphous and nanocrystalline silicon as a function of hydrogen dilution, substrate temperature, and gas pressure. Throughout the range of parameters investigated in this three-dimensional parameter space we find good correlations between post-deposition conductivity measurements and the thickness of the film at the transition from amorphous to nanocrystalline growth, as indicated by a smoothering transition in the surface roughness measured by RTSE during film growth. These results validate the use of RTSE as an in-situ diagnostic to elucidate the nature of HWCVD film growth, particularly as it relates to maximization of photovoltaic device efficiency. (C) 2003 Elsevier B.V. All rights reserved.