Over the past few years, we have applied real time spectroscopic ellipsometry (RTSE) in order to characterize and optimize hydrogenated amorphous silicon (a-Si:H)-based solar cell fabrication by plasma-enhanced chemical vapor deposition (PECVD). Recently, the RTSE approach has been expanded to include the characterization of fine-grained microcrystalline silicon p-layers(mu c-Si:H:B) deposited at low temperature (200 degrees C) on a-Si:H i-layer substrates in the Cr/(n-i-p) solar cell configuration. In this study, we explore the effects of a H-2-plasma pre-treatment of the underlying i-layer on the resulting microstructural evolution and the optical properties of intended mu c-Si:H:B p-layers, as deduced from a post-deposition analysis of the RTSE data. We find that film growth on an untreated underlying i-layer is in the form of a-Si:H:B even after 100 Angstrom, whereas nanocrystallite nucleation occurs immediately under the same conditions on a H-2-plasma treated i-layer. Unique optical properties observed for the 50-150-Angstrom thick mu c-Si:H:B layers in device structures have been attributed to size effects in Si nanocrystals.