Real time spectroscopic ellipsometry (SE) has been applied to obtain insights into the growth of hydrogenated amorphous silicon (a-Si:H) and microcrystalline silicon (mu c-Si:H) thin films by plasma-enhanced chemical vapor deposition as a function of the H-2-dilution gas flow ratio R = [H-2]/[SiH4], the accumulated film thickness d(b), and the substrate material. For depositions with 15 less than or equal to R less than or equal to 80 on clean amorphous semiconductor surfaces, for example, initial film growth occurs in a predominantly amorphous phase, as deduced from analyses of the real time SE data. However, after an accumulated thickness ranging from 3000 Angstrom for R = 15 to 30 Angstrom for R = 80, a roughening transition is observed in the SE analysis results as the Si film begins to develop a predominantly microcrystalline structure. We have identified this roughening transition as an amorphous-to-microcrystalline phase boundary in the deposition parameter space of d(b) and R. The thickness at which this boundary occurs decreases continuously with increasing R, and the position of the boundary is strongly substrate dependent. Based on these real time SE studies and detailed device analyses, we have found that the highest performance p-i-n solar cells are obtained in i-layer deposition processes maintained at the highest possible R value versus thickness without crossing the deposition phase boundary into the microcrystalline regime.