We applied ex situ spectroscopic ellipsometry (SE) on silicon thin films across the a-Si:H/mu c-Si:H transition deposited using different hydrogen dilutions at a high pressure by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD). The optical models were based on effective medium approximation (EMA) and effective to estimate the thickness of the amorphous incubation layer and the volume fractions of amorphous, microcrystalline phase and void in mu c-Si:H thin films. We obtained an acceptable data fit and the SE results were consistent with that from Raman spectroscopy and atomic force microscopy (AFM). We found a thick incubation layer in mu c-Si:H thin films deposited at a high rate of similar to 5 angstrom/s and this microstructure strongly affected their conductivity. (C) 2011 Elsevier B.V. All rights reserved.