Plasma enhanced chemical vapor deposition of silicon on glass substrates leads to formation of silicon amorphous films with partial crystallization of nano-grains in the amorphous matrix. We studied the transition of amorphous to microcrystalline silicon during such deposition. Formation of silicon nano-grains was detected by means of photoluminescence and Raman spectroscopy. The crystalline fraction and the mean size of the nano-grains were estimated by the position and the intensity of the peaks in the Raman spectrum. We showed that the fraction of crystalline silicon in the layers and the size of the nano-grains are strongly dependent on the growth conditions. The photoluminescence spectra exhibit distinct features related to recombination in the amorphous and in the crystalline phases. A significant narrowing of the photoluminescence peak related to the amorphous phase with increasing crystalline fraction indicates a structural modification in the amorphous silicon. It suggests an ordering process occurring before the start of the actual crystallization. A peak at about 1.4 eV was associated with isolated nano-crystalline grains within the amorphous matrix. A correlation between the peak energy and grain size was found, indicating effects of carrier quantum confinement. The experimental results confirm the established theoretical models for growth of microcrystalline silicon films.