We have investigated the phase separation and silicon nanocrystal (Si NC) formation in correlation with the optical properties of Si suboxide (SiOx, 0 < x < 2) films by thermal annealing in high vacuum. The SiOx films were deposited by plasma-enhanced chemical vapor deposition at different nitrous oxide/silane (N2O/SiH4) flow ratios. The as-deposited films show increased Si concentration with decreasing N2O/SiH4 flow ratio, while the deposition rate and surface roughness have strong correlations with the flow ratio in the N2O/SiH4 reaction. After thermal annealing at temperatures above 1000 degrees C, Fourier transform infrared spectroscopy, Raman spectroscopy, and transmission electron microscopy manifest the progressive phase separation and continuous growth of crystalline-Si (c-Si) NCs in the SiOx films with increasing annealing temperature. We observe a transition from multiple-peak to single peak of the strong red-range photoluminescence (PL) with increasing Si concentration and annealing temperature. The appearance of the single peak in the PL is closely related to the c-Si NC formation. The PL also redshifts from similar to 1.9 to 1.4 eV with increasing Si concentration and annealing temperature (i.e., increasing NC size). The good agreements of the PL evolution with NC formation and the PL peak energy with NC size distribution support the quantum confinement model. (c) 2006 Elsevier B.V. All rights reserved.