Abstract Si nanocrystals (nc-Si) were synthesized by rapid thermal annealing of magnetron-sputtered amorphous SiO x films. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy were employed to study the evolution of chemical structures and the rapid growth of nc-Si. XPS results reveal five chemical structures of Si in the as-deposited films corresponding to silicon oxidation states: Si n+ ( n=0, 1, 2, 3 and 4). Their bonding structures are found to be inhomogeneous, and far from that predicted by the random-bonding model. XPS and Raman spectra suggest that the formation of amorphous Si nanoclusters had already taken place in the as-deposited samples. Formation of nc-Si was realized through a combination of thermal decomposition and the diffusion process. Upon thermal annealing, Si suboxides (Si 2O, SiO and Si 2O 3) decompose into Si and SiO 2, resulting in the rapid growth of nc-Si.