Silicon nanocrystals (Si-NCs) in silicon carbide (SiC) matrix fabricated by rapid thermal annealing were characterized using grazing incidence X-ray diffraction and transmission electron microscopy. Two structures of Si-NCs were found in the thin films. Si-NCs in the surface layer had face-centered cubic (fcc) structure, whereas those in the inner layer had cubic diamond (cd) structure. The thermodynamics for the formation of fcc-structured Si-NCs was demonstrated. We suggested that two main factors, Gibbs free energy per atom and surface energy, determined what structure of Si-NC nucleus to be formed. Specifically, reducing the Gibbs free energy per atom of fcc-Si crystal and lowering the surface energy of fcc-Si crystal would be helpful for the formation of fcc-structured Si-NC nucleus. In addition, the effects of carbon and substrate on the formation of Si-NCs were discussed. The work suggested that fcc-structured Si-NCs were readily generated in the surface layer of Si-rich SiC thin films. (C) 2012 Elsevier B.V. All rights reserved.