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Precipitation and crystallization of Si nanocrystals have been monitored by means of Raman spectroscopy. SiO x films with different compositions have been deposited by low-pressure chemical-vapor deposition technique onto silica substrates and treated to temperatures exceeding 800 C⁠. The evolution of the Raman signal with the thermal budget reveals that the silicon transition from amorphous to crystalline state shifts to higher temperatures as the Si content in the layers is lowered. A rather complete crystallization of the nanoparticles is achieved after annealing at 1250 C for a Si excess lower than 20%, while for higher excesses the crystalline fraction reaches only 40%, suggesting the formation of a crystalline core surrounded by an amorphous shell. The Raman spectra have been analyzed by a phonon confinement model that takes into account stress effects. An increasing nanocrystal size, from 2.5 to 3.4 …