Semi-insulating polycrystalline silicon samples from a mixture of disilane and nitrous oxide were deposited with different oxygen content and annealed by RTA under different time and temperature conditions. Structural characterization by TEM showed that after annealing and in samples with high silicon content, the excess of silicon crystallizes in small clusters with diameters between 40-80 Angstrom. As shown by FTIR and XPS, the matrix surrounding the nanocrystals is composed of SiOx and SiO2 regions. A thermodynamic free energy model for the different silicon tetrahedral configurations was applied to the structure of this material. The model predicted that phase separation is favoured after annealing at high temperature, but some remaining concentrations of suboxide species are present in the whole range of oxygen content, all these results being in agreement with the shell model. Furthermore, photoluminescence experiments only showed bands owing to defects, in contrast with visible emission reported for nanocrystals of the same size.