Si nanocrystals in thermal oxide films (similar to 250 nm) were fabricated by 100 keV Si ion implantation followed by high temperature annealing. Two different doses were compared after annealing at 1050 degrees C for 2 h. A sample implanted with a dose of 1 x 10(17) cm(-2) shows a broad photo luminescence peak centered around 880 nm after annealing. A dose of 5 x 10(16) cm(-2) yields a considerable blue shift of about 100 nm relative to the higher dose as well as a reduction in intensity. Transmission electron microscopy study reveals a difference in the microstructure of the SiO2 films. Nanocrystals are clearly identified in the middle of the film for the highest dose, but not for the lower dose. The difference is discussed in terms of concentration dependent nucleation rate and differences in defect concentration. It is argued that the latter effect has a strong effect on the depth distribution of nanocrystals. (C) 2007 Published by Elsevier B.V.