We demonstrated that the timescale for Si quantum dot (Si-QD) formation in a SiOxNy layer is a few milliseconds by IR laser irradiation. The amount of Si agglomerated into QD in a laser irradiated SiOxNy layer is comparable to that calculated after furnace annealing at 1250 °C for 30 min. However, we found that crystalline Si-QD can be formed by laser only if the amount of Si atoms in excess is as high as 1×1022/cm3. The Si-QD contains impurities like N and O that prevent luminescence at 900 nm. The photoluminescence (PL) signal is recorded only after an additional annealing after laser irradiation at temperatures above 1000 °C when diffusion-assisted replacement of N and O occurs.