Origin of the blue and red photoluminescence from oxidized porous silicon

We have studied the origin of the visible photoluminescence (PL) from oxidized porous Si fabricated by rapid-thermal-oxidization processes. At low oxidation temperature (${\mathit{T}}_{\mathrm{ox}}$), the PL spectrum with a peak near 750 nm is observed, and silicon oxyhydrides and silicon oxides are formed at the surface of nanocrystallites. At high ${\mathit{T}}_{\mathrm{ox}}$ above 800 °C, strong blue PL is observed around 400 nm and the surface of crystallites is covered by ${\mathrm{SiO}}_2$. The electronic state of porous Si is very sensitive to the surface structure of crystallites. Spectroscopic data indicate that the red PL originates from the near-surface region in the crystallite, while the blue PL originates from the core region in the crystallite.