Optical properties of Si clusters and Si nanocrystallites in high-temperature annealed SiOx films

Structure, optical absorption and photoluminescence (PL) properties of ${\mathrm{SiO}}_x$ films subjected to thermal annealing at 750–1100 °C are investigated. Si crystallites with a few nanometers in size are observed in the ${\mathrm{SiO}}_{\mathrm{1.3}}$ and ${\mathrm{SiO}}_{\mathrm{1.65}}$ films annealed at 1100 °C. Threshold energies in optical absorption of the Si nanocrystallites are higher than that for bulk Si, suggesting a contribution from quantum confinement effects. The PL spectrum shows a remarkable increase in intensity after annealing at temperatures above 1000 °C. This PL behavior is closely related to the formation of Si nanocrystallites by the annealing. The PL peak energy of the annealed films shifts to higher energy with decreasing crystallite size but does not follow the blueshift for the absorption threshold energy. These results suggest that a localized state contributes to the PL mechanism. The ${\mathrm{SiO}}_{\mathrm{1.8}}$ film annealed at 1100 °C, which contains no Si crystallites, exhibits an intense PL similar to the annealed ${\mathrm{SiO}}_{\mathrm{1.3}}$ and ${\mathrm{SiO}}_{\mathrm{1.65}}$ films. It is implied that noncrystalline Si nanoparticles are formed in the ${\mathrm{SiO}}_{\mathrm{1.8}}$ film under high-temperature annealing.