Visible photoluminescence from Si clusters in γ-irradiated amorphous SiO₂

Visible photoluminescence (PL) bands around 2 eV were studied in ⁶⁰Co γ-irradiated (dose<1 MGy) oxygen-deficient-type amorphous SiO₂ (a-SiO₂) excited by 2-4 eV photons. In addition to the well-known 1.9 eV PL band due to nonbridging oxygen note centers, another PL band was observed at 2.2 eV when excited by 3.8 eV photons. The intensity of the 2.2 eV band increases with decreasing oxygen partial pressure during the sample preparation. Electron-spin-resoaance measurements show that the intensity of the 2.2 eV band is correlated with the concentration of the E'δ center. a paramagnetic state of a cluster of silicons. After much higher δ irradiation with a dose up to 10 MGy, a new PL band was induced at 1.75 eV under excitation by 2.5 eV photons, as well as the 1.9 and 2.2 eV PL bands. By comparing its spectral shape and excitation energy with known PL band in Si-implanted a-SiO₂, it is suggested that the 1.75 eV band is associated with Si nanocrystals formed from Si clusters in a-SiO₂ by the high-dose γ irradiation