Two types of oxygen-deficient centers in synthetic silica glass

The effects of ArF excimer laser irradiation on dehydrated high-purity silica glass were investigated on both the optical-absorption bands due to oxygen-deficient centers (ODC) and the formation of ${E}^{\ensuremath{'}}$ centers. With an intense uv flux from an excimer laser, an ${E}^{\ensuremath{'}}$-center density of the order of ${10}^{15}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ was created. The 7.6-eV absorption band remains at the original level, while the 5.0-eV absorption band having the 4.3-eV emission band decreased. Both bands were reduced by heat treatment in an ${\mathrm{O}}_{2}$ atmosphere. These results suggest that there exist two types of ODC: ODC(I), which is responsible for the 7.6-eV band, and ODC(II), for the 5.0-eV band. The concentrations of ODC(I) and ODC(II) were evaluated to be 1 \ifmmode\times\else\texttimes\fi{} ${10}^{18}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ through an analysis of the gas treatment data and of the order of ${10}^{14}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ through the growth curve of the ${E}^{\ensuremath{'}}$ centers, respectively. The structural origin of ODC(I) has been attributed to the Si-Si bond, judging from the fact that the peak energy and cross section of the absorption were in close agreement with those of the ${\mathrm{Si}}_{2}$${\mathrm{H}}_{6}$ molecule. Theoretical calculations on defect energy levels by O'Relly and Robertson supported the structural model for IDC(I) and suggested an unrelaxed oxygen vacancy for ODC(II). These assignments were also consistent with the results of a quenching experiment in which the fictive temperature of samples was changed; the equilibrium between concentrations of ODC(I) and ODC(II) shifted in a reasonable manner.