Nature and origin of the 5-eV band inSiO2:GeO2glasses

Abstract

The sources of an absorption band at \ensuremath{\sim}5 eV observed in ${\mathrm{SiO}}_{2}$:${\mathrm{GeO}}_{2}$ and ${\mathrm{GeO}}_{2}$ glasses have not been unambiguously identified. Results reported here are consistent with the source of two types of neutral oxygen vacancies. Samples of ${95\mathrm{S}\mathrm{i}\mathrm{O}}_{2}$:${5\mathrm{G}\mathrm{e}\mathrm{O}}_{2}$ and ${90\mathrm{S}\mathrm{i}\mathrm{O}}_{2}$:${10\mathrm{G}\mathrm{e}\mathrm{O}}_{2}$ were prepared by a chemical vapor deposition soot-remelting method. Optical-absorption and electron paramagnetic resonance spectra were measured. An absorption band centered at 5 eV in as-prepared ${\mathrm{SiO}}_{2}$:${\mathrm{GeO}}_{2}$ glasses is composed of two components. One has a peak at 5.06 eV and a FWHM (full width at half maximum) of 0.38 eV. Illumination with uv light bleached this band, and generated Ge E' centers. A linear relation was found between the decrement in the intensity of the 5.06-eV component and the concentrations of uv-induced Ge E' centers. This relation is a basis for attributing the defect responsible for this component to the precursors of uv-induced Ge E' centers. We propose that the 5.06-eV band is due to neutral oxygen monovacancies (NOV's) coordinated by two Ge ions. The oscillator strength of this band was evaluated to be approximately 0.4\ifmmode\pm\else\textpm\fi{}0.1 assuming that the NOV's are converted into Ge E' centers by absorption of uv quanta. The activation energy for this conversion process was of the order of ${10}^{\mathrm{\ensuremath{-}}2}$ eV.The second component of the absorption spectra has a peak at 5.16 eV and a FWHM of 0.48 eV. This band is not bleached but emits luminescence bands at 3.2 eV (intense) and 4.3 eV (weak) when irradiated with 5-eV light. Based on other research, we assign this band to ${\mathrm{Ge}}^{2+}$ ions coordinated by two oxygens and having two lone pair electrons (neutral oxygen divacancies). The concentrations of ${\mathrm{Ge}}^{2+}$ ions were much larger than those of the NOV's and the ratio of the NOV's to ${\mathrm{Ge}}^{2+}$ ions increases with increasing ${\mathrm{GeO}}_{2}$ content. A similarity was found in the characteristics of these two types of oxygen-deficient defects to those in ${\mathrm{SiO}}_{2}$ glasses.