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

Abstract

The sources of an absorption band at ∼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±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 ${10}^{\mathrm{-}2}$ eV.