NATURE AND ORIGIN OF THE 5-EV BAND IN SIO2-GEO2 GLASSES

The sources of an absorption band at approximately 5 eV observed in SiO2:GeO2 and GeO2 glasses have not been unambiguously identified. Results reported here are consistent with the source of two types of neutral oxygen vacancies. Samples of 95SiO2:5GeO2 and 90SiO2:10GeO2 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 SiO2:GeO2 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(-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 Ge2+ ions coordinated by two oxygens and having two lone pair electrons (neutral oxygen divacancies). The concentrations of Ge2+ ions were much larger than those of the NOV's and the ratio of the NOV's to Ge2+ ions increases with increasing GeO2 content. A similarity was found in the characteristics of these two types of oxygen-deficient defects to those in SiO2 glasses.