The optical transitions of radiation-induced paramagnetic Ge centers were investigated in Ge-doped samples which contained negligible numbers of native O coordination defects; whose optical absorption usually dominated the ultra-violet spectral range. Analysis of optical and electron paramagnetic resonance spectra, following various irradiation and thermal annealing treatments, showed that 2 optical absorptions (at 4.4 and 5.7eV) were related to an orthorhombic electron paramagnetic resonance signal, Ge(1), while a band at about 6.3eV reflected the evolution of an axial signal, Ge(3), which had been attributed to an axial E'-Ge center. No evidence was found for another orthorhombic electron paramagnetic resonance signal, Ge(2), which had been observed. The results suggested that the previous attribution of 4.4 and 5.7eV bands to Ge(1) and Ge(2) centers, respectively, should be reviewed. Both were here attributed to the sites which were responsible for the Ge(1) signal. This was checked by comparing the anisotropy and inhomogeneous dispersion of the principal g-tensor values with energy separation, and the relative bandwidth mid intensity of the optical bands. A structural model for the Ge site which was responsible for the Ge(1) signal was considered, and a structure like that proposed for the Eα' center was suggested; thus attributing the orthorhombic distortion to a nearby O excess group.

Optical Transitions of Paramagnetic Ge Sites Created by X-Ray Irradiation of Oxygen-Defect Free Ge-Doped SiO2 by the Sol-Gel Method. N.Chiodini, F.Meinardi, F.Morazzoni, A.Paleari, R.Scotti: Physical Review B, 1999, 60[4], 2429-35