Optical transitions of paramagnetic Ge sites created by x-ray irradiation of oxygen-defect-free Ge-doped SiO2 by the sol-gel method

Optical transitions of radiation-induced paramagnetic Ge centers have been investigated in Ge-doped SiO2 samples containing a negligible amount of native oxygen coordination defects, whose optical absorption usually dominates the UV spectral range. The analysis of optical and electron paramagnetic resonance (EPR) spectra following different irradiation and thermal annealing treatments shows that two optical absorptions (at 4.4 and 5.7 eV) are related to an orthorhombic EPR signal [Ge(1) signal in literature], while a band at about 6.3 eV follows the evolution of an axial signal [Ge(3) signal] already attributed to axial E'-Ge center. No evidence of another orthorhombic EPR signal observed by other researchers [Ge(2) signal] has been found in our samples. Our results suggest that the previous assignments of the 4.4- and 5.7-eV bands to Ge(1) and Ge(2) centers, respectively, be changed. Instead, both electronic transitions are likely to be ascribed to the sites responsible for the Ge(1) signal. This conclusion has been checked by comparing anisotropy and inhomogeneous dispersion of the principal g-tensor values with energy separation and the relative bandwidth and intensity of the optical bands. Structural models of the Ge site responsible for the Ge(1) signal have also been discussed. A structure like the one proposed for the E'(alpha) center is suggested, attributing the orthorhombic distortion to a nearby oxygen excess group.