Electron paramagnetic resonance study of Ge and Sn doping of SiO2 from sol-gel method

Electron paramagnetic resonance (EPR) data on X-ray irradiated Ge:Sn codoped SiO2 samples (0.1, 0.5, 1 wt% Sn and 1 or 3 wt% Ge) produced by sol-gel method are presented. Radiation-induced Ge and Sn related EPR signals are observed due to Ge- and Sn- E' centers, i.e. unpaired electrons in sp(3) orbital of three-fold coordinated Ge or Sn sites. The E' center creation yield is analyzed in samples with different Sn/Ge doping ratio. The concentrations of the two EPR species are not proportional to the dopant contents, the creation of Sn-E' centers being favored with respect to Ge-E'. Lower Ge-E' creation yield is observed in samples with higher Sn content at fixed Ge doping. A simple kinetic model shows that the competitive mechanism of Sn-E' creation cannot fully justify the data if the creation of Ge- and Sn-E' is supposed to be independent, as expected from randomly distributed non-interacting dopant species. Ge and Sn sites are thus proposed to be preferentially spatially related. (C) 1999 Elsevier Science Ltd. All rights reserved.