Characterization of E'delta and triplet point defects in oxygen-deficient amorphous silicon dioxide

We report an experimental study by electron paramagnetic resonance EPR of -ray irradiation induced point defects in oxygen deficient amorphous SiO2 materials. We have found that three intrinsic E , E , and triplet and one extrinsic AlO4 0 paramagnetic centers are induced. All the paramagnetic defects but E center are found to reach a concentration limit value for doses above 103 kGy, suggesting a generation process from precursors. Isochronal thermal treatments of a sample irradiated at 103 kGy have shown that for T 500 K the concentrations of E and E centers increase concomitantly to the decrease of AlO4 0. This occurrence speaks for a hole transfer process from AlO4 0 centers to diamagnetic precursors of E centers proving the positive charge state of the thermally induced E and E centers and giving insight on the origin of E from an oxygen vacancy. A comparative study of the E center and of the 10 mT doublet EPR signals on three distinct materials subjected to isochronal and isothermal treatments has shown a quite general linear correlation between these two EPR signals. This result confirms the attribution of the 10 mT doublet to the hyperfine structure of the E center, originating from the interaction of the unpaired electron with a nucleus of 29Si I=1/2 . Analogies between the microwave saturation properties of E and E centers and between those of their hyperfine structures are found and suggest that the unpaired electron wave function involves similar Si sp3 hybrid orbitals; specifically, for the E the unpaired electron is supposed to be delocalized over four such orbitals of four equivalent Si atoms. Information on the structural model of the triplet center are also obtained indicating that it could consist of the same microscopic structure as the E but for a doubly ionized state.