Characterization of $\mathrm{E}^{\ensuremath{'}}{}_{\ensuremath{\delta}}$ and triplet point defects in oxygen-deficient amorphous silicon dioxide

Characterization of $E^{'}_{δ}$ and triplet point defects in oxygen-deficient amorphous silicon dioxide

G. Buscarino, S. Agnello, and F. M. Gelardi

Phys. Rev. B 73, 045208 – Published 27 January 2006

Abstract

We report an experimental study by electron paramagnetic resonance (EPR) of $γ$ -ray irradiation induced point defects in oxygen deficient amorphous $Si O_{2}$ materials. We have found that three intrinsic ( $E^{'}_{γ}$ , $E^{'}_{δ}$ , and triplet) and one extrinsic $({[Al O_{4}]}^{0})$ paramagnetic centers are induced. All the paramagnetic defects but $E^{'}_{γ}$ center are found to reach a concentration limit value for doses above $10^{3} kGy$ , suggesting a generation process from precursors. Isochronal thermal treatments of a sample irradiated at $10^{3} kGy$ have shown that for $T ⩾ 500 K$ the concentrations of $E^{'}_{γ}$ and $E^{'}_{δ}$ centers increase concomitantly to the decrease of ${[Al O_{4}]}^{0}$ . This occurrence speaks for a hole transfer process from ${[Al O_{4}]}^{0}$ centers to diamagnetic precursors of $E^{'}$ centers proving the positive charge state of the thermally induced $E^{'}_{γ}$ and $E^{'}_{δ}$ centers and giving insight on the origin of $E^{'}_{γ}$ from an oxygen vacancy. A comparative study of the $E^{'}_{δ}$ 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 $E^{'}_{δ}$ center, originating from the interaction of the unpaired electron with a nucleus of ${}^{29}{Si}$ $(I = 1 ∕ 2)$ . Analogies between the microwave saturation properties of $E^{'}_{γ}$ and $E^{'}_{δ}$ centers and between those of their hyperfine structures are found and suggest that the unpaired electron wave function involves similar Si $s p^{3}$ hybrid orbitals; specifically, for the $E^{'}_{δ}$ 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 $E^{'}_{δ}$ but for a doubly ionized state.