Electron-spin-resonance analysis of the natural intrinsicEXcenter in thermalSiO2on Si

An electron-spin-resonance (ESR) analysis of the natural EX defect in thermal ${\mathrm{SiO}}_2$ on Si is presented. Its ESR spectrum consists of a central line amid a doublet of 16.1-G splitting. The Voigt shape of the central line is deconvoluted into a Lorentzian part originating from dipole-dipole interaction, and a residual Gaussian part, likely inhomogeneously broadened due to unresolved hyperfine interaction and/or a spread in the defect’s g factor. The 16.1-G doublet is found to arise from the ${}_{}{}^{29}{}_{}{}^{}\mathrm{Si}$ hyperfine interaction of the unpaired spin with three-or four equivalent Si sites, at first- or higher-order nearest-neighbor positions. The saturation behavior as well as the temperature (T) dependence of various ESR parameters of the EX signal are investigated: g and Δ${\mathit{B}}_{\mathrm{pp}}$ are found to be T independent in the range 4.3–77 K, while the behavior of the EX susceptibility indicates a weak ferromagnetic coupling at low T. It is outlined that the EX defect is not related to any known class of defects in a-${\mathrm{SiO}}_2$, including ${\mathit{P}}_{\mathit{b}}$, E’, the oxygen hole centers, and the self-trapped holes, nor can it be correlated with overcoordinated or undercoordinated O atoms. A preliminary model pictures EX as an unpaired electron delocalized over several atomic sites.