EPR of a Jahn-Teller distorted (111) carbon interstitialcy in irradiated silicon

An electron-paramagnetic-resonance (EPR) study of irradiated, $p$-type silicon doped with carbon enriched with ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ has revealed that the $\mathrm{Si}-G 11$ spectrum possesses a ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ hyperfine structure. Owing to the complexity and lack of resolution in the observed spectrum, we found it necessary to use a resolution-enhancement technique in order to unravel the angular dependence in the ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ hyperfine spectra. An analysis of the Zeeman and ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ hyperfine interactions indicates that the $\mathrm{Si}-G 11$ center consists of a vacancy occupied by two carbon atoms in a positive-charge state. The effects of stress applied at low temperature indicate that the $\mathrm{Si}-G 11$ center is distorted by the Jahn-Teller effect from ${\mathfrak{C}}_{3v}$ symmetry, a configuration in which both carbon atoms are situated near a vacancy and lie on the (111) axis, to ${\mathfrak{C}}_s$ symmetry, a configuration corresponding to a distorted (111) carbon interstitialcy. The activation energy for electronic reorientation from one Jahn-Teller distortion to another was found to be 0.20 eV. The effects of stress applied at high temperature indicate an atomic reorientation process which occurs with an activation energy of 1.21 eV.