An EPR signal observed in carbon-doped float-zone silicon after irradiation with 2-MeV electrons at room temperature has been investigated. It represents a defect with $S=\frac{1}{2},$ an apparently isotropic g factor (=2.0030), and a complicated hyperfine structure from ${}^{29}\mathrm{Si}$ nuclei in five shells that are consistent with an overall trigonal symmetry. Subtle asymmetries of the hyperfine pattern indicate the presence of a small trigonal component of the g tensor as well. An additional pair of satellite lines is identified by the relative intensity (1%) as arising from ${}^{13}\mathrm{C}$ in natural abundance, occupying two equivalent sites on the trigonal axis. Several defect structures that contain two equivalent carbon atoms on a trigonal axis were investigated by ab initio Hartree-Fock calculations. Only the negative charge state of a dicarbon center ${\mathrm{C}}_s-{\mathrm{C}}_s,$ in which the carbon atoms occupy adjacent substitutional sites, was found to be consistent with the EPR data.

• Received 31 August 1999

DOI:https://doi.org/10.1103/PhysRevB.61.12939