Dimer of substitutional carbon in silicon studied by EPR and ab initio methods

An EPR signal observed in carbon-doped float-zone silicon after irradiation with 2-MeV electrons at room temperature has been investigated. It represe nts a defect with S = 1/2, an apparently isotropic g factor (=2.0030), and a complicated hyperfine structure from Si-29 nuclei in five shells that are consistent with an overall trigonal symmetry. Subtle asymmetries of the hy perfine pattern indicate the presence of a small trigonal component of the g tensor as well. An additional pair of satellite lines is identified by th e relative intensity (1%) as arising from C-13 in natural abundance, occupy ing two equivalent sites on the trigonal axis. Several defect structures th at 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 C-s-C-s, in which the carbon atoms occupy adjacent substi tutional sites, was found to be consistent with the EPR data.