Dimer of substitutional carbon in silicon studied by EPR andab initiomethods

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=1/2, an apparently isotropic g factor (=2.0030), and a complicated hyperfine structure from ²⁹Si 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 ¹³C 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 Cₛ-Cₛ, in which the carbon atoms occupy adjacent substitutional sites, was found to be consistent with the EPR data.