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.