Infra-red absorption measurements of n-type material which was doped with C, and irradiated with electrons at room temperature, revealed new absorption lines (at 527.4 and 748.7/cm) which originated from the same defect. The 748.7/cm line was observed only when the sample was cooled in the dark, and the spectrum was measured through a low-pass filler with a cut-off frequency below 6000/cm. Light with a frequency above 6000/cm removed this line and generated the 527.4/cm line. A comparison with spectra from irradiated material which was doped with 13C showed that the 2 lines represented local vibrational modes of C. The annealing behavior of the 748.7/cm line was identical to that of the electron paramagnetic resonance signal which originated from the negative charge state of 2 adjacent substitutional C atoms (Cs-Cs)-. The 527.4 and 748.7/cm lines were attributed to the E modes of Cs-Cs in the neutral and negative charge states, respectively. The structure and local vibrational modes of (Cs-Cs)0 and (Cs-Cs)- were calculated by means of ab initio local density functional theory. The calculated structures agreed qualitatively with those previously obtained using Hartree-Fock methods, but the calculated Si-C and C-C bond lengths differed. The calculated local mode frequencies were in good agreement with those observed. The formation of Cs-Cs was also investigated. It was suggested that the center was formed when a vacancy was trapped by the metastable substitutional C-interstitial C center, Cs-Ci.

Local Vibrational Modes of Two Neighbouring Substitutional Carbon Atoms in Silicon. E.V.Lavrov, B.Bech Nielsen, J.R.Byberg, B.Hourahine, R.Jones, S.Oberg, P.R.Briddon: Physical Review B, 2000, 62[1], 158-65