It was recalled that interstitial C defects, Ci, exhibited various unexplained features. The Ci defect, in its neutral charge state, gave rise to 2 almost-degenerate vibrational modes at 920 and 931/cm. Their 2:1 absorption intensity ratio suggested that this was a trigonal defect, but this conclusion contradicted the results of uniaxial stress measurements. It was noted that the di-carbon defect, Cs-Ci, was bistable and that the energy difference between its A and B forms was small; even though the bonding was very different. In the B form of the neutral charge state, a Si interstitial was expected to be located near to a bond-centered site between two Cs atoms. This gave to vibrational modes that involved the motion of both C atoms. This appeared to conflict with the results of photoluminescence experiments. A first-principles local density functional cluster method was used here in order to calculate the structures and vibrational modes of the defects. It was found that the ratio of the absorption intensities of the local modes of Ci was in fair agreement with experimental data; even though the structure of the defect was not trigonal. It was also shown that modes in the vicinity of those that were detected, using photoluminescence techniques, for the B-form of the di-carbon center involved independent movement of the two C atoms.

P.Leary, R.Jones, S.Oberg, V.J.B.Torres: Physical Review B, 1997, 55[4], 2188-94