Peculiarities of interstitial carbon and di-carbon defects in Si

The C-i and C-s-C-i defects in Si exhibit several unexplained properties. In the neutral charge state, the C-i defect possesses two almost degenerate vibrational modes suggesting a trigonal defect in disagreement with the C-2v symmetry deduced from several experiments. The B-form of the second defect is believed to consist of a Si interstitial, Si-i, located near a BC site between two C-s atoms. in apparent conflict with the results of PL experiments which show that the C-related vibrational modes are decoupled. The structure and vibrational modes of both defects are analysed using LDF cluster theory. The degeneracy of the modes of Ci is attributed to an almost D-3h structure, with a 3-fold axis along [01 01(1) over bar]. The modes of the di-carbon interstitial lead to a resolution of the long standing problem concerning the almost zero-shifts due to mixed isotopes in the 580 and 543 cm(-1) local modes observed in PL studies.