It was noted that interstitial-substitutional C-pairs (CiCs) exhibited an interesting metastable behavior which was associated with 2 different structural configurations. Extensive ab initio calculations were performed here on this system. The results showed that the metastable configuration for the neutral charge state exhibited C1h symmetry and recalled the isolated interstitial C configuration: that is, a split interstitial C-Si pair with the substitutional C bonded to the Si interstitial. The ground-state configuration also had C1h symmetry, but consisted of a single Si interstitial which was 2-fold coordinated in an unusual bridge configuration between 2 substitutional C atoms. This configuration, with an activation energy of 0.07eV, became a motion-averaged state with C3v symmetry. The ground state was lower in energy by 0.11eV with respect to the metastable state. The jump from one configuration to the other corresponded to a simple bond-switching mechanism, with a calculated energy barrier of 0.13eV. Both configurations had 2 electronic states in the gap, and the gap-state wave functions were consistent with local bonding of the defect complex in each case. An analysis of local-mode vibrations on the ground-state configuration indicated a stronger component in one of the C atoms; thus explaining an experimentally observed isotope splitting. All of the results were in satisfactory agreement with experiment.

Theory of Carbon-Carbon Pairs in Silicon. R.B.Capaz, A.Dal Pino, J.D.Joannopoulos: Physical Review B, 1998, 58[15], 9845-50