It was noted that C was present at interstitial and substitutional sites in Si, and in complexes. Both the C and the Si interstitial atoms were mobile, and became trapped at other defects (thus altering their properties) or displaced impurity atoms into interstitial sites. An approximate molecular orbital procedure, which incorporated Car-Parrinello like dynamics, was used to model the structure of the Si and C interstitial atoms and of the C substitutional atom. By displacing them along their possible migration paths, and by allowing them to relax back into their stable sites, the details of low-energy migration routes were calculated. It was found that the neutrally charged Si interstitial occupied a (110) split site, but with the axis displaced (along the z-direction) away from the nearest pair of Si neighbors. The neutral C interstitial occupied a (001) split site, with the C being slightly nearer the site than the Si. Negative and singly positively charged interstitials occupied the same sites but, when doubly positively charged, the hexagonal sites were more stable. The migration path of the neutral Si interstitial involved a complex route, with an activation energy of 0.2eV. The lowest-energy C migration route was to pass via the hexagonal site, with a saddle-point energy of 0.77eV. Both the Si and C interstitials formed complexes, of which the metastable di-C center was the best understood. Similar calculations for this defect showed that the metastable Cs-Ci form had an energy which was about 0.8eV above the Cs-Sii-Cs form. An energy barrier of 0.1eV inhibited its re-ordering.

The Migration of Carbon and Self Interstitials in Silicon. A.Mainwood: Materials Science Forum, 1995, 196-201, 1589-94