The atomic structures and electronic properties of various defect configurations consisting of B and C atoms in Si pre-doped with C impurity were investigated using first-principles density-functional calculations. In the absence of Si self-interstitials (I), substitutional B and C atoms interacted repulsively with respect to each other; implying that B-C pairs at neighbouring substitutional sites did not behave as a trap for B dopants. In the case of I-B-C complexes, which could be formed in the presence of self-interstitials, it was found that a C-B split interstitial, where the B and C atoms shared a single lattice site along the [001] axis, was the most stable configuration. For several diffusion pathways, along which the B dopant diffused from the C-B split-interstitial configuration with [001] orientation to nearby tetrahedral and hexagonal sites, very high migration energies of some 3eV were found. The diffusing B atom could thus be easily trapped in the neighbourhood of C; resulting in reduced B diffusivity. The range of the C trap potential was estimated to be about 7Å. Diffusion of C from the stable C-B split interstitial, leaving B dopant at a substitutional site, was studied and it was found that the migration energy was reduced to 2.16eV. This indicated that, as the C atom was dissociated, it acted as a trap for self-interstitials; leading to a reduced number of self-interstitials available for B diffusion. Here, suppression of B diffusivity was expected, but without degradation of the electrical activity of the B dopants.

Chemical Bonding and Diffusion of B Dopants in C-Predoped Si. S.Jwa, J.Bang, K.J.Chang: Physical Review B, 2009, 80[7], 075206