First-principles calculations of interstitial boron in silicon

We perform first-principles total-energy calculations to identify the stabl e and metastable configurations of interstitial B in Si. We calculate forma tion energies and ionization levels for several equilibrium ionic configura tions in different possible charge states. In all charge states the ground state consists of a B atom close to a substitutional site and a Si self-int erstitial nearby. The binding energy of the self-interstitial to the substi tutional B is, however, rather weak, of the order of 0.2-0.3 eV. The ground state has negative-U properties in accordance with experiments. We find se veral charge-state-dependent metastable configurations of interstitial B en ergetically close to the ground state. We discuss on the basis of formation energies the role of excess Si interstitials in the activation of B diffus ion and the charge-assisted transport mechanism in the activation of B diff usion.