Ab initio pseudopotential calculations of B diffusion and pairing in Si

The ab initio pseudopotential method was used to study the boron diffusion and pairing process in crystalline silicon. The results show that substitutional B attracts interstitial Si with a binding energy of 1.1 +/- 0.1 eV. We show that B diffusion is significantly enhanced in the presence of the Si interstitial due to the substantial lowering of the migrational barrier through most likely a kick-out mechanism. The resulting mobile boron can also be trapped by another substitutional boron with a binding energy of 1.8 +/- 0.1 eV, forming an immobile and electrically inactive two-boron pair along a (001) direction. It is also found that the pairing of these two boron atoms involves the trapping of a Si interstitial. Alternatively, two B pairs that do not trap the Si interstitial were found to be energetically unfavorable. All of these findings are consistent with experimental results.