Atomic and electronic structures of a Boron impurity and its diffusion pathways in crystalline Si

We present extensive first-principles total-energy calculations for boron-silicon interstitial complexes with various configurations and charge states within the density functional theory. We find several stable and metastable configurations. We also find that the stability of each configuration is sensitive to its charge state: The most stable configuration is a pair of a substitutional B and an interstitial Si for positively charged state, whereas an interstitialcy configuration is the most stable for negatively charged state. The pair and the interstitialcy configurations have almost same formation energies in their neutral charge states. Examination of electron states induced by the B-Si complexes indicates that the neutral interstitialcy configuration is an active center for electron spin resonance measurements. It is also found that the B-Si complex is a negative-U system in which neutral charge states are only metastable with the Fermi energy at any position in the energy gap, corroborating the earlier experimental finding by Watkins and collaborators. Further, we present diffusion pathways and corresponding activation energies for the B-Si complex. It is found that the pathways and the activation energies are again sensitive to the charge state, opening a possibility of recombination enhanced diffusion. The calculated results are compared quantitatively with experiments and previous calculations available.