First-principles study of defect properties in Si and dielectric response in $MgB_2$

Abstract

We perform first-principles pseudopotential calculations to investigate the atomic structure and energetics of various defects consisting of B and P impurities and the effect of P on B diffusion in Si. In equilibrium case, we find that a B-P pair is energetically most stable when P is positioned at a second-neighbor site of B. When excess Si interstitials are generated by implantation, B and P impurities tend to form I$_s-B-P$ complexes with self-interstitials ($I_s$), where P still prefers to a second-neighbor site of B, particularly for high donor concentrations. Using the nudged elastic band method, we examine the diffusion of B and its energy barrier in the presence of P. We find that the diffusion pathways of B are similar to those reported for the $I_s-B$ pair without P, however, the energy barrier for B diffusion from the $I_s-B-P$ complex increases by about 0.2 eV. The increase of the activation energy and the formation of the B-P pair acting as a trap for B diffusion provide a clue for understanding the suppression of B diffusion observed in Si predoped heavily with donor impurities. We study the boron diffusion in Si on the biaxial stress through density-functional pseudopotential calculations. The formation energy of boron diffusing species is found to increase almost linearly as compressive stress increase. In the meanwhile, the migration path is split into three different paths due to symmetry breaking by strain. Two of which are on the plane containing the two stress direction vectors, and the other is perpendicular to the plane. We find that the migration energies for each path are also linearly increase with increasing compressive stress, while the increasing rates with respect to the stress are significantly different for the three paths, which means that boron diffuses well in a particular direction under stress. We suggest that our result can be applied to understand and control the boron diffusion in stressed Si such as STI (Shallow Trench I...