We investigate B diffusion in strained Si by using density functional theory calculations. We calculate the migration barriers and formation energies of the B-Si complexes at different charge states in the biaxial tensile strained ͕001͖ Si layer. The migration barriers and formation energies overall intend to decrease under strain at all charge states. For neutral and negatively charged B-Si complexes, the migration barrier reduces along the strain plane while the barrier in the direction perpendicular to strain plane remains unchanged, but there is no anisotropy in B diffusion for positively charged B-Si complexes.

Using gradient corrected density functional theory calculations, we have investigated the structure and diffusion of excess Si atoms in amorphous SiO 2 , with comparisons to their behavior in ␣-quartz. From the first principles calculations of their configuration, bonding, and energetics, we find that excess Si atoms can be fully incorporated into the amorphous oxide network while yielding oxygen vacancies. The incorporation turns out to gain energy as high as about 1.8 eV, relative to the bond center state where the excess Si atom is located at a Si-O bond center. Based on the results, we propose a novel mechanism for Si diffusion in amorphous SiO 2 in the presence of excess Si atoms, which involves the fourfold-coordinate Si 2+ state creation via oxygen vacancy diffusion and pairing and its reconfiguration to the bond center state. The overall diffusion barrier is approximated to be 4.5-5.0 eV, in good agreement with recent measurements. Our calculation results also predict that excess Si atoms, if they exist, may undergo diffusion with a moderate barrier of Ͻ3.0 eV in ␣-quartz.

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Arsenic enhanced or retarded diffusion is observed by overlapping the dopant region with, respectively, interstitial-rich and vacancy-rich regions produced by Si implants. Enhanced diffusion can be attributed to interstitial-mediated diffusion during postimplant annealing. Two possible mechanisms for diffusion retardation, interstitial-vacancy recombination and dopant clustering, are analyzed in additional experiments. The point defect engineering approach demonstrated in this letter could be applied to fabrication of n-type ultrashallow junctions.

PACS 68.35.Dv, 71.15.Mb, 73.20.Hb Using density functional theory slab calculations we have investigated the effect of surface passivation on vacancy and interstitial annihilation on the Si(001) surface. We find that interstitials and vacancies are both stabilized at the clean and H-/Cl-terminated surfaces, with an energy gain of about 2 -3 eV relative to those at the fifth subsurface layer where the surface effect becomes insignificant. This suggests that the Si surface is an effective sink for vacancies and interstitials, irrespective of surface passivation. However, our calculations show that the stability of vacancies and interstitials within the topmost three subsurface layers is greatly influenced by surface passivation, due to (i) strong interaction with surface Si dangling orbitals and (ii) rearrangement of surface atoms in the clean surface case. The chemical effect of adsorbates itself appears to be unimportant in the defect surface annihilation.