Investigation of off-center substitutional N in Si

Total-energy calculations for N as a function of displacement from the tetrahedral site in Si were computed by a Hartree-Fock technique applied to 17-atom hydrogen-terminated Si clusters containing N. Considering displacements of the central atom only, it was found that the total energy was minimized when N was on the Td-symmetric substitutional site. However, the total energy was minimized for 〈111〉 displacements of N if the four nearest-neighbor Si atoms relaxed either outward or inward. The N displacement when the Si atoms relax outward appears driven by a reduction in the N-Si force and the N deep level remains sp³ bonding in nature. In contrast, N displacement when the Si atoms relax inward appears driven by the proclivity of N to form trigonal sp² bonds. Additional cluster calculations suggest that such inward relaxation is likely. When the four nearest-neighbor Si atoms relaxed inward by 0.45 A{}, the total-energy minimum occurred with N displaced by 0.75 A{} in the 〈1{} 1{} 1{}〉 direction. Similar calculations for O revealed a 〈100〉 displacement of approximately 1.1 A{}.