Electronic structure of twofold-coordinated atoms in silicon-based amorphous semiconductors

Electronic states of the twofold-coordinated N atom in a-${\mathrm{Si}}_{\mathit{x}}$ ${\mathrm{Ni}}_{1\mathrm{-}\mathit{x}}$:H and twofold-coordinated P atom in P-doped a-Si:H have been calculated using the density-functional theory with a local-spin-density approximation. The calculated ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$ hyperfine parameters agree fairly well with those observed in N-rich a-${\mathrm{Si}}_{\mathit{x}}$ ${\mathrm{N}}_{1\mathrm{-}\mathit{x}}$:H by electron-spin-resonance (ESR) experiments, confirming the ESR center to be a twofold-coordinated N atom. On the other hand, the calculated ${}_{}{}^{31}{}_{}{}^{}\mathrm{P}$ hyperfine parameters are largely different from the observed values for the ESR center with a 250-G splitting in P-doped a-Si:H. Therefore the ESR center should not be identified as a twofold-coordinated P atom.