Role played by N and N-N impurities in type-IV semiconductors

Ab initio all-electron Hartree-Fock calculations within the molecular-cluster model are performed to analyze the role of N impurities, both isolated and complex, in type-IV semiconductors. The results are used to investigate the structural and electronic properties. For isolated impurities the N atom distorts in the 〈111〉 direction towards a vacancy leading to a final local ${\mathit{C}}_{3\mathit{v}}$ symmetry. The N atom forms ${\mathit{sp}}^2$ bonds with the host atoms and leaves an unoccupied N lone pair. For ${\mathrm{N}}_2$ both N atoms tend to form similar ${\mathit{sp}}^2$ bonds and move away from each other in the 〈111〉 direction without N-N bond formation. For the case of ${\mathrm{N}}_2^{+}$, localized N-N hole states are obtained. A detailed picture at the orbital level is given. These achievements should also be important for amorphous hydrogenated semiconductors.