The aggregation phenomenon of impurity atoms in Si and the properties of C-doped Si studied by first-principle calculations

Abstract

The formation energy, band structures and mechanical properties of carbon (C)-doped single-crystal silicon (Si) with doping concentration from 1.56% to 6.25% were investigated in this paper by means of first-principle calculations. The different performances of the doped structures with C atoms scattered distributing and aggregated distributing in the Si lattice were emphatically discussed. The low impurity formation energy of X4Si60 (X = carbon (C), phosphorus (P), boron (B), nitrogen (N), hydrogen (H), aluminum (Al) or germanium (Ge)) indicates that the impurity atoms have a tendency to aggregate in Si and form a local tetrahedral SiX4 structure. The analyses of partial density of state illustrate that the strong combination of the central Si atom with the four adjacent C atoms results in good stability of the local tetrahedral SiC4 structure. The increasing concentration of the scattered doping C atoms leads to the decrease of band gaps from 1.174 eV to 0.624 eV and the change of doped structures from indirect band gap semiconductors into direct band gap semiconductors. However, the C4Si60 with C atoms aggregated distributing is still indirect band gap semiconductor with a larger band gap of 1.027 eV. The doping of C atoms enhances the charge transfer among atoms, which increases the strength of covalent bonds and results in the improvement of bulk moduli of doped structures. But the doping concentration and the arrangement positions of C atoms have few influences on the mechanical properties of doped structures.