DFT study of undoped and As-doped Si nanowires approaching the bulk limit

The electronic properties of pure and As-doped Si nanowires with radii up to 9.53 nm are studied using large scale density functional theory (DFT) calculations. We show that, for the undoped nanowires, the DFT bandgap reduces with increasing diameter and converges to its bulk value, a trend in agreement with experimental data. Moreover, we show that the atoms closest to the surface of the nanowire contribute less to the states near the band edges, when compared with atoms close to the centre; this is shown to be due to differences in Si-Si atomic distances, as well as surface passivation effects. When considering As-doped Si nanowires we show that dopant placement within the nanowire plays an important role in deciding electronic properties. We show that a low velocity band is introduced by As doping, in the gap, but close to the conduction band edge. The dopant location affects the curvature of this band, with the curvature reducing when the dopant is placed closer to the center. We also show that asymmetry of dopant location with the nanowire leads to splitting of the valence band edge.