Size- and strain-dependent electronic structures in H-passivated Si [112] nanowires

Using first-principles calculations within density functional theory, we have investigated the electronic properties of H-passivated Si nanowires (SiNWs) oriented along the 112 direction, with the atomic geometries retrieved via global search using genetic algorithm. We show that [112] SiNWs have an indirect band gap in the ultrathin diameter regime, whereas the energy difference between the direct and indirect fundamental band gaps progressively decreases as the wire size increases, indicating that larger [112] SiNWs could have a quasi-direct band gap. We further show that this quasi-direct gap feature can be enhanced when applying uniaxial compressive stress along the wire axis. Moreover, our calculated results also reveal that the electronic band structure is sensitive to the change of the aspect ratio of the cross sections.