Surface and Electronic Properties of Hydrogen TerminatedSi [001] Nanowires

The calculated band gaps reported previously for silicon nanowires (SiNW) have disagreed with the experimental values both in magnitude and in the behavior with radius. We resolve this discrepancy here. We report ab initio quantum mechanical calculations of hydrogen terminated Si [001] nanowires (H-SiNWs) as a function of diameter (d) and hydrogen coverage using the B3LYP density functional. For smaller diameters (d <= 1.9 nm) we find that the most stable surface is fully saturated with hydrogen leading to direct band gaps. For larger diameters, the surface dangling bonds are not saturated, leading to surface LUMO and HOMO states that lower the gap and lead to an indirect band gap. This transition from direct to indirect gap resolves the previous disagreement in the scaling of band gap with diameter. We conclude that the electronic properties of Si NW depend sensitively on controlling the diameter and surface hydrogen coverage.