Ab-initio Study of Size and Strain Effects on the Electronic Properties of Si Nanowires

We have applied density-functional theory (DFT) based calculations to investigate the size and strain effects on the electronic properties, such as band structures, energy gaps, and effective masses of the electron and the hole, in Si nanowires along the <110> direction with diameters up to 5 nm. Under uniaxial strain, we find the band gap varies with strain and this variation is size dependent. For the 1 ~ 2 nm wire, the band gap is a linear function of strain, while for the 2 ~ 4 nm wire the gap variation with strain shows nearly parabolic behavior. This size dependence of the gap variation with strain is explained on the basis of orbital characters of the band edges. In addition we find that the expansive strain increases the effective mass of the hole, while compressive strain increases the effective mass of the electron. The study of size and strain effects on effective masses shows that effective masses of the electron and the hole can be reduced by tuning the diameter of the wire and applying appropriate strain.