  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.
