Band structure of Si/Ge core-shell nanowires along the  direction modulated by external uniaxial strain
Strain modulated electronic properties of Si/Ge core-shell nanowires along the  direction were reported, on the basis of first principles density-functional theory calculations. In particular, the energy dispersion relationship of the conduction/valence band was explored in detail. At the Gamma point, the energy levels of both bands are significantly altered by applied uniaxial strain, which results in an evident change of the band gap. In contrast, for the K vectors far away from Gamma, the variation of the conduction/valence band with strain is much reduced. In addition, with a sufficient tensile strain (~1%), the valence band edge shifts away from Gamma, which indicates that the band gap of the Si/Ge core-shell nanowires experiences a transition from direct to indirect. Our studies further showed that effective masses of charge carriers can also be tuned using the external uniaxial strain. The effective mass of the hole increases dramatically with tensile strain, while strain shows a minimal effect on tuning the effective mass of the electron. Finally, the relation between strain and the conduction/valence band edge is discussed thoroughly in terms of site-projected wavefunction characters.