Ab initiocalculations of the mechanical and electronic properties of strained Si nanowires

Abstract

This paper reports a systematic study of the mechanical and electronic properties of strained small diameter (0.7\char21{}2.6 nm) silicon nanowires (Si NWs) using ab initio density functional theory calculations. The values of Young's modulus, Poisson ratio, band gap, effective mass, work function, and deformation potentials are calculated for $⟨110⟩$ and $⟨111⟩$ Si NWs. We find that quantum confinement in $⟨110⟩$ Si NWs splits conduction band valleys and decreases transport effective mass compared to the bulk case. Consequently, additional tensile strain should not lead to further significant electron mobility improvement. An interesting finding we report in this paper is that under compressive strain, there is a dramatic decrease in deformation potentials of $⟨110⟩$ Si NWs, which may result in a strong increase in electron mobilities, despite a concurrent increase in effective mass. We also observe a similar strain-induced counterplay of hole deformation potentials and effective masses for both $⟨110⟩$ and $⟨111⟩$ Si NWs. Finally, we do not see any significant effect of tensile or compressive strain on electron effective masses and deformation potentials in $⟨111⟩$ Si NWs. The sudden changes in effective mass and deformation potentials are concurrent with a change in the conduction and valence band edge states. In $⟨110⟩$ NWs, this change corresponds to a transition from direct-to-indirect band gap under strain.