Surface Passivation and Orientation Dependence in the Electronic Properties of Silicon Nanowires

Different surface passivation configurations for silicon nanowires (SiNWs) have previously been studied for expanding their technological applications. Of note, methyl (CH$_3$) passivated SiNWs have enhanced ambient stability, while electronegative atoms/groups such as halogens are useful in band gap engineering and chemical post-processing. Thus far though, fundamental mechanisms for how such passivations alter the electronic properties of SiNWs have not been rigorously scrutinized. In this work, we address this issue through first-principles calculations on CH$_3$, fluorine (F) and hydrogen (H) passivated [110] and [111] SiNWs. In comparison to H passivation, we explain how CH$_3$ and F passivations cause significant band gap reductions in [110] SiNWs, through strain and quantum confinement respectively. Furthermore, we discuss how structural differences in [111] SiNWs mitigate these effects, thereby giving the electronic properties of [111] SiNWs greater stability against various surface passivations than those of [110] SiNWs.