Impact of Surface Passivation on the Electronic Structure and Optical Properties of the Si_1?xGe_x Nanowires

The electronic structures and optical properties of the 110?oriented Si_1?xGe_x nanowires (NWs) passivated with different functional groups (?H, ?F and -OH) are investigated by using first-principles calculations. The results show that surface passivation influences the characteristics of electronic band structures significantly: the band gap widths and types (direct or indirect) of the Si_1?xGe_x NWs with different terminators show complex and robust variations, and the effective masses of the electrons in the NWs can be modulated dramatically by the terminators. The study of optical absorption shows that the main peaks of the parallel polarization component of Si_1?xGe_x NWs passivated with the functional groups exhibit prominent changes both in height and position, and are red-shifted with respect to those of corresponding pure Si NWs, indicating the importance of both the terminators and Ge concentrations. Our results demonstrate that the electronic and optical properties of Si_1?xGe_x NWs can be tuned by utilizing selected functional groups as well as particular Ge concentrations for customizing purposes.