Effects of oxygen, fluorine, and hydroxyl passivation on electronic properties of ⟨001⟩-oriented silicon nanowires

Abstract

By means of ab initio calculations we show possible ways to band gap engineering in the ⟨001⟩-oriented silicon nanowires with a diameter of about 1.5 nm and terminated by hydrogen, oxygen, fluorine, and hydroxyl. The fluorine and hydroxyl functional groups are found to effectively decrease the gap value, if their coverage exceeds more than 50%, and to mostly preserve the direct nature of the gap in such nanostructures. Saturation by oxygen affects band dispersion near the gap region depending on the type of the bond between silicon and oxygen atoms. Estimates of the dipole matrix elements of the first direct transition mainly indicate marginal oscillator strength, however, some cases of silicon nanowires with the specific surface termination display the direct nature of the gap with appreciable oscillator strength. We also discuss the role of different terminating functional groups in the photoluminescence process from porous silicon considered as an ensemble of nanowires.