Surface passivation effects in silicon nanowires

We report first-principles simulation results for the electronic band structure of Si nanowires (SiNWs) aligned along the ⟨100⟩ and ⟨110⟩ directions with H, OH, and CH3 substituents passivating the surfaces. The ⟨100⟩ wires studied have {110} faces and square cross-sections with diameters up to 1.73 nm, while the ⟨110⟩ wires have {111} faces and diamond cross-sections with diameters up to 1.46 nm. We found that passivation using OH or CH3 groups reduced the band gaps compared to H-terminated ⟨100⟩ SiNWs, and passivation using CH3 groups produced systems with indirect gaps for all ⟨100⟩ SiNWs studied. All band gaps were direct in the ⟨110⟩ SiNWs independent of passivation. The near-gap orbitals are greatly affected by the different substituents. We also found that the carrier effective masses of ⟨100⟩ SiNWs are sensitive to the diameter and passivation, while those of ⟨110⟩ SiNWs are not.