First-principles study of the structural, electronic, and optical properties of oxide-sheathed silicon nanowires

Using a density functional theory approach, we examine the dielectric function (η(ω)) optical spectra and electronic structure of various silicon nanowire (SiNW) orientations (〈100〉, 〈110〉, 〈111〉, and 〈112〉) with amorphous oxide sheaths (-a-SiO _x) and compare the results against H-terminated reference SiNWs. We extend the same methods to investigate the effects of surface passivation on 〈111〉 SiNW properties using functional group termination (-H, -OH, and -F) and three different thicknesses of oxide sheath passivation. Oxide layer growth is evidenced in the spectra by concomitant appearance of tail oxide character with signatures of increased Si disorder. Suboxide contributions and increased Si disorder from oxidation average out the band structure dispersion observed in the reference SiNWs. Furthermore, we plot average Seraphin coefficients for 〈111〉 passivations that clearly distinguish functional group termination from surface oxidation and discuss the suboxide and disorder contributions on the characteristic intersection of these coefficients. The substantial difference in properties observed between 〈111〉-OH and 〈111〉-a-SiO_x SiNWs emphasizes the importance of using realistic oxidation models to improve understanding of SiNW properties.