Theoretical studies of H-passivated silicon nanowires, silicon surface systems and Si/Ge core/shell nanowires

摘要

Global structural optimization with Genetic Algorithm and first principle analysis have been performed on the Silicon nanowires, Ag induced Si surface reconstruction systems and Si/Ge core/shell nanowires. By using genetic algorithm combined with ab-initio calculation, we determined the atomic structures H-passivated \u3c110\u3e and \u3c112\u3e silicon nanowires. we found that at certain values of the hydrogen chemical potential the nanowires can take relatively stable structures in \u3c112\u3e SiNWs with rectangular cross sections bounded by monohydride {110} and {111} facets with dihydride wire edges. In \u3c110\u3e SiNWs cross section of the nanowire evolves from chains of six-atom rings to fused pairs of such chains to hexagons bounded by {001} and {111} facets. Second, with the structural models of SiNWs, we further analyzed their electronic properties. We showed that the \u3c112\u3e SiNWs have an indirect to quasi-direct band gap transition with the increasing sizes and the band gap properties under uniaxial stress and different aspect ratios. Third, we did a Ag-induced Si(111) (rt3xrt3) and (3x1) surface reconstruction search with our variablenumber GA with ab-initio relaxation. The (rt3xrt3) global search found the Inequivalent Triangle (IET) structure as the lowest energy. A model of combination of pure Ag films and IET structure is proposed to explain the islands-to-holes ratio (RIH) equals 3 situation observed in experiments. For the (3x1) reconstruction, a model with 2/3 ML Ag and 1Ml Si coverage has been found and it has lower surface energy than the widely accepted HCC model with only 1/3 ML Ag coverage. Finally, we did some DFT calculation on the Si/Ge and Ge/Si core/shell [112] nanowires. The charged localization inside the NWs reveals that the electrons and holes are seperated. The quantum confinement effect in the NWs is strongly modified by the band offsets. An indirect to quasi-direct band gap transition can be obtained with a compressive strain, and the depth of the quantum wells can be modulated by the aspect ratios.