First-principles Study For One-dimensional Semiconductor Nanostructures: Electronic Properties And Implication To Device Applications

The intrinsic(pure) and extrinsic(doped) electronic properties of several one-dimensional (1D) and quasi-1D semiconductor nanostructures have been studied using first-principles method based on density functional theory.Several device design proposals based on those nanostructures are presented from our theoretical calculations.Firstly,we studied the quantum confinement and electronic properties of singlecrystalline silicon nanotubes(sc-SiNTs) with both uniform(ideal) and nonuniform (more practical for experiments) thickness.(ⅰ) These pristine sc-SiNTs with sp~3 hybridization are found to be energetically stable.The electronic property is sensitive to the external diameter,tube-wall thickness,and tube-axis orientation due to quantum confinement effects.(ⅱ)For SiNTs with nonuniform thickness,the distributions of wavefunctions of the valence band maximum(VBM) and conduction band minimum (CBM) show that the carriers(electrons and holes) are mainly confined in the thicker sides,supplying an advantage to spatially separate the doping impurities from the conducting channel in doped SiNTs.And we proposed a new modulation doping method to reduce the impurity scattering through the nonuniformity of nanostructures instead of heterostructures.Secondly,we studied the impurity scattering for the quasi-1Dπ~* electrons at Si-Ge and Sn-Ge dimers on a Ge(001) surface by first-principles calculations,collaborated with STM experimental group.Phase-shift analysis of standing waves in dI/dV images reveals that Si and Sn atoms located in the conduction path ofπ~* electrons form potentials with the opposite sign to each other.Our density-functional calculations and model calculations based on the nearly-free-electron model explain the observed potential structures,well consistent with experiments.These results are qualitatively understood by relative p-orbital energy of the Si,Sn,and Ge atoms.Thirdly,we investigated electronic structures and electron field emission properties of Cs-doped boron-nitride nanotubes(BNNTs).We found that the nearly-free- electron(NFE) state of the BNNT couples with the alkali atom states,giving rise to a metallic band crossing the Fermi level.Our first-principles electron dynamic simulations under applied fields showed that the BNNT can generate an emission current two orders of magnitude larger than the carbon nanotube.We proposed that the alkali metal-doped BNNT should be an excellent electron emitter in terms of the large emission current as well as its chemical and mechanical stability.Finally,we studied the mechanical properties,electronic structure,and uniaxialstress effects ofβ-SiC nanowires(NWs).It is found that the band gap of SiC NWs becomes larger as their diameter decreases because of the quantum confinement effect, but increases(decreases) slightly with increasing tensile(compressive) stress up to about 12 GPa.The calculated Young's modulus and tensile strength are consistent with the experimental data.