【作者】 颜丙海；

【导师】 顾秉林； 段文晖；

【作者基本信息】 清华大学 ， 物理学， 2008， 博士

【摘要】 使用基于密度泛函的第一原理方法,我们研究了几种一维半导体纳米结构的本征的和掺杂状态的电子性质,由此提出了若干器件设计的构想。首先,我们研究了单晶结构的硅纳米管的电子性质和量子尺寸效应。我们提出的这种sp~3杂化的单晶结构硅纳米管模型具有非常稳定的结构,它们的电子性质主要与管壁厚度有关系。尤其对于厚度不均匀的硅纳米管,其带隙主要由较厚管壁区域的尺寸决定,价带顶和导带底的波函数主要分布在较厚的管壁上:这提供了把导电通路和掺杂杂质在空间上分离的可能性。藉此我们提出了新的调制掺杂方法:在较薄的管壁上进行选择性的杂质(p型或者n型)掺杂,就可以在空间上把杂质和导电通道分开,从而降低杂质散射,组装出高载流子迁移率的纳米器件。第二,我们深入探讨了半导体表面上一维输运通道中杂质的局域结构和杂质散射的关系。和东京大学的Komori扫描隧道显微镜实验组合作,我们研究了在重构的Ge(001)-p(2×2)(或-c(4×2))表面上Sn和Si杂质对一维导电通道(π~*电子)的散射机制。处在π~*电子通路上的Si和Sn原子形成了符号相反的散射势场:Si形成了势阱,而Sn形成了势牟。我们提出了密度泛函计算和近自由电子模型相结合的理论方法来计算散射势场,很好的验证了实验的结果。进一步研究发现,这种散射势形成的原因与Si,Sn和Ge原子的p轨道能量的相对大小有关。第三,我们研究了硼氮纳米管的场发射性质,利用其近自由电子态作为电子输运通道来设计高性能的场发射器件。碱金属原子掺杂的硼氮纳米管在费米面附近具有一条金属性的能带(具有接近自由电子的色散关系并且远离散射中心)。我们使用基于第一原理的动态模拟方法,计算了外电场下碳纳米管和掺杂的BN纳米管的场发射性质,发现硼氮纳米管的场发射电流要比碳管高两个数量级。最后,我们还研究了SiC纳米线的力学和电子性质,以及外应力对电子性质的影响。我们的计算得到了和此前实验一致的结论。更多还原

【Abstract】 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.更多还原