【作者】 张勇；

【导师】 陈克求； 唐黎明；

【作者基本信息】 湖南大学 ， 物理学， 2014， 博士

【摘要】 上世纪七十年代，石英光导纤维和GaAs激光器的出现，促进了光纤通讯的发展，使人类进入了信息时代。如今，纳米材料和纳米技术的发展，使人类从纳米水平上控制强大的新型器件，势必深刻的影响着世界政治，经济格局和军事对抗形式，彻底改变人类的生活方式。随着纳米材料尺寸的持续减小，表面效应变得愈发显著，对材料的电子结构和磁性产生了重要的影响。本论文基于密度泛函理论的第一性原理计算方法，针对具有应用前景的III-V族半导体GaAs，InSb纳米线以及氧化物半导体SnO2量子点的电子结构和磁性展开研究，获得了一些有意义的结果：模拟了裁剪(Ga,Mn)As薄膜成为纳米条带提高体系居里温度的实验结果。通过计算了Mn间隙原子在不同形状，不同方向，不同表面原子和不同尺寸纳米结构中的形成能，发现Mn间隙原子处在最外层由四个As原子组成的四面体间隙位，形成能最低。形成能的大小与纳米结构形状和取向无关，而与尺寸有关，随着尺寸减小形成能逐渐变大。结果表明通过纳米工程技术提高纳米结构比表面积，能够使Mn间隙原子扩散出去，降低Mn间隙含量，从而提高(Ga,Mn)As稀磁半导体的居里温度，合理的解释了实验现象。研究了表面钝化效应对(Ga,Mn)As纳米线磁性的影响，发现在裸纳米线和卤素钝化的纳米线中，Mn取代原子在表面位置形成能最小；赝氢钝化纳米线以后，Mn取代原子在纳米线内部位置形成能最小。研究还发现纳米线的铁磁性与晶体结构和表面钝化有关，闪锌矿型裸纳米线中存在铁磁性和反铁磁性，当纳米线表面被钝化后，出现了反铁磁性到铁磁性的翻转；然而在纤锌矿型纳米线中，无论其表面钝化还是非钝化，体系都将保持铁磁性，且表面钝化能够增强体系的铁磁稳定性。研究了表面钝化效应对InSb纳米线电子结构的调制作用，发现采用不同种类材料钝化InSb纳米线表面悬挂键，与表面原子发生电荷补偿，消除带隙中的表面态，不仅使InSb纳米线表现出其本质的半导体性质，而且能够提高InSb纳米线载流子迁移率，相比于赝氢钝化，卤素钝化效果更显著。研究了单轴应力对InSb纳米线电子结构的影响。通过压缩或者拉伸，发现不同晶体结构，不同尺寸InSb纳米线的电子结构得到了有效地调制。当压力作用在纳米线轴向上，可使闪锌矿型沿[111]方向生长的InSb纳米线电子结构从直接带隙向间接带隙发生转变，而纤锌矿型沿[0001]方向生长的纳米线从半导体向金属发生转变。研究结果还表明载流子的有效质量能够通过应力效应进行有效地调控；在闪锌矿型纳米线中，拉力能够降低空穴和电子的有效质量，压力能够增加空穴和电子的有效质量；在纤锌矿型纳米线中，拉力和压力能够增加空穴和电子的有效质量。研究了表面悬挂键对SnO2量子点磁性的影响，发现纳米结构表面悬挂键态密度的自旋劈裂是导致d0铁磁性的原因之一。阴离子悬挂键表面态主要出现在价带顶附近，类似于浅受主能级提供空穴；阳离子悬挂键表面态主要出现在导带底带隙中，类似于施主能级提供电子。采用带有分数电荷的赝氢原子来钝化体系的悬挂键，表面自旋态消失。过渡金属Co和Ni的3d态也出现在带隙中，当量子点表面存在悬挂键时，阴阳离子的悬挂键分别起受主和施主能级的作用，表面态与Co-和Ni-3d态相互耦合。悬挂键的出现使得SnO2量子点表现出丰富的磁特性，磁矩的变化采用载流子调制模型得到解释。更多还原

【Abstract】 The emergence of quartz optical fiber and GaAs laser promote the developmentof the fiber-optic communications and make our passage into the information ageduring the1970s. The development of nano material and nanotechnology, whichmakes people contral the new devices from the nano-level, has a profound effect onthe political, economic and military, and changes people life in the most radical ways.With the reduction of nano-material size, the surface effect of nano-materials resultsin the importance of surface characteristics in determining the physical porperties ofnano-materials, such as electrical, magnetism etc. In this dissertation, usingfirst-principles calculations, we have studied the electronic structures and magneticproperties of III-V semiconductor nanowires and oxide semiconductor quantum dot,such as GaAs, InSb nanowires and SnO2quantum dot, which have wide applicationprospects, and get some meaningful results.We have simulated a experimental result, which is that a consistent increase inCurie temperature (Tc) with decreasing (Ga,Mn)As wire width was observed in theexperiment. We calculate the formation energy of Mn interstitials located at differentsites in the GaAs nanostructure with different shapes, orientations, surface atoms andsizes. We nd that the Mn atom is located at the outmost tetrahedral interstitial site andthe As atoms occupy this tetrahedral outside surface, the formation energy is thesmallest regardless of the shape and orientation, but it increases with reducing size ofthe nanowire. Therefore, increasing the free surface by nanostructure engineeringallows the Mn interstitials to diffuse out at the sidewalls. Reducing theself-compensating Mn interstitials will effectively increase the Tcof the (Ga,Mn)Asmagnetic semiconductor, which gives a reasonable explanation to the previousexperimental observation.We have investigated the effect of surface passivation on the magnetic propertiesin (Ga,Mn)As nanowires. We find that the Mn atom prefers to stay near the surface in anonpassivated and halogen passivated GaAs nanowire. The Mn atom can stay close tothe center when the nanowire is passivated by pseudo-hydrogen (PH) atoms. Themagnetic coupling in the nanowires depend on the crystallographic structure andsurface passivation. The nonpassivated nanowire with zinc-blend structue has theferromagnetic and antiferromagnetic coupling. After the zinc-blend nanowire is passivated, the antiferromagnetic coupling is translated into ferromagnetic coupling.Both nonpassivated and passivated nanowire with wurtzite structure has theferromagnetic coupling, which is strengthened by surface passivation.We have investigated the surface passivation effects on the electronic propertiesof InSb nanowires. We find that surface passivation effects can make InSb nanowireexhibit intrinsic electronic properties. The band gap modulation is base on chargecompensation between passivation atoms and surface atoms, which is eliminating thesurface states. Surface passivation effects can enhance the carrier mobilities of InSbnanowires. Comparing with the PH passivation, halogen passivation is more distinct.We have studied the uniaxial strain effects on the electronic properties of InSbnanowires, We find that applying strain (compression or tension) can effectivelymodulate the electronic properties of nanowires with various size, direction and crystalstrucutre. After the compression is applied on the growth axie of nanowire, the bandgap of InSb zinc-blend(ZB) nanowires grown along the [111] direction experiences adirect-to-indirect transition, the band gap of InSb wurtzite(WZ) nanowires grownalong the [0001] direction experience a transition from semiconductor to metal.Effective mass of carriers in InSb nanowire can be modulated by strain. Tension canreduce the effective mass of holes and electrons and the compression can increase theeffective mass of holes and electrons in ZB nanowires. Both tension and compressioncan increase the effective mass of holes and electrons in WZ nanowires.The effect of surface dangling bonds on the magnetism of a SnO2quantum dot hasbeen investigated. We find that the spin splitting states of surface dangling bonds is asource of d0magnetism. The anionic surface states mainly are distributed above the topof valence band and are similar to accepter level providing holes, the cationic surfacestates mainly are distributed below the bottom of conduction band and are similar todonor level providing electrons. After the surface dangling bonds are fully passivatedby PH, the surface spin states are eliminated. Co-and Ni-3d states appear in band gap,and the surface states arising from dangling bonds also appear in the band gap. Thesestates couple with each other, and induce a large magnetic moment so that the SnO2quantum dot shows very abundant variation of the magnetic properties. The changes ofmagnetic moments can be explained well by a carrier modulation model.更多还原