准一维纳米结构中的电子输运研究.doc

论文题目：准一维纳米结构中的电子输运研究作者简介：廖志敏，男， 1981年6月出生，2002年9月师从于北京大学俞大鹏教授，于2007年1月获博士学位。中 文 摘 要当物质的尺寸降到纳米量级时，量子尺寸效应、量子干涉效应、库仑阻塞效应以及多体关联效应都会表现得越来越明显，纳米体系将会呈现出许多不同于宏观物体，也不同于单个孤立原子的奇异现象。由于纳米结构的多样性以及电子相互作用的复杂性，纳米系统中新的物理效应不断被发现，人们对纳米材料的电子输运特性远未达到一个清楚认识的地步，在这个领域还有广阔的研究空间。本论文介绍了作者对几种纳米结构中的电子输运特性研究，主要内容包括：(1) 利用电子束、离子束微/纳加工技术制备了各种纳米结构，分析了实验参数对纳米结构与器件加工的影响；(2) 从实验测量和理论分析两个方面研究了无序纳米系统中的量子干涉效应、逾渗纳米系统中的量子干涉效应、强局域化系统中的跳跃电导和库仑充电效应、磁性纳米结构中自旋极化的电子输运、以及光激发下半导体纳米线中的电子输运；(3) 制备出基于单根Fe3O4纳米线的自旋过滤器件，基于单根ZnO纳米线的气敏器件、整流二极管和光伏器件。取得了以下主要研究成果：1. 详细研究了聚焦离子束沉积金属纳米结构过程中的刻蚀效应，讨论各工艺参数对金属沉积的影响，用聚焦离子束制作基于单个ZnO纳米梳子的整流器件。2. 研究了聚焦离子束沉积的Pt(Ga)/C纳米线中的量子干涉效应。从四电极测量的电阻温度依赖关系清楚地观察到起源于系统无序的弱局域化效应。更为有趣的是，我们测量到不同于常规弱局域化体系的正磁电阻效应，根据我们样品的成分和显微结构特征，能很好地用弱局域化框架下的自旋轨道散射和电子之间的库仑相互作用来解释上述现象。3. 详细地研究了退火对Pt(Ga)/C纳米线复合系统的表面形貌、显微结构和电子输运的影响。观察到随着退火温度的升高，Pt颗粒发生迁移并凝聚成更大的颗粒。退火样品的电阻随温度变化曲线显示出两个金属绝缘体转变，低温下由于弱局域化效应和电子电子相互作用，系统电阻随着温度的下降又增大。我们用磁阻实验进一步揭示退火后的Pt(Ga)/C纳米线逾渗系统的量子干涉效应。经过900oC退火后，样品完全表现为金属的行为，由于电子的平均自由程和样品的直径相当，实验结果显示出表面和界面的散射对系统的电子输运特性产生重要的影响。4. 研究了Pt量子点阵列中的强局域化效应，电阻随温度变化的实验结果显示：从室温到64 K曲线可以用e指数的T-1/2函数进行很好的拟合，而低温下用e指数的T-1进行很好的拟合。这是因为电子对颗粒的充电效应能够抑制隧穿电导，库仑相互作用对于局域化的电子在费米能级附近会打开一个库仑能隙，在高温下电子在Pt颗粒之间的隧穿符合库仑相互作用修正的变程跳跃电导模型，从而产生一个e指数的T-1/2的温度依赖关系；而在低温下，电子在Pt颗粒之间跳迁（Hopping）对Pt颗粒充电会造成库仑阻塞效应，由于库仑阻塞效应会产生一个类似于热激活的温度依赖关系，理论分析与实验结果非常吻合。5. 表征了Fe3O4纳米线的显微结构和磁学特性，并且采用电子束曝光技术，在直径约30纳米的单根Fe3O4纳米线上制作了金属微电极。在此器件的基础上，研究了它的自旋依赖的电子输运。对单根纳米线器件的磁电阻系统、深入的分析，发现了电阻随着外加磁场的增加而增大的正磁阻效应，120K时磁阻效应达到15，室温下也有7.5的磁阻，并且观测到一个磁阻的回滞现象，表明磁阻与Fe3O4纳米线的磁化过程有关。利用自旋过滤效应来解释所观测到的正磁电阻效应，对于Fe3O4由于AB位置上磁性离子磁矩反平行排列而相互不能抵消，所以呈现亚铁磁性。Fe3有5个d电子，它们之间相互平行，形成一个满的子壳层。Fe2有一个附加的自旋向下电子，它能够很容易地跃迁到与它自旋平行的邻近的Fe3离子。在磁有序态，仅仅是B位上自旋向下的电子能够很容易地移动，导致一个自旋极化的电子输运。由于自旋极化电子与外加磁场方向相反，所以Fe3O4为负的自旋极化。器件可以看作为一个负自旋极化的纳米线和两个正常金属的接触，当有电流时，自旋向上的电子会受到强烈的散射，只有自旋向下的电子才能够通过纳米线，这样使得界面的电阻大大增加。器件的磁电阻还能够由外加偏压来控制。6. 系统研究了单根ZnO纳米线的发光、电子输运及光电导性质，观察到在强激光激发下，载流子对能带的填充效应导致ZnO纳米线由激子发射转变到带间跃迁，用能带填充效应对实验结果进行了很好的解释。测量了不同气氛下单根ZnO纳米线的I-V特性，理论分析了表面态对纳米线电子输运有着重要的影响。实验上研究了单根ZnO纳米线的光电导特性，观察到持续光电流、激发功率和偏振光依赖的光电导等实验现象；实现了光电导增益、光电开关和光能存储等器件功能。关键词：纳米线，电子输运，量子效应，自旋输运，光电导Study on electron transport in quasi-one dimensional nanostructuresLiao ZhiminABSTRACTAs the size of materials below 100 nm, the quantum confinement effect, quantum interference effect, Coulomb blocked effect and many-body correlated effect influence intensively to the physical properties of materials, which are different from both macroscopical objects and single atoms. For the complex interactions in nanosystems and the diversity of the nanostructures, the properties of electron transport in nanostructures are far from well understood. In this dissertation, the electron transport properties in quasi 1-D nanowires, including disordered system: Pt(Ga)/C nanowires, percolation system: Pt nanowires, strong localization system: Pt/C nanowires, ferromagnetic system: Fe3O4 nanowires and semiconductor system: ZnO nanowires were investigated. The main research works in this thesis are summarized as follows: 1. The process of focused ion beam (FIB) induced platinum deposition was examined by employing atomic force microscopy (AFM) working under the tapping mode. A diode based on single ZnO nanocomb was fabricated by FIB, and the I-V characteristics were measured by conductive AFM tip. 2. The electron transport behavior in single Pt(Ga)/C nanowire fabricated using FIB was studied. Low temperature resistivity of the nanowire increased with decrease of its temperature, following a law, which could be interpreted by electron-electron interaction in weak localization regime. An anomalous positive magnetoresistance was observed as well, which was ascribed to quantum interference effect arising from Coulomb interactions and weak localization in the strong spin-orbit scattering limit. 3. The annealing effects on surface morphology, microstructure and electron transport in Pt(Ga)/C nanowires have be studied in details. The Pt atoms agglomerate and form into the coherent platinum metal throughout the entire nanowire after 900 oC annealing treatment. Form the dependence of resistance with temperature and magnetoresistance at different temperature, we found the quantum interference with a percolation behavior in the annealed sample. After 900 oC annealing, the sample showed a metal behavior, and the surface scattering and boundary scattering contributed a lot to the conductance for the diameter with the nanowire comparable to the electron mean free length at low temperature. 4. An array of Pt quantum dots connected by amorphous carbon was fabricated by electron beam induced deposition. A crossover of the resistivity from the exp(T0/T)1/2 behavior to the activated behavior exp(T*/T) is observed at the critical temperature about 64 K. This crossover behavior is well described by the variation of Coulomb interaction depending on temperature, that is, the interaction with a soft Coulomb gap between the charged grains changes to the strong Coulomb blockade regime with the inelastic tunneling and small mean energy level spacing in a single grain. 5. Spin-dependent electron transport in individual magnetite (Fe3O4) nanowires contacted with normal metallic electrodes was investigated. Such configured device demonstrated a spin-filter effect, this is, only the minority spin carriers can transport through the magnetite nanowire for its negative spin polarization. An anomalous positive magnetoresistance 7.5% is observed at room temperature. Moreover, the magnetoresistance can be controlled via bias voltage. 6. Near-band-edge emission from single ZnO nanowires has been studied as a function of temperature. A transformation from excitonic emission to interband recombination was observed with the increase of temperature as well as the excitation intensity. The broadening of the interband emission is discussed by the screening of Coulomb interaction, band filling effect, and carriers scattering. The current-voltage (I-V) characteristics of single Z