【作者】 王敬蕊；

【导师】 叶志镇；

【作者基本信息】 浙江大学 ， 材料物理与化学， 2008， 博士

【摘要】 氧化锌（ZnO）是一种直接宽带隙化合物半导体材料,其室温禁带宽度为3.37eV,激子束缚能为60meV,是制备半导体发光器和半导体激光器的理想材料。ZnO纳米材料具有丰富的结构形态、广阔的应用前景和科学研究价值。实现纳米光电器件和电子器件的重要一步是实现不同ZnO纳米结构的可控制备。另一方面,ZnO纳米结构随材料尺度的减小会表现出许多不同于块体材料的特殊物理效应,如量子尺寸效应、宏观量子隧道效应、库仑阻塞效应、小尺寸效应、表面效应等,各种新颖的纳米结构的制备、结构表征以及性能分析成为纳米ZnO研究的一个热点。此外,ZnO纳米结构的掺杂与合金化研究是最终实现ZnO纳米光电子器件的一个重要方向,引起了人们的广泛关注。本文采用金属有机物化学气相沉积（MOCVD:Metal Organic Chemical VaporDeposition）方法探索了不同形式（量子点、纳米晶颗粒、以及一维纳米结构）和不同形貌（纳米棒,纳米管以及纳米墙）的ZnO纳米结构的生长参数,研究了不同流量和温度对形成不同纳米结构的影响。在此基础上,实现了自组装ZnO纳米形貌的可控生长,并且深入分析了其形成机理;进行了掺Mg合金化纳米结构的研究,深入探讨了合金元素对纳米结构的形貌、微结构和光学性能等的影响。现简要介绍如下:1.利用低压MOCVD方法探索ZnO纳米结构的生长工艺。本文通过改变生长温度、反应物摩尔流量等参数,分析了ZnO量子点、纳米晶薄膜、以及一维纳米结构等三种形式的纳米结构的制备条件及生长机理。在流量较小时,生长受动力学控制,晶粒随衬底温度的升高而增大,从量子点向纳米晶薄膜转变。当流量增大时,纳米晶薄膜向纳米棒转变。随着流量的进一步增大,c轴择优取向生长进一步增强,纳米棒的直径减小,形成ZnO纳米线。2.通过对富锌情况下生长的样品研究,揭示了不同形貌的ZnO纳米结构（纳米棒,纳米管以及纳米墙）之间的关系,以及它们形成的原因。由于ZnO_x（x＜1）的熔点较低,所以当衬底温度较高时,该晶核层会发生蒸发和再吸附现象,从而造成了纳米棒结构向空心的纳米管结构和纳米墙结构转变。3.优化生长工艺,制备出形貌新颖的层片状ZnO纳米棒阵列阵列。XRD、Raman测试表明,层片状纳米棒具有很好的晶体质量。XPS表明样品表面存在比较多的表面态。TEM分析表明层片状ZnO纳米棒是一个单晶整体。PL光谱研究表明,近带边峰主要来自于表面激子发射。通过场发射性能的研究,提出样品表面较多的表面态引起所载流子局域化效应是影响场发射性能的重要原因。4.采用一种全新的掺杂方法,即用六水硝酸镁为Mg源,实现了ZnMgO纳米结构的制备。制备出了直径分布均匀,排列整齐的ZnMgO纳米棒;实现了Mg成分在单根纳米棒内的均匀分布。Mg的含量可以达到12.7at.%。我们对单根纳米棒进行成分分析,发现Mg含量偏差在0.5at.%之内。通过对制备过程的剖析,我们分析了实验所涉及到的化学反应过程,并提出了ZnMgO纳米棒的生长机理。5.在Si衬底的同一区域制备出ZnMgO折叠纳米片和准纳米管的混合结构。基于微结构的分析发现,提出ZnMgO纳米片（（?）2（?）0）面的折叠是由于生长前沿的反应物的浓度起伏,生长方向沿<10-10>晶向的六个对称方向随机改变造成的。6.改变有机源DZE载气的流量,得到Mg含量分别为2.5、1.2、0at.%的ZnMgO折叠纳米片和准纳米管的混合结构。随着Mg含量的增多,PL谱近带边发射峰发生了微小的蓝移,这是由于Mg的掺入引起的禁带宽度增大;带边峰/可见光峰强度之比降低,这表明Mg掺入会造成样品晶体质量的下降。Raman测试表明:第一,E₂（high）峰随样品中Mg含量的增多,发生了微小的宽化和红移;第二,E₁（LO）振动模的强度随Mg含量的增大而增强。这是因为替代的Mg²⁺与Zn²⁺相比具有不同的半径,Mg杂质的掺入打破了晶体原有的平移对称性,从而在晶格中引入了氧空位等微缺陷而造成的。7.在Mg含量为2.5at.%的ZnMgO纳米片中观察到柏格斯矢量为1/3<11（?）3>的具有[0001]刃型分量的位错。结合PL谱、Ranman的测试结果,我们分析认为,位错的形成与掺入的Mg元素有很大的关系:晶格中的氧空位浓度随着纳米片中Mg含量的增大而增大,当超过它在ZnO晶格中的固溶度时,氧空位产生凝聚;氧空位凝聚会造成晶格的坍塌,引起（0001）面的消失,从而产生了柏格斯矢量为1/3<11（?）3>的位错。利用能带结构模型,计算得到位错引起的能带弯曲势垒为2.38V。更多还原

【Abstract】 Zinc oxide（ZnO）is a semiconductor with a direct wide band gap of 3.37 eV and large exciton binding energy of 60 meV at room temperature.Therefore,ZnO is a potential candidate for applications in short-wave optoelectronic devices,such as blue/violet light emitting diodes and laser diodes.Nanostructured ZnO has a diverse group of growth morphologies,which is regarded as the richest family among all the nanomaterials.One important step for fabricating nano-optoelectronics and nanoelectronics devices is to realize the controllable growth of different ZnO nanostructures.It is also necessary to fabricated and characterize complicated structured ZnO nanomaterials to investigate possible distinguished performance in electronics,optics and photonics.In order to widen the spectral range of emission from ZnO based materials and obtain higher luminescence efficiency,alloying ZnO with Mg or Cd is imperative to modulate the band gap.In this thesis,metal organic chemical vapor deposition（MOCVD）method was used to grow nanostructured ZnO.By optimizing the growth parameter,different forms（nanorods,nanocrystals,nanodots）and different morphologies（nanowires, nanowalls,and nanotubes）of ZnO nanostructrus are fabricated.Based upon that,we successfully fabricated self-assembly ZnO-based nanostructures and the growth mechanisms were proposed.1.Different forms of ZnO nanostuctures（nanorods,nanocrystals,nanodots） have been grown by MOCVD method on Si substrates.It was demonstrated that,at small flow rate of the source materials the growth process was controlled by growth kinetics and the obtained ZnO samples transformed from nanodots to nanocrystals at elevated temperatures;while,the growth process was controlled by the growth thermodynamics and the obtained product transforming from nanocrystals to nanorods at elevated flow rate.2.Different morphologies of ZnO nanostructures have been grown on silicon substrates by MOCVD.It is proposed that the reason for the one-dimensional nanostructures to the nanowalls or nanotubes is the zinc-rich growth condition at relative high temperature.3.Lamellated ZnO nanorods have been grown on silicon substrate by MOCVD. The ZnO nanorods have single hexagonal wurtzite structure and prefered orientation along c axis direction.4.ZnMgO nanorod arrays have been synthesized by metal-organic chemical vapor deposition using diethylzinc and magnesium nitrate as the precursors.No oxidant source was used.The ZnMgO nanorods were homogeneous and uniform.The growth process was investigated in detail and a possible growth mechanism was proposed.It is believed that this growth method may avail the uniform distribution of Mg in ZnMgO nanorod.The blueshift of near-band-edge emission of PL spectrum could be readily identified as compared with ZnO crystals,indicating the band-gap engineering in the ZnMgO nanoscale system.5.ZnMgO pleated nanosheets and quasi-nanotubes were synthesized in the same region on Si substrate using[Mg（H₂O）₆]（NO₃）₂ and zinc diethyl as the reactant source. The nanosheets are periodically pleated with the angles of 120°or 60°between two adjacent pleats.Some of the nanosheets fold into quasi-nanotubes.Homoepitaxial interconnections are observed at the boundaries of the pleats,and the whole pleated nanosheet is a single wurtzite crystal.The growth mechanism is discussed,which is proposed to be a combination of vapor-solid process of two-dimensional growth of the（1210）planes and their random along <1010> directions.6.ZnMgO pleated nanosheets and quasi-nanotubes with different Mg content were obtained by changing the zinc diethyl flow rate via metalorganic chemical vapor deposition.The thickness of the nanosheets decreased with the increase of Mg content in the ZnMgO alloy.The Mg incorporation caused the blue-shift of near-band-edge emission of photoluminescence,also（002）peaks position shifted to larger value in X-ray diffraction.The mechanism of the red-shift and broadening of the E2 modes of Raman spectra were discussed.Two additional bands at 387 cm^-1and 622 cm^-1were observed,which were presumably attribute to the host lattice defects induced by Mg incorporation.7.Dislocations with Burgers vector of 1/3<11（?）3> were observed by high-resolution transmission electron microscopy in the ZnMgO nanosheets with Mg content of 2.5 at.%.The Mg incorporation enhances oxygen vacancies in the crystal lattices,thus introducing the dislocations in the ZnMgO pleated nanosheets.The electrostatic potential in the vicinity dislocations was determined to be 2.38 V.更多还原