【作者】 卢焕明；

【导师】 叶志镇；

【作者基本信息】 浙江大学 ， 材料物理与化学， 2006， 博士

【摘要】 氧化锌（ZnO）是一种直接宽带隙半导体材料，室温下的禁带宽度为3.37eV，激子束缚能为60meV，远高于其他宽禁带半导体材料，也高于室温下的热运动能，可以实现室温或更高温度下的激子诱发的受激紫外辐射发光，因此，ZnO是制备室温或更高温度下半导体发光管及激光器的理想材料。ZnO还有丰富的纳米结构，由于量子约束效应，ZnO纳米材料会具有特殊的光电特性，禁带宽度会增加，激子束缚能将进一步提高，因此，ZnO纳米材料在制备纳米光电子器件方面有很好的应用价值，此外还可以在场发射、生物传感等领域得到应用。目前，缺乏稳定可靠的p型ZnO材料是实现ZnO基发光器件突破的瓶颈，因此，改善ZnO材料的晶体质量、探索有效的p型掺杂技术是目前ZnO薄膜研究中的两个重要方面，是目前迫切需要解决的问题。提高ZnO薄膜的晶体质量是获得性能良好的p型ZnO材料的基础，这方面的突破将铺平ZnO走向短波长光电子器件实际应用的道路，带动信息技术的革命。现代高分辨电镜，使我们可以在原子尺度研究材料的显微结构，从而极大地加深了我们对材料的结构与性能的理解。本文的主要目的就是，利用透射电子显微镜，从微米、纳米到原子尺度对硅基ZnO薄膜及一维ZnO纳米材料的结构、形貌、性能及生长机理进行分析和探讨。研究磁控溅射生长工艺影响ZnO薄膜显微结构的本质，探索提高晶体的可能途径，研究ZnO薄膜中原生缺陷与诱生缺陷的结构形态与形成机理、以及硅基ZnO薄膜中的晶界结构；研究MOCVD法生长的ZnO纳米材料的生长机理，探索控制ZnO纳米材料结构与自组装生长的方法。本文的研究内容分磁控溅射法生长的ZnO薄膜与金属有机化学气相沉积法（MOCVD）生长的ZnO纳米材料两个方面：（1）在优化的工艺条件下，ZnO薄膜呈柱状晶粒生长，并具有高度c轴垂直取向性。阐明了形成择优取向的主要原因是ZnO的[0001]晶向的选择性生长，因为在非晶氧化硅上形成的ZnO晶核具有c轴垂直取向，从而在最初生长阶段的ZnO就具有择优取向性。ZnO薄膜有高密度的原生缺陷，在生长过程中极易形成层错或位错。大量的层错与位错等缺陷使得柱状晶呈层状的镶嵌结构（Mosaic）。研究阐明了ZnO薄膜原生的层错是单错排层的层错，扩展位错的柏氏矢量为：b=1/6[0（?）23]。ZnO薄膜的晶界是柱状晶绕[0001]晶向倾转的晶界结构，按取向差大小可分成三种：晶粒之间取向差比较低的小角度晶界，晶界由不规则排列的位错组成，其柏氏矢量为b=1/3[2（?）0]；晶粒之间取向差接近30°的大角度晶界，晶界沿着一侧晶粒的{10（?）0}面；大部分大角度晶界，晶界两边的晶体相对于晶界对称，晶界沿着接近{11（?）0)的晶面。用重位点阵晶界模型分析了大角度晶界的对称结构。本文还研究了掺Cd的Zn_1-xCd_xO薄膜的显微结构。Zn_1-xCd_xO薄膜Cd组分可达5 at．％，当Cd组分进一步增加时，薄膜中形成了CdO相，成为Zn_1-xCd_xO与CdO的两相混合物。（2）研究了磁控溅射生长工艺对ZnO薄膜显微结构的影响，在这基础上提出了改善硅基ZnO薄膜晶体质量的途径。提高溅射功率，对提高ZnO的晶体质量不利。最佳衬底生长温度约400℃；在较低的衬底生长温度下，ZnO薄膜仍具有高度的c轴垂直取向，但晶粒内的层错等晶体缺陷密度很高；超过400℃的衬底生长温度下，ZnO薄膜的c轴取向偏差增加。（3）退火处理能提高ZnO薄膜晶体的完整性，但过高的退火温度，引起了间隙Zn原子的大量沉淀，形成了新的诱生缺陷，并消耗了相应的氧原子，造成高密度的氧空位，使得n型载流子浓度增加。本文对ZnO薄膜高温退火后形成的诱生缺陷进行了深入的研究与讨论，诱生缺陷是间隙原子沉淀形成的3层错排层的插入型层错，研究还发现，Zn原子优先在原生层错上沉淀时，形成只有2层错排层的层错。通过对原生的层错与退火后诱生的层错的显微结构研究，澄清了ZnO薄膜中三种层错的不同的形成机理，以及影响ZnO薄膜性能的原因。（4）研究了MOCVD法生长的硅基ZnO纳米线与纳米管阵列的显微结构，ZnO纳米线具有六角型的截面形状，纳米线的侧面是{10（?）0}晶面。验证了通过实时控制生长工艺，增加反应物流量使纳米线的直径减少。这种排列高度整齐、尖端只有10nm左右的ZnO纳米线阵列，在场发射器件方面具有很大的应用价值。ZnO纳米管是在岛状ZnO缓冲层上外延生长的六角型的空心管，六角型的侧面是{2（?）0}晶面。形成ZnO纳米管的原因是ZnO纳米管被限制在直径在40～60nm的纳米岛范围内带状生长，使得ZnO纳米带成为六角形的纳米管。如果ZnO纳米管没有封闭，就成为截面为多边形的纳米带。本文为进一步研究ZnO纳米管的可控生长，最终为达到ZnO纳米管的自组装奠定了一定基础。更多还原

【Abstract】 Zinc oxide （ZnO） is a semiconductor with a wide direct wide-bandgap of 3.37eV at room temperature, and exciton binding energy of 60 meV which is much larger than other wide-gap semiconductors such as GaN and 2.3 times that of the room-temperature thermal energy （ k_B = 26meV ）. It is a potential candidate for applications in short-wavelength optoelectronic devices, including light emitting diodes （LEDs） and laser diodes （LDs）, because its large exciton binding energy could lead to lasing action based on excition recombination even above room temperature. Furthermore, many significant exciton effects may be expected in low-dimensional ZnO nanostructures due to quantum confinement effects, and so the improvement in device performance can be predicted. ZnO nanostructures have promising potentials in extensive applications in nano-optoelectronics and nano-electronics devices, nanosized gas sensors, and field emitters etc.However, its applications to optoelectronic devices has not yet materialized due chiefly to the lack of reliable and high quality p-type epitaxial layers of ZnO. An imperative issue is that crystal quality of epitaxial ZnO layers should be improved and p-type conductivity films with good properties should be achieved, which could potentially make inroads into the world of optoelectronics..In this thesis, to make use of modern transmission electron microscopy, ZnO films by magnetron sputtering and nano-ZnO by Metal Organic Chemical Vapor Deposition （MOCVD） have been investigated for structure, properties and developing mechanism on micrometer, nanometer and atomic scales. The grain boundary, as-growth defects and processing defects by annealing also have been investigated. The influences of sputtering conditions and post-annealing on microstructures for ZnO films have been investigated in order to find way to improve crystallinity, and the developing mechanism for nano-ZnO materials have been investigated to control structure and self-assembled crystal.The research topics and main results are as follows:1. In optimized condition, ZnO films possess columnar grains with strong[0001] preferred orientation. The reason of formation for texture is mainly that only ZnO nucleus with selecting [0001] orientation grow on amorphous oxidation on Si due to the lower surface free energy of （0001） plane. It is indicated that single stacking fault is a kind of as-growth defect which could be surrounded by partial dislocations with Burgers vector b =1/6[02|-23]. The [0001]-tilt grain boundaries in ZnO films have been investigated symmetrically. It is indicated that boundaries can be classified into three types: low-angle boundaries described as an irregular array of edge dislocations with Burgers vector b =1/3[21|-1|-O], boundaries of near 30 degreeangle with {101|-0} facet structures, and large-angle boundaries with symmetric structure which have been investigated by low Σ Coincidece Site Lattice as a low free energy ones to explain why it is straight or zigzag arrangements. Zn_1-xCd_xO films have been investigated, too. It is indicated that the wurtzite-type structure of Zn_1-xCd_xO can be stabilized up to Cd content x = 0.05 without a cubic CdO phase separation.2. It is indicated that films grow still with preferred orientation but with dense defects at low temperature, and deviation angles of ZnO [0001] orientation from normal increase at temperature more than 400℃. And ZnO films degrade with too larger power due to having random oriented polycrystalline ZnO layer at the film-substrates interfaces. A growth process with two steps is suggested that c-oriented layer is deposited at low temperature and then epitaxial ZnO film is developed at temperature more than 400 °C with low defects and preferred c-oriented.3. The crystallinity of ZnO films could be improved by annealing, however, in too high temperature, high concentration of oxygen vacancies formed because oxygen atoms have to diffuse from the surrounding lattice to stacking fault when Zn atoms precipitate which lead to that n-type carrier concentration increases. The three layer stacking fault is a kind of processing defects by annealing because interstitial Zn atoms precipitated. Besides, double stacking fault is formed when interstitial Zn atoms prefer to precipitate on as-growth single stacking fault. It is proposed that the precipitation is Zinc-blende ZnO of about 1nm thick.4. Microstructures of well-aligned ZnO nanowires, quasi-alignedone-dimensional nano-ZnO composed of nanotubes and nanobelts grown by MOCVD method on Si substrates have been investigated. It has hexagonal cross sections, surrounded by {101|-0}planes. The diameter of ZnO nanowires has been controlled by simply varying the reactant sources flow rate, and nanowires with a 10 nm diametical tip possess of excellent field emission properties. The nanotubes have hexagonal cross sections with sheets of about 10 nm thick, but surrounded by {21|-1|-0} planes. A new mechanism growth is proposed, which can be used to control other nanotubular and self-assembled structures, that the ZnO nanobelts have been limited to grow at top of ZnO islands with a diameter of 40～60 nm on Si substrates to lead to form nanotubes.更多还原