【作者】 唐海平；

【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】 第三代半导体材料中的ZnO和GaN的禁带宽度都在3.4 eV左右，它们的发光波长在紫外波段，这个属性使得它们在半导体材料中处于不可取代的位置。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，远高于其它半导体材料，如已经在蓝紫光波段发光器件方面得到广泛应用的GaN材料的激子束缚能只有26meV；由于ZnO中的激子能够在室温及以上温度下稳定存在，而且由激子．激子散射诱发的受激辐射的阈值要比电子-空穴等离子体复合的受激辐射阈值低的多，故ZnO是制备室温和更高温度下的半导体激光器(LDs)的理想材料。ZnO还是目前所有材料中纳米结构最为丰富的材料，现在诸多研究者已成功生长了如纳米线、纳米管、纳米带、纳米环等，它们会具有量子限域效应；ZnO的纳米结构在制备纳米光电子器件和纳米电子器件方面有很好的应用价值，另外，ZnO的纳米结构还可以在场发射、医疗、生物传感等领域得到应用。为了实现纳米ZnO在微纳电子器件和发光器件方面的应用，采用非金属催化剂的自催化生长(采用金属催化剂，金属杂质会扩散进入材料中，并最终形成深能级缺陷，有可能影响器件的稳定性)，获得高质量的纳米ZnO材料这也是值得研究的。在此基础上，对纳米结构进行有意的元素掺杂，提高ZnO纳米结构的导电性能以及稳定性也是以后制作微纳电子器件和发光器件所必需的。近年来，场发射显示技术作为一种新型的自发光平板显示技术，其具备以下优点：(1)冷阴极发射；(2)自发光和高亮度；(3)宽视角；(4)高速响应等。ZnO纳米阵列具有良好的场发射性能，是一种良好的场发射阴极材料，一些研究者进行了这一方面的研究，但仍处于探索阶段。如何进一步降低场发射阈值场强，如何进一步提高其场致发射性能的技术指标和稳定性并制作原型器件等等都需要研究。另外ZnO纳米线LED简单原型已有报道，将来有可能实现ZnO的纳米发光器件，为了拓宽纳米ZnO发光波长范围及获得更高效的发光，对其掺入Mg和Cd进行能带调控也是值得进行研究的。本文第一部分以热蒸发法实现了纳米ZnO非金属催化剂的自催化生长，分析了我们所采用的方法的生长机理。在此基础上，对生长的纳米ZnO阵列进行了Al掺杂的研究，深入分析了Al在ZnO禁带中的能级位置。在生长了掺Al纳米ZnO阵列的基础上，我们采用不同的缓冲层优化了纳米ZnO阵列的场发射性能，并制作了场发射显示原型器件。为了实现能带调制，进行了合金化Mg和Cd的研究。生长了两种具有特殊形貌的ZnMgO和ZnCdO纳米结构。主要结果如下：(1)首次采用醋酸锌(ZA)作为自催化生长纳米ZnO的原料，在两种不同的初始温度得到了辐射状和准阵列的ZnO纳米棒。分析认为不同形貌的直接原因在于两种样品的初始生长温度不同，形成了两种不同的形核层，从而使得最终的形貌呈现出两种结构。(2)在此基础上，首次生长了掺Al的准阵列的ZnO亚微米棒。C-AFM(导电探针原子力显微镜)的分析测试表明Al的掺入很明显的提高了ZnO亚微米棒的电学性能。光学性能的研究发现表明：①Al更确切的施主能级约～90meV；②在Al掺杂ZnO亚微米结构中，室温NBE是由束缚在表面缺陷上的激子复合和其一级LO声子叠加而成；③掺Al引起了吸收边蓝移，而室温PL却发生了红移。这些新的发现帮助我们更清楚地理解了Al在ZnO中的杂质能级。(3)以磁控溅射Au层作为初始的过渡层生长了Al掺杂的ZnO纳米阵列，测试了其场发射性能，其阈值电场为4.5V／μm。与最近一些文献报道的指标(阈值场强4.3V／μm-19.1V／μm)在同一水平，并获得了场致发射显示。(4)首次生长了树枝状的ZnMgO纳米棒。研究表明初始掺入的Mg是以MgO的分相形式存在的，经过800℃退火后MgO最终转变为六方相ZnMgO，并使其相应的PL谱中有蓝移现象，退火样品的带边辐射蓝移了6nm(0.05eV)。采用高纯Zn粉，醋酸锌(ZA)和氯化镉的混合粉生长了宝塔型的ZnCdO亚微米棒。掺入0.9at.％的Cd使得有效带隙成为3.23eV，小于ZnO的有效带隙。GaN作为已在商业领域获得很大成功的第三代半导体，依然是学术界研究的热点。如何在Si衬底上生长高质量GaN为以后的光电集成奠定基础也是GaN的研究者们所努力探索的一个方向。本文第二部分采用MOCVD法进行了Si衬底上生长GaN的研究。生长了无微裂的高质量GaN外延膜，并对其进行了结构、形貌等的测试分析研究，最后运用高分辨XRD对于位错以及外延关系进行了研究。现简要介绍如下：(5)预沉积合适的TMAl可以有效的防止Si衬底的氮化，采用1050℃生长的AlN可以很好的作为外延膜的缓冲层；采用AlN缓冲层生长完，并预沉积Ga的AlN／GaN多缓冲层，可以很好的增加初始Ga的表面浸润性，可以很好的实现早期三维生长向二维生长的转变，并生长出无微裂的GaN外延膜。更多还原

【Abstract】 ZnO and GaN, the two outstanding materials among the third generation semiconductors, are indispensable materials because of their wide band gaps of about 3.4 eV, which lead to emission in the ultraviolet spectral range.Zinc oxide （ZnO） is a semiconductor with a direct wide band gap of 3.37eV at room temperature. Its exciton binding energy is 60 meV, much larger than that of GaN （26 meV）, another wide-gap semiconductor （Eg～3.40eV at room temperature） which is widely used for production of blue-ultraviolet and white light-emitting devices. The larger exciton binding energy makes ZnO more competitive in obtaining efficient lasing by excitonic emission compared to other wide-band-gap semiconductors. Because exciton-exciton scattering-induced stimulated emission occurs at a threshold lower than that for the electron-hole plasma recombination, ZnO is an ideal material for fabricating semiconductor laser devices operating at room temperature and higher temperature. On the other hand, nanostructured ZnO has a diverse group of growth morphologies, which is regarded as the richest family among all the nanomaterials. Many kinds of ZnO nanostructures such as nanowires, nanotubes, nanobelts and nanorings have been obtained so far, and have attracted increasing attention. ZnO nanostructures have promising potentials in extensive applications and are the fundamental building blocks for fabricating nano-optoelectronics and nano-electronics devices, nanosized gas sensors, transducers, and field emitters etc.In order to realize the application of ZnO nanostructures in nano-optoelectronic and nano-electronic devices, it is necessary to obtain high quality ZnO nanostructures by using non-metal catalyst or catalyst-free method. Since, the metal impurities may diffuse into the nanostructures and lead to deep level defects （these are fatal to the stability of devices.） Also, it is necessary to increase the conductivity of ZnO nanostructures for fabricating nano-optoelectronic and nano-electronic devices by intentional n-type doping. Recently, the field emission display technology—a new kind of self-emitting panel display technology has obtained great attentions becauseof its merits—cold cathode emission, self-emission, high-brightness, broad visual angle and quick responsibility. Although many efforts have been made because of its good field emission property, the research of ZnO nanostructures’ field emission is still on the groping way. It still needs further studies on many issues such as further lowering the threshold field of ZnO nanostructures, enhancing the stability and fabricating the prototype of the field emission display device, etc. On the other hand, the ZnO nano-LEDs （light emitting diodes） have been reported. It is possible to realize the actual ZnO nano-LEDs in future. 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 for adjusting the bandgap.In the first section of this thesis, the thermal evaporation using non-metal catalyst was used to grow ZnO nanostructures. The growth mechanisms of the two kinds of nanostructures were also studied. On these basis, we have doped ZnO nanoarray with Al. The exact defect level of the Al impurity was also identified. In the next segment of the section, we prepared Al-doped quasi-alined ZnO nanorods using two kinds of buffer layer to optimize the field emission property. The prototype of the field emission display device was made and tested. In the last segment, two kinds of ZnMgO and ZnCdO nanostructures with peculiar shape were prepared. The main results are as follows:（1）Two kinds of ZnO nanostructures were grown at different initial temperature by using zinc acetate dihydrate （ZA） as self-catalysis materials. We suggest that two kinds of initial layer were formed because of different initial temperature, hence lead to the different morphology of ZnO nanostructures.（2）The quasi-aligned Al-doped ZnO submicro-rods were prepared and its conductivity were also characterized by C-AFM （conductance atomic force microscopy）. It indicated that the conductivity of ZnO submicro-rods was enhanced evidently by Al-doping.Meanwhile, the investigation of the optical properties of the rods indicates: ①The exact donor-level of Al is about ～90meV. ②At room temperature, an excitonicemission and its first LO-phonon replica dominates the NBE emission. Also, we suppose that the excitonic emission can be attributed to exciton bound to surfacedefects. ③ The absorption edge of the Al-doped samples blueshifts, while the nearband edge emission redshifts.These new findings can help us to understand the defect levels of Al in ZnO.（3）Al-doped quasi-aligned ZnO nanorods were prepared on Si substrates with Au buffer layer. Field emission measurements were also conducted. The threshold field is 4.5V/μm. It is comparable to the results（the threshold field 4.3V/μm-19.1V/μm） reported in the literatures. Also, field emission display was obtained.（4）Dendritic ZnMgO nanostructures were grown on Si substrates. The investigations indicate that Mg exist in separate phase （MgO） in as-grown ZnMgO nanostructures. It can be transformed into ZnMgO （hexagonal phase） by annealing at 800°C. The photoluminesence of ZnMgO nanostructures have a blue-shift of about 6nm （0.05eV）.The pagoda-like ZnCdO micro-needles were prepared on Si（111） substrates using Zn, ZA and Cadmium chloride 2.5-hydrate （CdCl₂·2.5H₂O） as the source materials without using metal catalysts. Alloying ZnO with 0.9at.% Cd have changed the effective bandgap to 3.23 eV, which is smaller than that of the pure ZnO.GaN is still a highlight in semiconductor research at present. It is one outstanding material of the third generation semiconductors for its success in applications in our lives. Great efforts have been made to grow high-quality GaN on Si substrates for the potential integration between optical and electrical devices by many researchers. The second section of this thesis focuses on this issue. Crack-free GaN layers on Si substrates were grown by MOCVD. Meanwhile, the structure and morphology of the layers were investigated. The dislocation density and the epitaxial relation between the substrate and the epilayer were investigated by high resolution XRD. Detailed results are as follows:（5）The nitrification of Si substrates can be avoided by suitable predeposition of the TMAl. AlN grown at 1050°C can be a good buffer layer for the growth of GaN. Afterthe growth of A1N, by adding a predeposition step of TMGa （it can increase the wetting-ablity of Ga on the buffer layer） and using AlN/GaN multi-buffers, early transformation from 3-D grwoth to 2-D growth can be induced. Crack-free GaN can be obtained by these processes.更多还原