【作者】 孙合辉；

【导师】 赵连城； 国凤云；

【作者基本信息】 哈尔滨工业大学 ， 信息功能材料与器件， 2013， 博士

【摘要】 GaN基多量子阱薄膜在紫外和红外探测领域有着重要的应用前景，同时其探测性能又在很大程度上受制于外延薄膜的晶体质量，所以对外延薄膜的晶体质量和缺陷展开研究具有重要的意义。本文设计生长了基于GaN/AlGaN多量子阱的外延薄膜，研究了不同中间层生长工艺对外延薄膜的形貌、缺陷和界面结构的影响，同时对多量子阱薄膜的光学和电学性能进行了表征，获得了基于量子阱子带能级跃迁的中波红外吸收，并综合分析了薄膜的微观缺陷对量子阱子带间跃迁的影响。论文的主要研究内容包括：利用薛定谔方程和泊松方程自洽求解的方法计算了不同工艺条件下量子阱中的导带和子能级的分布特性。当量子阱阱层掺杂浓度在10¹⁹-10²⁰cm-3范围时将显著改变子带能级的分布，引起子带间跃迁能级的偏移；盖层Al组分的增加改变了极化电场，使得子带间吸收波长发生蓝移；当多量子阱薄膜的位错密度低于10¹⁰cm^-2时，位错密度的变化不能改变量子阱子带的能级分布；在相同厚度和等效Al组分情况下，GaN/AlN短周期超晶格中间层比AlGaN中间层所需的弛豫厚度大，受此影响的量子阱层周期厚度波动范围加大，由此降低了量子阱能级的对称性，从而降低了子带间吸收的强度。利用金属有机物化学气相沉积方法生长了具有不同中间层结构的GaN/AlGaN多量子阱薄膜，通过光学显微镜和扫描电镜对表面裂纹进行分析后发现，裂纹主要沿<11-20>方向，中间层的插入在一定程度上释放了应变，降低了裂纹密度。原子力显微镜观察证明了表面的台阶流生长模式，同时观察到螺位错导致的六方形螺旋小丘表面。通过高分辨X射线衍射和高分辨透射电镜对样品的晶体质量和界面结构进行了分析，发现外延薄膜各层与衬底间属于赝晶生长，插入AlGaN中间层的样品其量子阱周期性良好，界面平直且陡峭，通过Z衬度像对量子阱层各层成分进行分析后发现各层成分分布均匀，GaN层和AlGaN层间没有Al元素的扩散。利用高分辨透射电镜和傅立叶变换处理技术对各层界面微结构进行了分析。发现GaN缓冲层与α-Al₂O₃衬底界面处存在大量的位错和层错等缺陷，很好的释放了失配应力；AlGaN中间层的引入阻挡了来自于GaN缓冲层的位错，同时又产生了新的失配位错，但均被限制在界面处，从而提高了量子阱功能层的晶体质量；GaN/AlN短周期超晶格中间层的引入并没有对位错传播和膜层应力状态产生明显影响；分析了V型缺陷和位错类型之间的关系。通过弱束暗场技术对位错分布、密度和类型进行了分析，发现位错以刃型位错为主，位错密度在10⁸-10⁹cm^-2范围内，穿透位错起源于GaN缓冲层与α-Al₂O₃衬底界面，并向上传播，在不同界面处受到一定程度的阻挡。表征了四种位错反应类型；利用几何相位分析技术分析了界面处应力场的变化，通过明暗衬度确定了位错的应变状态。利用阴极荧光和扫描电镜技术，研究了表面裂纹、V型坑和位错对多量子阱薄膜发光特性的影响。研究表明多量子阱薄膜的发光谱中峰值波长在550nm的发光峰来源于杂质原子和点缺陷等形成的缺陷能级辐射复合发光所致；表面位错作为非辐射复合中心，表面V型坑不发光，但降低了载流子浓度和迁移率，进而降低了子带间吸收的强度。利用傅立叶红外光谱技术，研究了GaN/AlGaN多量子阱薄膜的红外吸收特性。测试结果显示插入AlGaN中间层的样品存在峰值波长在3.75μm的中波红外吸收，分析认为其对应于量子阱1、2子能级的跃迁。综合分析后认为高的位错密度和量子阱周期厚度的不均匀引起的极化电场减弱和载流子浓度降低是导致子带间跃迁消失的主要原因。通过对GaN/AlGaN多量子阱薄膜显微结构的深入分析，揭示了显微缺陷与薄膜制备工艺之间的关联规律，并在“能带工程”基础上进一步分析了显微结构与薄膜的光学和电学性能之间的内在关系，为基于GaN/AlGaN多量子阱材料实现中波红外探测奠定了基础。更多还原

【Abstract】 Nowadays, GaN-based Multiple Quantum Well （MQW） films have beenattracted much attention as a candidate for photodetector which could be used inultraviolet and infrared region. Detector performance is highly influenced by thecrystal quality of epitaxial films, therefore, the study on the crystal quality anddefects of epitaxial MQW films is very important. In this letter, GaN/AlGaN MQWfilms are designed, the influence of different intermediate layers on the properties ofepitaxial thin film morphology, defects and interface structures are investigatived.The optical and electrical properties of MQW films are characterized, themid-infrared absorption of MQW films have been obtained. The impact ofmicroscopic defects was researched on the intersubband transition in MQW films.The main content of this thesis including:The energy levels of GaN/AlGaN MQW are calculated through self-consistentsolutions of Schr dinger equation and Poisson equation. When the doping density ofwell layers was between10¹⁹cm-3and10²⁰cm-3，the energy level of MQW areobviously varied, which will caused the change of intersubband absorption. Thechanging of Al content of cap layer on the top will lead to blue-shifted ofintersubband absorption wavelength. When the dislocation density was lower than10¹⁰cm^-2, the intersubband energy level will not vary with the change of dislocationdensity. With the same thickness and Al content, the affected thickness ofGaN/AlGaN MQW films with the AlGaN intermediate layer is thicker than the onewhich with the GaN/AlN short period superlattice intermediate layer. Therefore, itwill decrease the symmetry of quantum well energy levels, and then reduce theintensity of intersubband absorption.We have grown GaN/AlGaN MQW films with different intermediate layers byMetal-Organic Chemical Vapor Deposition （MOCVD）. Surface cracks wereinvestigated through optical microscope and scanning electron microscopy. Cracksmainly along the <11-20> direction, distribution of cracks are not correlation withsurface dislocations, the inserting of intermediate layer release the strain in a certainextent and reduce the crack density. The step flow growth mode was proved throughatomic force microscope, at the same time hexagonal spiral hillocks arised fromscrew dislocations were observed.The crystal quality and interface structure of films were analyzed byhigh-resolution X-ray diffraction and high-resolution transmission electronmicroscopy, it is observed that the growth of epitaxial layers and substrates wasbelong to pseudomorphic growth. The film with the AlGaN intermediate layer shown well periodicity, straight and sharp interface. Z-contrast image analysisindicated that compositions were uniform. No diffusion of Aluminum element wasfound between the GaN layer and the AlGaN layer.Interface microstructures of different layers were examined by high-resolutiontransmission electron microscopy and Fourier transform processing technology. It isfound that a large number of dislocations existed at the interface between the GaNbuffer layer and the α-Al₂O₃substrate, which play a key role to release misfit stress.The AlGaN intermediate layer blocked threading dislocations which origined fromthe GaN buffer layer, however, new misfit dislocations generated at the interface.Misfit dislocations were confined at the interface. So the crystal quality of quantumwell layers was improved, the introduction of GaN/AlN short period superlatticeintermediate layer did not affect dislocation distribution and films stress state.Furthermore, the relationship between V-typed defects and dislocations wasanalyzed.Dislocation density, distribution, and types were investigated through weakbeam dark field image. It is found that dislocations are mainly edged-typedislocations, with a density between10⁸cm^-2and10⁹cm^-2. Threading dislocationsorigined at the interface between the GaN buffer layer and the α-Al₂O₃substrate,and then spreaded upwards. Dislocations were blocked at different interfaces. Fourtypes of dislocation reaction were mannered. Stress field changes were analyzed bygeometric phase analysis technique，strain state of dislocations were recongnizedthrough the light and dark contrast.The effect of Surface cracks, V-typed pits and dislocations on luminescenceproperties of MQW films have been studied by cathodoluminescence spectra andscanning electron microscopy. It is shown that the peak located at550nm originedfrom radiative recombination, which resulted from the defect state caused byabsorpation of donor and point defects. The surface dislocation and V-typed defectsacted as non-radiative recombination centers, decreased the carrier density, and thenreduced the rate of intersubband transition.Infrared absorption properties of GaN/AlGaN MQW films were studied byFourier transform infrared spectroscopy. The result shown the sample with AlGaNmiddle layer had an infrared absorption with a peak wavelength corresponding to3.75μm, it is believe that the absorption arised from the transition between first andsecond subband level. The disappearance of intersubband transition in sample2wasdue to high defect density and reduced polar field caused by nonuniform thicknessof quantum wells.Through the analysis of microstructure of MQW films, the association wasshown between microscopic defects and film growth. According to the furtheranalysis of the intrinsic relationship between the microstructure and the optical-electrical properties of films, some improvement was maded on GaN/AlGaNMQW films to achieve the mid-wave infrared detection.更多还原