Stress Evolution during Group III-Nitride Heteroepitaxy

技术：本项目旨在对金属有机气相外延生长第三族氮化物过程中应力的来源、大小和演化有新的认识和理解。这项研究建立在先前关于在Si(111)衬底上生长GaN过程中的应力演变的原位研究的基础上，其中薄膜破裂是一个问题。本课题将原位应力测量与生长后表征技术相结合，研究了AlGaN和InGaN生长过程中本征生长应力与微观结构演变之间的关系。研究了薄膜成分、生长条件和缓冲层结构对薄膜应力、表面粗糙度和穿丝位错密度的影响。表面活性剂可以改变表面能和动力学过程，将被用来扰乱薄膜的生长模式，以研究导致应力产生和松弛的机制过程。将研究缓解应力的方法，以形成厚的、应变弛豫层，用于UV和绿色发射器。第三族氮化物的异质外延生长通常是在晶格不匹配的衬底上进行的，例如蓝宝石和碳化硅。由于薄膜和衬底之间存在较大的晶格失配和热膨胀失配系数，在外延层中产生宏观应力。应力和由此产生的晶格应变在决定III族氮化物层和异质结构的结构、电学和光学性质方面起着重要作用，并最终通过应变诱导的压电极化和产生位错和缺陷的应力松弛来影响器件性能。除了外延应力和热应力外，由于薄膜形态的发展而产生的生长应力也起着重要的作用，但人们对此还没有很好的理解，特别是对于III族氮化物系统。非技术性：该项目解决具有高度技术相关性的材料科学专题领域的基础研究问题。这项研究将在基础水平上为潜在的下一代电子/光子设备的新理解和能力贡献基本材料科学知识。该计划的一个重要特点是通过在一个具有根本意义和技术意义的领域对学生进行培训，将研究和教育结合起来。除了机理研究，从应用的角度来看，缓解压力的方法也是有价值的。能够在不需要使用多层中间层的情况下制备应变松弛的高Al含量的AlGaN厚层，将有利于紫外光发射器和探测器的发展。同样，应变松弛InGaN层将为进一步改善绿光发射体的性能提供新的途径。这项研究为两名研究生的博士工作奠定了基础。本科生通过暑期研究经验和高级论文项目积极参与该项目。首席调查员将继续参与针对初中生和高中生的夏令营和活动。

Technical: This project aims for new knowledge and understanding of the origin, magnitude and evolution of stress during group III-nitride growth by metalorganic vapor phase epitaxy. The research builds on prior in-situ studies of stress evolution during GaN growth on Si (111) substrates in which film cracking is a problem. In this project, in-situ stress measurements are combined with post-growth characterization techniques to study the relationships between intrinsic growth stress and microstructure evolution during AlGaN and InGaN growth. Effects of composition, growth condition and buffer layer structure on film stress, surface roughness and threading dislocation density will be investigated. Surfactants, which alter surface energy and kinetic processes, will be used to perturb the growth mode of the films in order to study mechanistic processes responsible for stress generation and relaxation. Methods to mitigate stress will be studied for formation of thick, strain-relaxed layers for UV and green emitters. The heteroepitaxial growth of group III-nitrides is typically carried out on lattice-mismatched substrates such as sapphire and SiC. Macroscopic stress is generated in the epitaxial layer due to the large lattice mismatches and coefficient of thermal expansion mismatches that exist between the film and substrate. Stress and the resulting lattice strain play an important role in determining structural, electrical and optical properties of the group III-nitride layer and heterostructrures, and ultimately impact device performance through strain-induced piezoelectric polarization and stress relaxation that produces dislocations and defects. In addition to the epitaxial and thermal stresses, growth stresses resulting from developing film morphology play an important role but are not as well understood, particularly for the group III-nitride system. Non-technical: The project addresses basic research issues in a topical area of materials science having high technological relevance. The research will contribute basic materials science knowledge at a fundamental level to new understanding and capabilities for potential next generation electronic/photonic devices. An important feature of the program is the integration of research and education through the training of students in a fundamentally and technologically significant area. In addition to mechanistic studies, methods to mitigate stress are valuable from an applications perspective. The ability to fabricate thick strain-relaxed high Al-content AlGaN layers without the need to use multiple interlayers would benefit the development of UV emitters and detectors. Likewise, strain-relaxed InGaN layers would provide a new approach to further improve the properties of green light emitters. This research forms the basis for the doctoral work of two graduate students. Undergraduates are active in the program through summer research experience and senior thesis projects. The principal investigator will continue to be involved in summer outreach camps and activities targeted at female middle school and high school students.