Stress Evolution during Group III-Nitride Heteroepitaxy

技术：本项目旨在通过金属有机气相外延对iii族氮化物生长过程中应力的来源、大小和演化有新的认识和理解。该研究建立在先前的原位研究的应力演化过程中，GaN生长在Si（111）衬底，薄膜开裂是一个问题。在本项目中，将地应力测量与生长后表征技术相结合，研究了AlGaN和InGaN生长过程中固有生长应力与微观结构演化之间的关系。研究了成分、生长条件和缓冲层结构对薄膜应力、表面粗糙度和螺纹位错密度的影响。改变表面能和动力学过程的表面活性剂将被用来干扰薄膜的生长模式，以研究负责应力产生和松弛的机制过程。将研究减轻应力的方法，以形成厚的应变松弛层，用于UV和绿色发射器。iii族氮化物的异质外延生长通常在晶格不匹配的衬底上进行，如蓝宝石和碳化硅。由于薄膜与衬底之间存在较大的晶格失配和热膨胀系数失配，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、电学和光学性能方面起着重要作用，并最终通过应变诱导的压电极化和应力松弛产生位错和缺陷来影响器件性能。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，但尚未得到很好的理解，特别是对于iii族氮化物体系。非技术：该项目涉及材料科学主题领域的基础研究问题，具有很高的技术相关性。该研究将在基础水平上贡献基础材料科学知识，为潜在的下一代电子/光子器件提供新的理解和能力。该计划的一个重要特点是通过培养学生在一个基础和技术上重要的领域的研究和教育的整合。除了力学研究外，从应用的角度来看，减轻应力的方法也很有价值。在不需要使用多个中间层的情况下制造厚应变松弛高铝含量AlGaN层的能力将有利于UV发射器和探测器的发展。同样，应变松弛的InGaN层将提供一种新的方法来进一步改善绿光发射器的性能。这项研究构成了两位研究生博士工作的基础。本科生通过暑期研究经历和毕业论文项目积极参与该项目。首席研究员将继续参与针对女初高中学生的夏令营和活动。

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.