Stress Evolution during Group III-Nitride Heteroepitaxy

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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.