Design and Development of Stress Engineering Techniques for III-Nitride Epitaxy on Si

摘要

III-Nitrides have been a heavily researched material system for decades. Their material properties are favorable for a number of applications, most commonly in the optoelectronic and power device industry. Currently a majority of commercialized devices are fabricated on sapphire and SiC substrates but these are expensive and limit the widespread commercialization of the technology. There is substantial ongoing research geared toward the development of GaN on Si substrates because of the significant cost saving that would be realized through the inexpensive, large wafer and maturity of Si fabrication. Significant challenges with the deposition of GaN on Si have, thus far, prevented its wide-spread commercialization specifically the large lattice mismatch and thermal expansion coefficient mismatch. Both of these issues can be overcome by engineering the stress levels in the films. In this thesis work close examination and exploration of the stress formation and evolution in GaN-on-Si is performed. Methods of improving stress levels are developed in addition to providing a deeper understanding of the stress evolution process.;A commonly used methodology of engineering stress levels is to use an AlGaN multi-layer stack. The first layer in the stack is an AlN buffer layer. Typical deposition methods for AlN leaves the surface rough and not ideal for subsequent epitaxy. Here, two specific modifications to the conventional deposition process are made which yield dramatic improvement in material quality and stress levels of an overgrown GaN layer. Full width at half maximum measurements from HRXRD rocking curve of GaN grown on the modified buffers show a 2x reduction and ~0.45 GPa greater built-in compressive stress in the films.;A semi-empirical model to predict stress evolution in III-Nitrides is established using both fundamentals and experimental data. The model will allow researchers determine the desired stress levels in the films in advance of Epitaxy. This will substantially reduce time to discovery and optimization of complex structures. Here an initial data set is developed using this model for a single AlGaN film on various AlN buffer layers. The impact of doping on stress evolution in these films are also investigated for the first time and it is seen that Mg doping plays a significant role in stress development and relaxation, similar to Si.