| Group III-nitrides are the key materials for the fabrication of high-efficiency, high-brightness optoelectronic semiconductor devices operating in the visible and ultra-violet regions of the electromagnetic spectrum. In the last decade, intensive research and development efforts around the globe have been made to successfully improve the luminescence efficiency of nitride-based light-emitting devices, which have found varieties of applications in everyday life. In the near future, nitride-based solid-state-lighting technology is expected to bring a revolution in general lighting application. However, there are still many issues with these materials that remain to be solved. This dissertation addresses some important challenges facing the development of high-quality III-nitride semiconductor materials. At first, plastic strain relaxation of heteroepitaxial indium gallium nitride (InGaN) films has been extensively explored by means of transmission electron microscopy. Generation of misfit dislocation arrays via slip on the inclined secondary system occurs when low-defect-density bulk gallium nitride (GaN) substrate is used. Basal-plane slip has been found to be active in the presence of surface indentations on the growth surface irrespective of the threading dislocation density in the substrates. The important strain relaxation mechanisms of nitrides heterostructures provide useful insights to improve the crystal quality of InGaN alloys especially with high indium composition used in the longer wavelength regime. Secondly, the influence of growth conditions on the crystallinity of InGaN quantum wells has been studied by microstructural characterization. The quality of InGaN quantum wells and top GaN capping layers are found to be strongly affected by the growth temperature, i.e., the indium composition. Thirdly, unique structural and optical properties of stacking faults in GaN have been examined using combination of transmission electron microscopy and cathodoluminescence. A direct correlation has been established between basal and prismatic stacking faults and their respective luminescence characteristics. Additionally, dislocation generation in lateral epitaxy of aluminum nitride films has been probed by transmission electron microscopy. Thermal gradient present in the film during growth has been found to result in the generation of high-density dislocations in the relatively defect-free lateral growth area when two crystallites coalesce. These findings bring understanding to the nature of defects in nitride semiconductors, particularly lattice mismatch relaxation, and to their influence on the electrical and optical properties. |
