Abstract
The main focus of this dissertation is on the growth of GaN on Si(111) through different approaches using plasma-assisted molecular beam epitaxy (PAMBE). Since the nearly strain-free and low dislocation density III-nitride nanorods (NRs) can be grown on Si(111), the use of NRs as the buffer layer becomes first approach in this study. During the growth, the unavoidable surface nitridation of silicon is the first problem to be solved. After that, the AlN is successfully grown in a narrow growth window to obtain smooth and droplet-free layer with desired polarity for the following GaN epilayer. Then, uniform GaN NRs oriented along <111> are grown using the AlN as a buffer layer. To change the growth mode from three-dimension (3-D) to two dimension (2-D), various growth conditions are used to investigate the growth mode of GaN. Next, the coalescence of GaN NRs into a continuous GaN film is demonstrated by growing under a 2-D growth mode. However, Raman scattering spectroscopy analysis results reveals that the strain-free condition exists only in GaN NRs. Whenever the NRs begin to coalesce, the tensile stress is detected at the same time. Therefore, a strain-relaxing layer is still necessary for the growth of GaN on Si. Therefore, the techniques of superlattices (SLs) is applied to solve residual strain problem. By optimizing the growth conditions of AlGaN/GaN SLs, a flat GaN surface can be obtained with an average roughness of less than 0.5 nm. By using high resolution X-ray diffraction measurements, the mole fraction of Al in AlGaN is determined to be 12%. The thickness of each layer in SLs is within the critical thickness determined by the improved Matthews and Blakeslee model. The results of Raman spectroscopy of a 3 μm crack-free top GaN epilayer show reduced tensile stress to nearly strain-free. To further improve the crystalline quality, a nano-holes pattern is fabricated into voids in the epilayer to block the propagation of dislocations. The patterned nano-holes are fabrication in a 2 inch wafer through soft nano-imprint lithography. Finally, SLs and nano-holes are combined together to improve the crystalline quality of GaN. These two methods are optimized separately first before used together by using various structures through analysis of scanning electron microscopy (SEM), Raman scattering spectroscopy, photoluminescence spectroscopy (PL), high resolution X-ray diffraction (HRXRD) and transmission electron microscopy (TEM).