The chemistry and surface microstructure of silicon-based substrates and their effect on the evolution of the microstructures of III-nitride films grown via metalorganic vapor phase epitaxy

The present research was undertaken with the goals of understanding the evolution of defects and strain in heteroepitaxial AlN and GaN films deposited via metalorganic vapor phase epitaxy and minimizing those defects through manipulation of the substrate. As observed with atomic force microscopy (AFM), AlN initially grew in the form of flat-topped islands on as-received SiC substrates. Threading dislocations (TDs) observed in transmission electron microscopy (TEM) images initiated at the AlN/SiC interface as the result of defects at the surface of the mechanically polished substrate and/or condensation of point defects. GaN initially grew in the Stranski-Krastanov mode on AlN/SiC before transitioning to the dislocation-mediated step flow mode. The TDs in GaN resulted from the propagation of the TDs present in the AlN layer. The biaxial strain in the GaN layers varied with buffer layer material and layer thickness yet all samples investigated remained in residual compression due to incomplete relaxation of the coherent strain. The presence of strain during the initial growth of AlxGa1-xN layers directly on as-received SiC also resulted in phase-separated regions of Al-rich and Al-poor film. A high temperature hydrogen etch was then used to remove mechanical polishing scratches from the SiC substrates. Subsequently deposited AlN layers featured reduced pit density and the elimination of scratch-induced undulations. GaN layers deposited with AlN buffer layers on these substrates resulted in slightly reduced TD densities as observed by AFM, TEM, and high resolution X-ray diffraction (HRXRD). Regions of dramatically reduced dislocation densities were observed by HRXRD, TEM, and cathodoluminescence for GaN layers on stripe-patterned Si substrates. However, long growth times resulted in outdiffusion of Si from the substrate and subsequent film roughening. Finally, it was demonstrated that the presence of ammonia during heating of GaN templates to the growth temperature for homoepitaxy resulted in removal of carbon- and oxygen-based contaminants from the template surface.