The successful growth of 2-in. -Ga 2 O 3 crystals by the edge-defined, film fed growth (EFG) method was demonstrated. The optimization of growth conditions for larger single crystalline -Ga 2 O 3 is discussed in detail. The seeding conditions of temperature and neck width were found to be the most important factors to grow single crystals. X-ray rocking curve measurements of -Ga 2 O 3 crystals were conducted to estimate the dislocation densities of the grown crystals. Etch pit densities (EPDs) of the -Ga 2 O 3 crystals were also measured using KOH solution to measure the dislocation densities. The results were discussed combining with crystal growth parameters such as neck width to clarify the mechanisms of propagation and the origin of dislocations in crystals from phenomenological and crystallographic points of view.

The possibility of chemical mechanical polishing (CMP) as an intermediate ex-situ surface planarization process for thin-film epitaxy devices has been investigated. The surface quality of the homoepitaxial GaN after a surface pretreatment of CMP on the GaN-on-sapphire template drastically improved as compared to that with the regular homoepitaxial process. In addition, it was found that CMP contributed to a reduction in the dislocation densities in the subsequent homoepitaxial GaN layers. The CMP-treated GaN surface exhibited pits, with an average diameter of 0.3 μm, due to the chemical etching effect of the CMP slurry. These pits are thought to enhance the epitaxial lateral over growth of the GaN thin films, leading to a reduction in the dislocation densities in the homoepitaxial GaN layers.

Patterned sapphire substrate (PSS) was fabricated by wet etching solutions with different mixture ratios of H2SO4 to H3PO4 and different temperatures to investigate the fabrication mechanisms. It was found that the mixture ratio and temperature of the etching solutions affect the ratio of the pattern diameter to the pattern depth. In addition, the observed pattern shape was strongly affected by the mixture ratio. To discuss the reaction mechanisms of sapphire with H2SO4 and H3PO4 separately, we estimated the activation energies and reaction frequency factors for each reaction. By the estimated results, the behavior of the observed pattern shape for the etching conditions was well explained. To confirm the fabrication mechanism of pattern shape in the microscopic scale, the electron probe microanalysis (EPMA) inspection of the sapphire surface after H2SO4 and H3PO4 etching were carried out. As a result, it was indicated that the pattern shape is controlled by the step flow reaction with and without impurities in the etching solutions. From the observation of the pattern shape, the estimation of the activation energies and reaction frequency factors, and EPMA inspection of the reaction products for each reaction, a schematic model of the fabrication mechanisms for the wet etching of a PSS was established.

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Reduction of the bowing of GaN-on-sapphire and GaN-on-silicon substrates was demonstrated with an internally focused laser processing. Stress implantation was successfully achieved inside the sapphire and silicon substrates by the internally focused laser process to compensate for the strain generated by the GaN/sapphire and GaN/Si systems which resulted in substrate bow reduction. This new approach gives us a larger flexibility in the design engineering of epitaxial and device fabrication processes and thus accelerates the realization of a larger diameter device process with GaN-on-sapphire and GaN-on silicon.

The importance of the atomically well-controlled surface of sapphire substrate with slight misorientation and ideally minimized surface roughness for III–nitride epitaxy is discussed in detail. An atomically controlled surface of sapphire substrate with slight misorientation angle is modeled and an almost ideal level of atomic surface roughness of sapphire substrate is found to be obtained by a chemical mechanical polishing (CMP) with colloidal silica. Cathodoluminescence (CL) imaging indicated the complete absence of subsurface damage induced by mechanical polishing. GaN and AlN thin films are grown on misoriented sapphire substrate with an atomically controlled surface by the CMP to investigate the misorientation angle of both sapphire and grown GaN and AlN thin films. An interface model is proposed to explain the difference in misorientation angle between sapphire and III–nitride thin films, providing strong evidence of the necessity of atomically controlled surface of sapphire substrate for III–nitride epitaxy.

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