Thick (20–30 µm) layers of highly pure GaN with device-quality smooth as-grown surfaces were prepared on freestanding GaN substrates by using our advanced hydride-vapor-phase epitaxy (HVPE) system. Removal of quartz parts from the HVPE system markedly reduced concentrations of residual impurities to below the limits of detection by secondary-ion mass spectrometry. Appropriate gas-flow management in the HVPE system realized device-quality, smooth, as-grown surfaces with an excellent uniformity of thickness. The undoped GaN layer showed insulating properties. By Si doping, the electron concentration could be controlled over a wide range, down to 2 × 1014 cm−3, with a maximum mobility of 1150 cm2·V−1·s−1. The concentration of residual deep levels in lightly Si-doped layers was in the 1014 cm−3 range or less throughout the entire 2-in. wafer surface. These achievements clearly demonstrate the potential of HVPE as a tool for epitaxial growth of power-device structures.

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Recently, <001>-oriented nanometer-sized pores (nano-pores) without side branches, unlike porous Si, have been realized by our group on (001) n-InP surfaces by means of electrochemical anodization in 1M HCl solution. However, they exhibit large structural nonuniformity including the presence of an irregular top layer, random pore positioning and wavy pore walls. In this study, attempts have been made to improve pore uniformity and to clarify their optical properties. Anodization in 1 M HCl+HNO3 solution realized highly uniform nano-pore arrays consisting of square-shaped straight pores defined by four crystalline (011) facets. This improvement is explained in terms of preferential etching along the vertical <001>-direction at the pore tip due to the slow etching rate along the lateral <011>-direction as well as the uniform supply of reactant species to pore tips realized by the removal of the irregular top layer during the anodization process. The nano-pore arrays show strong blue- and red-shifted photoluminescence emissions at high and low temperatures, respectively. These are assigned as emissions due to the transition between electron and hole quantum states inside the pore walls and that involving a broad surface state continuum at pore wall surfaces, respectively.

On the basis of a novel crystal hardness control, we successfully realized macrodefect-free, large (2-6 in.) and thick +c-oriented GaN bulk crystals by hydride vapor phase epitaxy. Without the hardness control, the introduction of macrodefects including inversion domains and/or basal-plane dislocations seemed to be indispensable to avoid crystal fracture in GaN growth with millimeter thickness. However, the presence of these macrodefects tended to limit the applicability of the GaN substrate to practical devices. The present technology markedly increased the GaN crystal hardness from below 20 to 22 GPa, thus increasing the available growth thickness from below 1 mm to over 6 mm even without macrodefect introduction. The 2 and 4 in. GaN wafers fabricated from these crystals had extremely low dislocation densities in the low-to mid-10 5 cm %2 range and low off-angle variations (2 in.: <0.1°; 4 in.: >0.2°). The realization of such high-quality 6 in. wafers is also expected.