Taichiro Konno

Hydride-vapor-phase epitaxial growth of highly pure GaN layers with smooth as-grown surfaces on freestanding GaN substrates

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 cm2V−1s−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.

Macrodefect-free, large, and thick GaN bulk crystals for high-quality 2–6 in. GaN substrates by hydride vapor phase epitaxy with hardness control

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-105 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.

Roughening of GaN homoepitaxial surfaces due to step meandering and bunching instabilities and their suppression in hydride vapor phase epitaxy

In this letter, we have shown that well-known surface instabilities in crystal growth, i.e., in-phase step meandering and step bunching, are the main causes of surface roughening of GaN homoepitaxial layers on vicinal +c-oriented GaN substrates. Both instabilities were effectively suppressed in hydride vapor phase epitaxy (HVPE) under appropriate conditions, which produced highly smooth as-grown surfaces suitable for reproducible device fabrication. Wavy surface morphologies, often observed in homoepitaxial GaN layers grown not only by HVPE but also by metal-organic chemical vapor deposition (MOCVD), were found to be covered by an array of meandered bunched steps consisting of m- and a-oriented sections. Because the bunched steps meandered in an in-phase fashion, facets consisting of m- and a-oriented bunched steps formed narrow macro-steps and wide macro-terraces, respectively, leading to the formation of ridges and grooves in-between the facets. Although the use of a substrate with large off-angles (≥0.4°) effectively suppressed the step meandering, i.e., wavy surface morphology, it induced a strong tendency for step bunching. Only growth on surfaces having relatively small off-angles (0.25°), under conditions providing large degrees of adatom desorption, i.e., at high temperatures or low V/III-ratios, suppressed both the meandering and bunching instabilities simultaneously and produced highly smooth as-grown surfaces. The use of the HVPE method seems better than the use of the MOCVD method in this strategy to increase the surface flatness of GaN homoepitaxial layers because it can maintain a sufficiently high growth rate even with a large degree of adatom desorption.In this letter, we have shown that well-known surface instabilities in crystal growth, i.e., in-phase step meandering and step bunching, are the main causes of surface roughening of GaN homoepitaxial layers on vicinal +c-oriented GaN substrates. Both instabilities were effectively suppressed in hydride vapor phase epitaxy (HVPE) under appropriate conditions, which produced highly smooth as-grown surfaces suitable for reproducible device fabrication. Wavy surface morphologies, often observed in homoepitaxial GaN layers grown not only by HVPE but also by metal-organic chemical vapor deposition (MOCVD), were found to be covered by an array of meandered bunched steps consisting of m- and a-oriented sections. Because the bunched steps meandered in an in-phase fashion, facets consisting of m- and a-oriented bunched steps formed narrow macro-steps and wide macro-terraces, respectively, leading to the for...