Takehiro Yoshida

Vertical GaN p-n Junction Diodes With High Breakdown Voltages Over 4 kV

Vertical structured GaN power devices have recently been attracting a great interest because of their potential on extremely high-power conversion efficiency. This letter describes increased breakdown voltages in the vertical GaN p-n diodes fabricated on the free-standing GaN substrates. By applying multiple lightly Si doped n-GaN drift layers to the p-n diode, the record breakdown voltages (VB) of 4.7 kV combined with low specific differential ON-resistance (RON) of 1.7 mΩcm2 were achieved. With reducing the Si-doping concentration of the top n-GaN drift layer adjacent to the p-n junction using well-controlled metal-organic vapor phase epitaxy systems, the peak electric field at the p-n junction could be suppressed under high negatively biased conditions. The second drift layer with a moderate doping concentration contributed to the low RON. A Baligas figure of merit (VB2/RON) was 13 GW/cm2. These are the best values ever reported among those achieved by GaN p-n junction diodes on the free-standing GaN substrates.

Thermal and Electrical Properties of High-Quality Freestanding GaN Wafers with High Carrier Concentration

The thermal and electrical properties of high-quality bulk GaN crystals fabricated by hydride vapor phase epitaxy were investigated in the high-carrier-concentration region between 1.0×1018 and 1.24×1019 cm-3. The carrier concentration was almost identical to the Si doping concentration. An electrical resistivity as low as 2.5 mΩ cm was obtained at the carrier concentration n=1.24×1019 cm-3. The electron mobilities from Hall measurements were 441 and 200 cm2 V-1 s-1 for n=1.0×1018 and 1.24×1019 cm-3, respectively, which are significantly higher than those reported in the literature. The thermal conductivity measured by the laser flash method was consistently high in the measured carrier concentration range, i.e., about 2.0 W cm-1 K-1 for n=1.0×1018 cm-3 and 1.87 W cm-1 K-1 for n=1.24×1019 cm-3.

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.

Freestanding GaN Wafers by Hydride Vapor Phase Epitaxy Using Void-Assisted Separation Technology

An outline is presented of the fabrication technique of freestanding GaN wafers by hydride vapor phase epitaxy using the void-assisted separation method and the properties of resulting crystals. A thick GaN layer of large area can be separated with excellent reproducibility from a base substrate by the application of thermal stress. This process is assisted by numerous voids formed near the interface between the thick GaN layer and the base substrate. By using this method, high-quality GaN wafers of large area with diameters of over 3 in. have been prepared.

Wafer-level nondestructive inspection of substrate off-angle and net donor concentration of the n−-drift layer in vertical GaN-on-GaN Schottky diodes

In the mass production of GaN-on-GaN vertical power devices, a nondestructive simple inspection of the net donor concentration (N D − N A) of the n−-drift layer in the range of 1015 cm−3 is required. In this study, we demonstrate the wafer-level nondestructive inspection of GaN Schottky barrier diode epi-structures grown by metal organic vapor phase epitaxy (MOVPE) on free-standing GaN substrates. We found that the normalized yellow (YL) photoluminescence peak intensity of the near band edge (NBE), I YL/I NBE, is strongly related to the acceptor concentration N A of the n−-drift layer. This means that the N D − N A of the n−-drift layer can be inspected by photoluminescence measurement at a high speed, when Si concentration is not varying across the GaN wafers. Noncontact capacitance–voltage and secondary ion mass spectrometry measurements were used to investigate the cause of N D − N A variation across the GaN wafers. The discrepancy between C and N A indicates that compensation could be due to another electron trap.

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.

Nondestructive measurement of homoepitaxially grown GaN film thickness with Fourier transform infrared spectroscopy

In vertical devices containing GaN homoepitaxial layers on GaN substrates, the layer thickness is a key parameter that needs to be clarified before starting the device process. We applied Fourier transform infrared spectroscopy (FT-IR) to a homoepitaxially grown GaN film that consisted of an n−-GaN layer. The estimated film thickness from the FT-IR spectrum agreed well with the results of cross-sectional scanning electron microscope cathodoluminescence images. This is the first report of nondestructive film thickness measurements for homoepitaxially grown GaN and indicates the applicability of FT-IR to the nondestructive inspection of vertical GaN power devices.