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.
A high breakdown voltage of 5.0 kV has been achieved for the first time in vertical GaN p-n junction diodes by using our newly developed guardring structures. A resistance device was inserted between the main diode portion and the guard-ring portion in a ring-shaped p-n diode to generate a voltage drop over the resistance device by leakage current flowing through the guard-ring portion under negatively biased conditions before breakdown. The voltage at the outer mesa edge of the guard-ring portion, where the electric field intensity is highest and the destructive breakdown usually occurs, is decreased by the voltage drop, so the electric field concentration in the portion is reduced. By adopting this structure, the breakdown voltage (V B ) is raised by about 200 V. Combined with a low measured on-resistance (R on ) of 1.25 mΩ cm 2 , Baliga's figure of merit ðV 2 B =R on Þ was as high as 20 GW/cm 2 .
K,Na)NbO 3 (KNN) films with high transverse piezoelectric coefficients were successfully deposited onto Pt/Ti/SiO 2 /Si substrates by RF magnetron sputtering. These films were polycrystalline and had pseudo-cubic perovskite structures with preferential h001i orientation. To improve their piezoelectric properties, we investigated the effects of annealing after the deposition and the Na=ðK þ NaÞ ratio of the films. Annealing in air at 750 C led to a decrease in the residual strain in the KNN crystal and the disappearance of openings at the grain boundary, thereby improving the transverse piezoelectric coefficient and leakage current properties. We also investigated the transverse piezoelectric coefficient and dielectric constant as a function of the Na=ðK þ NaÞ ratio; both had maximum values at a ratio of approximately 0.55. For the KNN films, e à 31 ranged between À10:0 and À14:4 C/m 2 ; thus, it was superior to previously reported values for lead-free piezoelectric films and was comparable to the best commercially available Pb(Zr,Ti)O 3 films. #
In the mass production of GaN-on-GaN vertical power devices, a nondestructive simple inspection of the net donor concentration (ND − NA) 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), IYL/INBE, is strongly related to the acceptor concentration NA of the n−-drift layer. This means that the ND − NA 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 ND − NA variation across the GaN wafers. The discrepancy between C and NA indicates that compensation could be due to another electron trap.
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