High-resolution X-ray diffraction measurements of GaN and AlGaN grown on 4H-and 6H-SiC(0001) vicinal substrates with misorientation angles of up to 9 are presented. Growth of (Al)GaN was carried out by plasma-assisted molecular beam epitaxy. The c-axis tilt, i.e., inclination of the (Al)GaN c-axis relative to that of SiC, was systematically investigated. The inclination angle clearly depended on the SiC substrate misorientation angle, while it was independent of the (Al)GaN growth temperature, SiC polytype, and substrate misorientation direction. The behavior observed for both GaN and AlGaN is in excellent agreement with the model proposed previously by Nagai for the InGaAs/GaAs system.

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The growth, electrical characterization, and electroluminescence (EL) of GaN/SiC heterojunction bipolar transistors (HBTs) are presented. GaN grown on off-axis SiC by molecular beam epitaxy showed step bunching owing to the large off-angle of SiC substrates, which contributed to the annihilation of edge dislocations. We investigated the impact of base doping concentration and SiC polytype (4H and 6H) on the characteristics of GaN/SiC heterojunction diodes. By utilizing a reduced doping concentration of 1×1018 cm-3 instead of 1×1019 cm-3, we suppressed the tunneling current via interface traps, resulting in an improved rectifying behavior in the diodes. Capacitance–voltage (C–V) and EL characteristics revealed that the band lineup of GaN/SiC is of type II, and 6H-SiC is better for electron injection. In accordance with diode characteristics, the fabricated GaN/SiC HBTs showed an improved common-base current gain of 0.03 by employing a reduced base doping concentration of 1×1018 cm-3 and 6H-SiC, whereas a current gain below 1×10-4 was obtained in the HBTs with a base doping concentration of 1×1019 cm-3.

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