The effects of high-temperature-grown AlN multiple intermediate layers (HT-AlN-MILs) on the crystal quality of Ga-polarity GaN layers grown on (0001) Al 2 O 3 substrates by molecular beam epitaxy using rf-plasma nitrogen source were investigated. The high-temperature-grown AlN intermediate layers (HT-AlN-ILs) with different thicknesses were found to play different roles in the improvement of crystal quality. The 8-nm-thick HT-AlN-ILs brought about improvement of electrical properties. On the other hand, the 2-nm-thick HT-AlN-ILs improved the surface morphology. The combination of these 8-nm-HT-AlN-ILs and 2-nm-HT-AlN-ILs improved both the electrical properties and the surface morphology concurrently.

High-speed GaN growth of 1.0 µm/h with migration enhanced epitaxy (MEE) by molecular beam epitaxy using rf-plasma nitrogen (RF-MBE) was demonstrated. The electron mobility of MEE-GaN was 362 cm 2 /V·s for the electron density of 1.7 × 10 17 cm −3 at room temperature. The threading dislocation density of MEE-GaN was estimated to be 1.0-3.0 × 10 10 cm −2 based on the cross-sectional transmission electron microscope (TEM) image. The remarkable improvement of electrical properties was obtained by the introduction of a high-temperature (750 • C) grown AlN/GaN multiple intermediate layer (AlN-MIL). The cross-sectional TEM image showed that threading dislocations were bent or terminated at the AlN-MIL. The highest room temperature mobility of 668 cm 2 /V·s was obtained at the electron density of 9.5 × 10 16 cm −3 . The low-temperature peak mobility was 2340 cm 2 /V·s at 90 K.

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