High‐quality AlGaN with high Al content was grown on high‐quality epitaxial AlN film on sapphire (0001) by low‐pressure metal organic vapour phase epitaxy (LP‐MOVPE). The obtained AlxGa1—xN (0.2 < x < 0.8) thicker than 0.9 μm had atomically smooth surface without any cracks. Full width at half maximum (FWHM) values of X‐ray rocking curve (XRC) from the AlGaN were less than 200 arcsec for (0002) diffraction. It is considered that in‐plane compressive stress in the AlGaN due to larger in‐plane lattice constant of the AlGaN than that of the underlying AlN plays an important role in restraining generation of cracks.

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We demonstrate high-quality epitaxial AlN films on C-plane sapphire as a new buffer layer technique for the growth of high-quality GaN. The obtained GaN films were atomically flat and the full width at half maximum (FWHM) values of the X-ray rocking curve (XRC) were 120arcsec and 510arcsec for (0004) and (10-10), respectively. XRC-FWHM values of the underlying AlN film were 90arcsec and 1800arcsec for (0004) and (10-10), respectively. The XRC results of the GaN indicated that tilted mosaicity of the GaN was much smaller than that of conventional GaN and that small tilted mosaicity coincided with small twisted mosaicity. It was characteristic that the twisted mosaicity drastically reduced from the AlN to the GaN. According to TEM results, it was clarified that the high-quality AlN layer had an effect in reducing the amount of screw-type threading dislocations and that the GaN/AlN interface played an important role in reducing the amount of edge-type dislocations.