A technique for separating a thick AlN layer grown by hydride vapor phase epitaxy (HVPE) on a (0001) sapphire substrate was developed. By heat treatment at 1450 °C in a gas flow containing H2 and NH3, many voids could be formed at the interface between a thin (100 nm) AlN layer grown at 1065 °C and the sapphire substrate due to the preferential decomposition of sapphire. During the cooling process after the subsequent growth of a thick (85 µm) AlN layer, the thick AlN layer separated from the sapphire substrate with the aid of the interfacial voids. The freestanding AlN substrate thus obtained had a smooth (0001) surface, a dislocation density of 1.1×109 cm-2, and an optical transparency for wavelengths above 208.1 nm.

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A thin protective AlN layer was grown on a (0001) sapphire substrate at 1065 °C for main AlN layer growth at high temperature (> 1300 °C) by hydride vapor phase epitaxy (HVPE). The formation of surface pits on the surface of the epitaxial AlN layer, grown directly on the sapphire substrate at 1320 °C, could be prevented by growing the protective layer. The full‐width at half‐maximum (FWHM) of X‐ray diffraction (XRD) rocking curves of asymmetric (10 $\bar 1$ 0) and symmetric (0002) AlN decreased to 19.8 and 9.6 min, respectively. The concentration of oxygen impurities in the layer grown at 1320 °C was also reduced from 3 × 1019 to 4 × 1018 cm−3 by protecting sapphire substrate at 1065 °C. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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