Direct heteroepitaxial growth of thick AlN layers on sapphire substrates by hydride vapor phase epitaxy

Thick AlN layers were grown directly on sapphire substrates by hydride vapor phase epitaxy at growth rates of 40–60 μm/h. The resulting films were colorless, smooth and specular. Subsurface cracking, attributed to the plastic relief of tensile strain from island coalescence, was observed by Nomarski optical contrast microscopy and cross-sectional scanning electron microscopy. Typical full-widths at half-maximum of X-ray rocking curves for the on-axis (0 0 0 2) and off-axis (2 0 2¯ 1) reflections of the AlN films were 310–640′′ and 630–800′′, respectively. The threading dislocation density was 2×109 cm−2 as determined by plan-view transmission electron microcopy. Convergent beam electron diffraction was used to determine that the surfaces of our AlN films are Al-face. These results represent substantial progress towards rapid production of AlN templates and free-standing AlN substrates of high optical quality.