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Preparation of A Crack-Free AlN Template Layer on Sapphire Substrate by Hydride Vapor-Phase Epitaxy at 1450 °C

A crack-free aluminum nitride (AlN) template layer was grown on a sapphire substrate at 1450 °C using a thin (100 nm) protective AlN layer grown at 1065 °C by hydride vapor-phase epitaxy (HVPE). Full-width at half-maximum (FWHM) values of X-ray rocking curves (XRCs) for planes of the AlN layer were 378 and 580 arcsec, respectively. The formation of voids was observed at the interface between the thin protective AlN layer and the sapphire substrate due to decomposition reaction of sapphire during heating up to 1450 °C. The voids relaxed the tensile stress in the AlN layer, which resulted in the suppression of cracks.

Using a novel ultrahigh vacuum transmission electron microscope (TEM) with in situ reactive molecular beam epitaxy system, we report the successful synthesis of epitaxial AlN on the sapphire surface upon exposure to ammonia. The substrate, an electron transparent sapphire foil, was annealed in oxygen at high temperature to eliminate damage induced during preparation and to create large atomically flat regions. The sample was then held at 950 °C in flowing ammonia inside the microscope for 2 h during which periodic observations of microstructural development were made. We report direct observation of the formation of epitaxial AlN with the orientation relationship AlN//sub,AlN//sub and present both TEM and atomic force microscope images of the sample before and after nitridation. The results are consistent with a diffusion-limited reaction model involving transport of oxygen and nitrogen ions through the growing AlN epilayer between the free surface and the unreacted α-Al2O3.


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