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Strain-free ultrathin AlN epilayers grown directly on sapphire by high-temperature molecular beam epitaxy

High-quality AlN ultrathin films on sapphire substrate were grown by molecular beam epitaxy using an in situ high-temperature annealing approach. From transmission electron microscopy studies, it was found that the AlN epilayers are strain relaxed within the first nm, thus growing nearly strain free. Many of the dislocations generated at the AlN/sapphire interface are reduced within the first 50 nm of growth. Epitaxial films grown directly on sapphire, which are ∼100 nm thick, show X-ray diffraction (002) and (102) rocking curve peaks with full widths at half maximum of less than 150 and 1400 arc sec, respectively, which are the narrowest linewidths reported for AlN of this thickness. Detailed photoluminescence studies further showed that such AlN epilayers exhibit relatively high luminescence efficiency and strong near-band edge emission without defect-related transitions.