Native cation vacancies in Si-doped AlGaN studied by monoenergetic positron beamsIdentification of vacancy-oxygen complexes in oxygen-implanted silicon probed with slow positrons Vacancy-type defects in AlN grown by metal-organic vapor phase epitaxy ͑MOVPE͒ and lateral epitaxial overgrowth ͑LEO͒ using halide vapor phase epitaxy were probed by a monoenergetic positron beam. Doppler broadening spectra of the annihilation radiation were measured and compared to the spectra calculated using the projector augmented-wave method. For MOVPE-AlN, the concentration of vacancy-type defects was high near the interface between AlN and the GaN buffer layer, and the defect-rich region expanded from the interface toward the surface when the NH 3 flow rate increased. For the sample grown on the AlN buffer layer, however, the introduction of such defects was suppressed. For LEO-AlN, distinct deep emission peaks at 3-6 eV were observed in cathodoluminescence spectra. From a comparison between Doppler broadening spectra measured for LEO-AlN and computer simulated ones, an origin of the peaks was identified as complexes of Al vacancy ͑V Al ͒ and oxygen atoms substituting nitrogen sites such as V Al ͑O N ͒ n ͑n = 3 and 4͒.

Ammonothermal GaN growth using a novel apparatus has been performed on c-plane, m-plane, and semipolar seed crystals with diameters between 5 mm and 2 in. to thicknesses of 0.5-3 mm. The highest growth rates are greater than 40 m/h and rates in the 10-30 m/h range are routinely observed for all orientations. These values are 5{100Â larger than those achieved by conventional ammonothermal GaN growth. The crystals have been characterized by X-ray diffraction rocking-curve (XRC) analysis, optical and scanning electron microscopy (SEM), cathodoluminescence (CL), optical spectroscopy, and capacitance-voltage measurements. The crystallinity of the grown crystals is similar to or better than that of the seed crystals, with FWHM values of about 20-100 arcsec and dislocation densities of 1 Â 10 5-5 Â 10 6 cm À2. Dislocation densities below 10 4 cm À2 are observed in laterally-grown crystals. Epitaxial InGaN quantum well structures have been successfully grown on ammonothermal wafers.

A novel, highly scalable apparatus has been employed to perform high temperature ammonothermal growth of (0001) GaN bulk crystals up to 52 mm in diameter and to a thickness of greater than 2 mm. X‐ray characterization of the crystals shows excellent crystallinity with radii of curvature > 20 m and a 201 rocking curve FWHM of <30 arcsec. Hall effect and optical absorption measurements performed on the crystal show a free carrier concentration of 1.1 × 1018 cm−3 and an absorption coefficient of 1 cm−1 at 450 nm and 2 cm−1 at 410 nm, meeting the substrate requirements for use as a native substrate in blue or violet LED devices. To the best of our knowledge, this is the lowest optical absorption coefficient reported for ammonothermally‐grown GaN having a carrier concentration above 1018 cm−3.

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The plane dependence of GaN grown in supercritical basic ammonia was investigated. Seed crystals with various surface crystallographic orientations were prepared and loaded for four separate growth runs. The growth thickness and crystal quality of GaN grown on each seed was evaluated by caliper and X-ray diffraction (XRD), respectively. These parameters were highly dependent on the orientation of the seed crystal. We achieved high crystalline quality with high growth rates utilizing semi-polar seed crystals. Also, the non-polar m-plane showed the narrowest XRD full width at half maximum (FWHM).