Growth of AlN bulk single crystals on 4H-SiC substrates and analyses of their structural quality and growth mode evolution

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Abstract

Aluminium nitride (AlN) bulk single crystals were grown on off-oriented 4H-SiC substrates by the sublimation method. High-quality crystals with about 25 mm in diameter and up to 5 mm in thickness were obtained with an optimized growth process. The crystals show hexagonal symmetry with well developed {10equation image1} side facets. Confocal-Raman spectroscopy and double-crystal X-ray diffraction rocking curve (DCXRD-RC) measurements confirm the high structural quality of the grown crystals. In Raman spectroscopy, the full-width at half maximum (FWHM) of E2(high) phonon mode decreases from 40 cm−1 for thin layers (800 µm) to 22 cm−1 for a 5 mm thick crystal, showing the improvement in quality with thickness. The DCXRD rocking curve FWHM obtained for a 5 mm thick crystal is about 120 arcsec. A basal plane bending of 100 arcsec is observed by rocking curve measurements while scanning 17 mm of the sample's diameter. Two kinds of growth modes namely, step flow growth and spiral growth modes were observed under similar growth conditions. This may be attributed to a slight variation of super-saturation in the gas phase. In both cases, a single active hexagonal growth centre is formed. As-grown surfaces have been found to be always Al-polar. Wet chemical etching revealed both, threading type and basal-plane dislocations (BPD) on the surfaces of the samples with total etch pit densities (EPDs) in the range of 8 × 105–1 × 106 cm−2.

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