Sumathi, R. Radhakrishnan; Gille, Peter (2012): Sublimation growth of c -plane AlN single crystals on SiC substrates. In: Crystal Research and Technology, Vol. 47, Nr. 3: S. 237-246
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1-inch diameter, (0001)-plane AlN single crystals were grown on 6H-SiC substrates by the sublimation method. With the optimised growth conditions, 1-3 mm thick, yellow-coloured crystals with smooth c -facet and good structural properties were obtained. X-ray diffraction rocking curves show a full width at half maximum (FWHM) value of about 100 arcsec. The crystal surface exhibits 3D-microstructures with different morphologies as observed by laser scanning microscopy. All these structures reflect the hexagonal symmetry. The origin of these structures was further investigated by atomic force microscopy and an overlapping double growth spiral was observed in the middle of these structures that is attributed to a dislocation core. Green colouration observed in some of the grown crystals was analysed with the electron probe microanalysis and the results infer that more incorporation of carbon may be the cause. Further a high silicon content of up to 5.5 wt\% was observed at the top layer of 3 mm thick green-coloured crystals whereas 3 wt\% were measured for the yellow-coloured ones. The distribution of Si is uniform over the entire diameter of the crystals. Yellowish crystals show improved structural properties compared to the greenish ones. The FWHM value and the peak positions of Raman phonon modes represent a high crystalline quality and also confirm that the crystals are relaxed from stress. Raman micro-mapping analysis indicates the presence of small, localized tensile stress on the growth spiral region, whereas this is not observed at smooth areas of the crystal surface without spirals. Uniform E 2(high) FWHM and phonon intensity over the entire mapped area further prove that the grown crystals are structurally very homogeneous.