Bulk AlN single crystal growth on foreign substrate and preparation of free-standing native seeds

Abstract

Physical vapour transport growth of aluminium nitride (AlN) single crystals on silicon carbide (SiC) substrates has been optimised and crack-free, large-area, free-standing (0001) AlN wafers were prepared from the grown template crystals. 28 mm diameter single crystals without any polycrystalline surroundings were obtained. Different off-oriented substrates give rise to different growth modes. Sharp and symmetric line shapes of X-ray diffraction (XRD) rocking curves and high intense Raman phonon mode peaks prove high structural quality and homogeneity of the grown crystals. Full width at half maximum of the rocking curves is 72 arcsec for symmetric 00.2 reflection and 200 arcsec for asymmetric 10.3 reflection, representing a low screw as well as edge type threading dislocations. Wet chemical etching results also confirm the above XRD results and the estimated etch pit density is as low as 2–5 × 10⁵ cm⁻². The growth surfaces of all the crystals show only Al-polarity as inferred by the etching analysis. The concentration of silicon and carbon impurities incorporated from the SiC substrate decreases with the growth length of the AlN crystals. These impurities might play a decisive role in determining the optical properties of the crystal and be responsible for the absence of near-bandgap excitonic luminescence. Confocal Raman spectra show only the phonon modes allowed by the selection rules for the measured symmetry. The observed E₂(high) phonon mode frequency very closely matches the reported stress-free phonon frequency of AlN. This work demonstrates that AlN templates prepared on SiC as a foreign substrate can be used as native seeds for the growth of further homo-epitaxial layers and crystalline boules.