Large-diameter aluminium nitride (AlN) single crystals are essential for producing deep ultra violet light-emitting diodes and laser diodes, which are expected to have a major environmental impact by replacing the presently used toxic mercury lamps, both in water and air purifications. Besides that, currently AlN is also being exploited for its efficient use in the development of new power-semiconductor technology for the electric power grid modernisation or “smart grid”. Various growth techniques have been attempted for producing large-diameter, high-quality AlN single crystals namely hydride vapour phase epitaxy, physical vapour transport (PVT) and solution growth methods. Until now, only PVT growth of AlN has been successfully demonstrated for producing bulk crystals with realistic growth rates of hundreds of microns per hour. However, most of the currently available commercial wafers are smaller than 25 mm in size and are in very limited quantities, besides the high cost. We have successfully grown AlN single crystals exceeding 1-inch diameter, on silicon-terminated, both 6H-silicon carbide (SiC) [1], 4H-SiC [2] substrates with very good structural quality by hetero-epitaxial seeding method. Most of the encountered problems in this hetero-epitaxial method namely, cracks (due to thermal mismatch), micro-pipes (propagation from the substrate), 3D-nucleation (preferable spiral growth due to screw dislocation), very high dislocation density (due to lattice mismatch) were effectively mitigated. While the other remaining issues such as presence of low-angle grain boundaries, high incorporation of unintentional impurities, very low electrical conductivity will be addressed in this work. Synchrotron white beam Laue topography was made in the transmission geometry to investigate lattice imperfection. Unintentional silicon and carbon concentration, incorporated from the SiC substrate, was about 1.5 – 2 wt% observed on the top part of the grown crystals. Incorporation of these impurities seemed to vary for different facets (c-, r- and m-facets) as revealed by EDX and EPMA measurements and also by different CL intensities.

In this paper, we also present the growth of AlN on the carbon-terminated SiC substrates for obtaining N-polar AlN substrates and to explore the influence of polarity on the properties. But, the growth on C-face was not straightforward because of obvious reasons. Usually our (0001) AlN single crystals were more than 5 mm in thickness, 28 mm in diameter and flat c-facets were obtained for all substrate off-orientations. Depending on the off-orientation of the substrate and supersaturation in the gas phase, 3D island nucleation as well as 2D step-flow growth modes were observed. AlN growth on carbon-face SiC was dominated by multi nucleation [Fig.1] and further, an abrupt interface was observed between the AlN layer and the substrate, in contrast to the growth on Si-face, where there was a formation of hexagonal hillocks at the interface. Very high supersaturation resulted in a spiral growth mode even on highly off-oriented substrates and the whole crystal grew as a single spiral [Fig.2],  exhibiting a single nucleation centre with well pronounced hexagonal morphology. Broad XRD rocking curves of the sample (FWHM ~ 300 arcsec), indicated a high density of misfit dislocations near the interface and further a 2 cm^-1 shift of E₂(high) phonon mode in the Raman measurements showed a significant misfit stress. By defect-selective etching with molten KOH:NaOH, the dislocation density was found to be in the range of (1 – 4) x 10⁵ cm^-2 in the middle of the grown crystal. We will report the detailed results on the morphology and structural imperfection of the crystals grown on C-face SiC substrates. These results will further be presented in comparison with the growth on Si-face SiC.
Reference

[1] R. R. Sumathi, P. Gille, R. U. Barz and T. Straubinger, Phys. Status Solidi C 8 (2011) 2107-2109.

[2] R. R. Sumathi, R. U. Barz, A. M. Gigler, T. Straubinger and P. Gille, Phys. Status Solidi A 209 (2012) 415-418.

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