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Abstract: The influence of AlN buffer growth temperature on the quality and stress of 4.5μm GaN epilayer on 6H–SiC substrate by organometallic vapor phase epitaxy (MOVPE) has been investigated. The crystalline quality and the atomic surface morphology were improved, the density of the pits and the stress of the GaN epilayer were reduced by increasing the growth temperature of the AlN buffer in the range from 950°C to 1100°C. By employing the optimized 1100°C growth temperature of AlN buffer, very high quality of GaN epilayer was achieved. The X-ray full width of half maximums (FWHMs) of (002) and (102) reflection rocking curves of the GaN epilayer have been improved to 159arcsec and 194arcsec, respectively, and the surface RMS to only 0.31nm in the 5μm×5μm atomic force microscopy (AFM) scan. The stress of GaN epilayer was investigated by X-ray diffraction and Raman scattering as well. The degree of the tensile stress in GaN epilayer could be suppressed by increasing the growth temperature of AlN buffer. Finally, a high quality of crack-free 4.5μm thick GaN epilayer was obtained on 6H–SiC substrate using the optimized 1100°C AlN growth temperature. [Copyright &y& Elsevier]