The crystalline quality of AlGaN/GaN heterostructures was improved by optimization of surface pretreatment of the SIC substrate in a hot wall metal organic chemical vapor deposition reactor. X-ray photoelectron spectroscopy measurements revealed that oxygen- and carbon related contaminants were still present on the SIC surface treated at 1200 degrees C in H-2 ambience, which hinders growth of thin AIN nucleation layers with high crystalline quality. As the H2 pretreatment temperature increased to 1240 degrees C, the crystalline quality of the 105 nm thick AIN nucleation layers in the studied series reached an optimal value in terms of full width at half maximum of the rocking curves of the (002) and (105) peaks of 64 and 447 arcsec, respectively. The improvement of the AIN growth also consequently facilitated a growth of the GaN buffer layers with high crystalline quality. The rocking curves of the GaN (002) and (102) peaks were thus improved from 209 and 276 arcsec to 149 and 194 arcsec, respectively. In addition to a correlation between the thermal resistance and the structural quality of an AIN nucleation layer, we found that the microstructural disorder of the SiC surface and the morphological defects of the AIN nucleation layers to be responsible for a substantial thermal resistance. Moreover, in order to decrease the thermal resistance in the GaN/SiC interfacial region, the thickness of the AIN nucleation layer was then reduced to 35 nm, which was shown sufficient to grow AlGaN/GaN heterostructures with high crystalline quality. Finally, with the 35 nm thick high-quality AIN nucleation layer a record low thermal boundary resistance of 1.3 x 10(-8) m(2) K/W, measured at an elevated temperature of 160 degrees C, in a GaN-on-SiC transistor structure was achieved. (C) 2015 Elsevier B.V. All rights reserved.