Using low-pressure MOVPE, with a close-coupled showerhead reactor, Si-doped GaN layers were grown on 4H-SiC substrates. On substrates cut 8degrees off the (0 0 0 1) plane, Al0.6Ga0.4N nucleation layers were grown at 1020 degreesC with thicknesses ranging from 8 to 250 nm. A thickness of at least 250 nm was required to obtain specular, crack-free GaN layers 3 mum thick. At this thickness the FWHM of the GaN (0 0 0 2) rocking curve was 286 arcsec. A clear relation between AlGaN layer thickness and GaN FWHM exists. However, the GaN surfaces showed undulations with an rms roughness of 8 nm and a peak to valley distance of 40-90 nm. We have investigated lower Al-content layers on exactly oriented 4H-SiC in order to prepare devices with vertical conduction. Si-doped GaN layers were grown on Al0.3Ga0.7N and Al0.08Ga0.92N nucleation layers deposited at temperatures varying from 800 to 1000 degreesC. Atomically smooth GaN surfaces were obtained. The lowest FWHM of the GaN (0 0 0 2) rocking curve we achieved using Al0.08Ga0.92N was 189 arcsec. The XRD peaks of all layers grown on Al0.3Ga0.7N layers were significantly broader, indicating a more defective crystal structure. Remarkably, the measured electron mobilities in these samples were systematically higher than in the GaN layers grown on Al0.08Ga0.92N. It appears that the higher mobilities are obtained with higher compressive stress in the GaN layer. (C) 2004 Elsevier B.V. All rights reserved.