Heteroepitaxial growth of AIN buffer layers directly on (10 (1) over bar0) sapphire substrates by metalorganic vapour phase epitaxy has been investigated. A single-step growth procedure without a sapphire nitridation was employed resulting in mirror-like crack free approximate to 1.1-1.6 mu m thick AIN layers of single phase (11 (2) over bar2) orientation. Trimethylaluminum pre-dose time and reactor pressure were optimized for surface roughness and crystal quality. The crystal quality was found to degrade with increasing pre-dose time and also reactor pressure. The smallest full width at half maximum value for on-axis X-ray rocking curve of the (11 (2) over bar2) AlN layers was about 610 arcsec and 1480 arcsec along [(1) over bar(1) over bar 23](AIN) and [1 (1) over bar 00](AIN), respectively. The surface roughness, measured by atomic force microscopy using a 10 x 10 mu m(2) area, was in the range 2.6-3.5 nm. A basal stacking fault density of (7 +/- 1) x 10(5) cm(-1) was estimated by transmission electron microscopy. (C) 2014 Elsevier B.V. All rights reserved