AlN layers were grown by metalorganic vapor-phase epitaxy at high temperatures up to 1500 degrees C. Nucleation layer growth parameters and flow conditions before nucleation were changed and the effect on the AlN layer grown on top was studied. Structural analysis performed by high-resolution X-ray diffractometry and transmission electron microscopy showed that pregrowth conditions affect the material quality drastically. The best structural quality as indicated by a screw (including mixed) dislocation density of 8 x 10(8) cm(-2), together with smooth surface morphology was found to result from simultaneous switching on of ammonia and TMAl at the beginning of nucleation layer growth. (C) 2008 Elsevier B.V. All rights reserved.