Highly oriented wurtzite-type GaN layers were grown on Si(001) substrates by metalorganic vapor-phase epitaxy. The use of a high-temperature AlN seed layer at more than 1100 degrees C allows growing solely c-axis-oriented crystallites. However, due to the symmetry of the Si(001) surface two in-plane alignments of the GaN occur, twisted by 30 degrees. Therefore, to obtain a coalesced GaN surface, the selection of only one certain orientation is necessary. The approach of 4 degrees off-oriented substrates enabled to grow a flat, highly mono-oriented GaN layer on Si(001), with both kinds of in-plane alignments realizable. The crystalline quality of the GaN was investigated by X-ray analysis and scanning electron microscopy. (c) 2006 Elsevier B.V. All rights reserved.