We report on the epitaxial growth of hexagonal c-axis GaN on Si(001) substrates by metalorganic vapor-phase epitaxy (MOVPE). High-temperature (HT) AlN buffers were used. The use of 4 degrees misoriented Si(001) substrates allows the growth of GaN layers with a single crystal orientation and low roughness and mosaicity. Coalescence of the GaN films is obtained for thicknesses of about 1 mu m. Crystal quality, strain state, polarity, and optical properties, assessed by transmission electron microscopy, X-ray diffraction and photoluminescence, are discussed and compared with those of GaN layers grown on Si(111) substrates. (c) 2005 Elsevier B.V. All rights reserved.