Hexagonal GaN films on Si(111) substrates have been grown by metalorganic vapor phase epitaxy (MOVPE). High-temperature A1N buffers provided a reliable diffusion barrier to avoid meltback etching. By introducing a thick, graded A1GaN buffer layer, the critical thickness for cracking has been increased to at least 2 mu m. The films have been characterized by optical microscopy, transmission electron microscopy (TEM) and X-ray diffraction. (c) 2005 Elsevier B.V. All rights reserved.