A thin epitaxial GaN layer was deposited on a TiN/c-plane Al2O3 substrate by low-energy ion beam assisted molecular beam epitaxy. The TiN single layer on Al2O3, as well as the GaN/TiN-bilayer was investigated with respect to their orientation relationships and their crystalline quality by X-ray diffraction (XRD) and by high resolution transmission electron microscopy (TEM). The results show that the cubic TiN film is epitaxial to the c-plane Al2O3 substrate and that it consists of rotation-twinned cubic crystallites. The GaN layer deposited on the TiN film is epitaxial, too, and it solely consists of the wurtzitic GaN polytype. The mosaicity and the surface roughness, derived from atomic force microscopy (AFM), of the GaN film is higher than that of the TiN film. (C) 2003 Elsevier B.V. All rights reserved.