Wurtzite GaN was grown by gas source molecular beam epitaxy on (0001) sapphire substrates. Taking advantage of the catalytic decomposition of ammonia on the growing surface, high growth rates (>1 mu m/h) were achieved for substrate temperatures ranging between 800 and 850 degrees C. Surface morphology, structural, and optical properties of thick (2-4 mu m) GaN films were investigated versus the growth temperature of the GaN buffer layer. It is shown that this parameter has a drastic influence on the GaN properties. (C) 1997 American Institute of Physics.