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 &mgr;m/h) were achieved for substrate temperatures ranging between 800 and 850 °C. Surface morphology, structural, and optical properties of thick (2–4 &mgr;m) GaN films were investigatedversusthe growth temperature of the GaN buffer layer. It is shown that this parameter has a drastic influence on the GaN properties. ©1997 American Institute of Physics.