In this article, we present our results on gallium nitride (GaN) films grown by chemical beam epitaxy (CBE) using triethyl gallium (TEG) and ammonia (NH3), on Al2O3(0001) substrates. In situ characterization of the GaN surface was performed by reflection high energy electron diffraction and time-of-flight mass spectroscopy of recoil ions. In order to initiate the growth, a 200 Angstrom thick buffer layer was, grown using electron cyclotron resonance plasma activated nitrogen and TEG. During CBE, growth of GaN, growth rates were in the range of 1000-4000 Angstrom/h limited only by the pumping capacity of the growth reactor. It is only in a narrow temperature window of 800-825 degrees C that two-dimensional smooth single crystal layers are obtained. A clear correlation between crystal quality and surface carbon was observed. GaN films grown at 800 degrees C consist of hexagonal hillocks less than 1 mu m in size with surface root mean square roughness of 40 Angstrom/l mu m.