GaN has been grown by MBE using NH3 and elemental Ga on c-plane sapphire. Based on RHEED measurements during growth and film thickness measurements we have constructed a kinetic growth model in which the key feature is the relation between surface Ga coverage and the dissociation of NH3. The model is used during growth on smooth buffers; the initial growth on rougher surfaces is somewhat different. For the initial growth on an AlN buffer we have observed a critical Ga flux, that depends on substrate temperature and NH3 flux, below which the growth mode is 3D and the surface remains rough. For the films grown at too low a Ga flux, AFM images show deep columnar voids. Above this critical flux, the surface becomes smooth after 20 nm of growth and AFM shows step trains of single atomic layer high steps separated by terraces that have average lengths of about 70 nm. On these smooth surfaces, RHEED is used to determine the condition where unincorporated Ga is accumulated on the surface. The substrate temperatures and fluxes are consistent with the kinetic framework developed. The results indicate that excess Ga inhibits NH3 dissociation.

*Partially supported by the ONR