The kinetics of surface processes during the growth of GaN by molecular-beam epitaxy (MBE) with ammonia as the source of reactive nitrogen is studied theoretically and experimentally. A model of surface processes is developed taking into account specific effects of the blocking of NH3 adsorption sites by Group III and Group V surface species. Parameters of the model (respective kinetic rate constants) are determined from comparison with experimental data. It is shown that the evaporation rate of GaN in ammonia atmosphere is much lower than that in vacuum. Kinetics of GaN growth under gallium-rich and nitrogen-rich conditions are compared. Under nitrogen-rich conditions the GaN surface is predicted to be enriched by NH, surface radicals, in contrast to the case of growth under gallium-rich conditions or of free evaporation in vacuum. It is shown that use of the nitrogen-rich conditions allows one to increase the growth temperature by similar to 80-90 degrees C compared with the case of gallium-rich conditions or plasma-activated MBE. The increased growth temperature is favorable in improving the optical and electrical properties of the material grown. (C) 2000 Elsevier Science B.V. All rights reserved.