Ga adsorption and desorption kinetics onM-plane GaN

We study the adsorption and desorption kinetics of Ga on GaN(1100) in molecular-beam epitaxy using in situ reflection high-energy electron diffraction. Two stable surface phases are identified, which manifest themselves at low temperature by a (1×2)/(4×2) reconstruction at bilayer coverage and a (4×4) reconstruction at trilayer coverage. At growth temperature, Ga adsorbs layer-by-layer up to bilayer coverage after which Ga cluster and eventually droplet formation occurs. The bilayer desorption is delayed by ``feeding'' from this excess Ga. The optimum growth conditions with regard to surface morphology are those giving rise to trilayer coverage at low temperature. This finding is in contrast to the case of GaN(0001) where the optimum growth conditions are related to the formation of a Ga bilayer at the growth front.