The growth of GaN and InxGa1-xN on c-plane sapphire substrates was carried out by molecular beam epitaxy using NH3. bl situ reflection high-energy electron diffraction (RHEED) was used to monitor the growth process. Oscillations of the specular beam intensity were observed during both GaN and InxGa1-xN deposition. This allows determining in real time the composition of InxGa1-xN alloys. The effects of the growth temperature and the Ga flux on the In incorporation rate were investigated. The critical thickness for InGaN islanding as a function of In mole fraction is also easily deduced from RHEED experiments. (C) 1998 American Institute of Physics.