MICROSTRUCTURE OF GAN EPITAXIAL-FILMS AT DIFFERENT STAGES OF THE GROWTH-PROCESS ON SAPPHIRE(0001)

The microstructure of GaN films at different stages of a classical two steps growth process is studied using TEM. The buffer layer grown at low temperature (600 degrees C) exhibits a mixed cubic-hexagonal colum nar microstructure. Numerous defects are present to accomodate the mis orientations between micrograins. During the following annealing step up to 1050 degrees C, the microstructure drastically changes: cubic is lands remain on the top of a him with hexagonal structure. The buffer layer at this stage is still highly polycrystalline. The microstructur e of micrometer thick films grown at 1050 degrees C could be separated in two zones. Close to the interface with sapphire, misfit dislocatio ns, basal stacking faults and nanocavities are observed. We propose a mechanism of relaxation of the strain due to the difference of thermal expansion coefficients which could explain the presence of stacking f aults. The existence of nanocavities is supposed to be related to a co ntamination by oxygen. After a thickness of 0.5 mu m, two types of thr eading defects remain: edge dislocations with 1/3[<11(2)over bar 0>] B urgers vector which accomodate slight misorientations between grains, and nanopipes. These nanopipes are identified as open core dislocation s with (0001) Burgers vector. They have an alternating behaviour: clos e core, open core. The microstructure of this bulk zone duplicates the microstructure of the buffer layer at a higher scale, pointing out th e crucial importance of the first steps of the growth.