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 columnar microstructure. Numerous defects are present to accomodate the misorientations between micrograins. During the following annealing step up to 1050 degrees C, the microstructure drastically changes: cubic islands remain on the top of a him with hexagonal structure. The buffer layer at this stage is still highly polycrystalline. The microstructure of micrometer thick films grown at 1050 degrees C could be separated in two zones. Close to the interface with sapphire, misfit dislocations, 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 faults. The existence of nanocavities is supposed to be related to a contamination by oxygen. After a thickness of 0.5 mu m, two types of threading defects remain: edge dislocations with 1/3[<11(2)over bar 0>] Burgers vector which accomodate slight misorientations between grains, and nanopipes. These nanopipes are identified as open core dislocations with (0001) Burgers vector. They have an alternating behaviour: close core, open core. The microstructure of this bulk zone duplicates the microstructure of the buffer layer at a higher scale, pointing out the crucial importance of the first steps of the growth.