P. Vennegues et al., MICROSTRUCTURE OF GAN EPITAXIAL-FILMS AT DIFFERENT STAGES OF THE GROWTH-PROCESS ON SAPPHIRE(0001), Journal of crystal growth, 173(3-4), 1997, pp. 249-259
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.