By using high-resolution electron microscopy, atomistic modelling and image simulation, typical contrasts were predicted for most of the extended defects which formed in epitaxial GaN layers. There were 3 types of defect which propagated into the active layers. These were threading dislocations, prismatic stacking faults and inversion domains. The atomic structure of the a-type pure edge threading dislocations was shown to exhibit 5/7, or 8 atom cycles. The 2 configurations were observed with similar frequencies for isolated dislocations and low-angle grain boundaries. Coincidence grain boundaries were made up of pure edge a-type dislocations having the above atomic structures. A topological analysis of high-angle grain boundaries was carried out in order to determine the defect content at the interfaces. Reconstruction of some boundaries was possible only by taking account of the occurrence of structural units which exhibited 4-atom ring cycles in the dislocation cores. The {1¯2•0} stacking fault had 2 atomic configurations in wurtzite (Ga,Al,In)N with 1/2<10•1> and 1/6<20•3> displacement vectors. It originated from steps at the SiC surface and could form on a flat (00•1) sapphire surface. The 2 atomic configurations had comparable energies in AlN, whereas the ½<10•1>{1¯2•0} atomic configuration was expected to be more stable in GaN and InN. Observations which were carried out in plan-view revealed the 1/2<10•1>{1¯2•0} configuration in GaN layers. The 1/6<20•3> configuration was found within the AlN buffer layer in cross-sectional observations. It folded rapidly to the basal plane, and when back in the prismatic plane, it had the 1/2<10•1>{1¯2•0} configuration. The {10•0} inversion domains in GaN layers grown on sapphire were identified in multi-beam dark-field images and by convergent-beam electron diffraction. Holt and V models were shown to form in the samples; depending upon the growth conditions. Samples containing Holt inversion domains exhibited a flat surface morphology, whereas the V inversion domains were observed in the centres of small pyramids (100nm high) which protruded from the sample surface. The Holt inversion domains were always smaller (less than 20nm), and present in quite high densities (2.5 x 1010/cm2). The V inversion domains ones could reach 50nm and one order of magnitude lower density. The inversion domains were generated mainly at sapphire surface steps, where they minimized the large (20%) misfit along the c-axis.

Atomic Structure of Extended Defects in Wurtzite GaN Epitaxial Layers. P.Ruterana, G.Nouet: Physica Status Solidi B, 2001, 227[1], 177-228