The {1010} inversion domains in GaN/sapphire layers: An electron microscopy analysis of the atomic structure of the boundaries

Abstract

The {1010} inversion domains in GaN layers grown on sapphire substrate have been investigated by transmission electron microscopy. They are easily identified in multiple-beam dark-field images and by convergent-beam electron diffraction. Using dark field imaging, it was shown that the domain boundaries do not exhibit any translation in the basal plane. The atomic structure of the boundaries has been determined by comparison with geometrical models. In fact, three types of model can be used to interpret the observations; they correspond to the Holt, Austerman-Gehman and V or IDB∗ models. For each type of model, the boundary plane can be located in two positions, dependent on the cutting plane. Using the AB stacking sequences as seen in high-resolution electron microscopy images, two models were identified. The atomic structure of the boundaries was then determined in comparison with simulated images. In the investigated samples, Holt and V models are shown to exist in the two positions of the boundary plane. However, it was noticed that one does not switch from a V to a Holt boundary structure which seem to form in different samples depending on the growth conditions. The samples containing Holt inversion domains exhibit a flat surface, whereas the V inversion domain boundaries were observed in the centres of small pyramids (100nm high) protruding at the sample surface. Moreover, the Holt inversion domains were always smaller (less than 20 nm) and formed at quite high densities (2.5 × 10¹⁰ cm⁻²) whereas the V inversion domains could reach 50 nm and a density one order of magnitude lower.