Dislocations at the interface between sapphire and GaN

GaN layers grown by metal organic vapour phase epitaxy on sapphire were imaged by synchrotron radiation X-ray topography. The threading dislocations could not be resolved in the topographs due to their high density, but a smaller density of about 105 cm-2 misfit dislocations were seen in the interface between GaN and sapphire by using large-area backreflection topography. The misfit dislocation images in the topographs form a well-resolved cellular network, in which the average cell size is roughly 30 μm. Different cell shapes in the misfit dislocation networks are observed on different samples.