X-ray scattering from GaN epitaxial layers - an example of highly anisotropic coherence

GaN(0001) epitaxial layers were grown by molecular beam epitaxy on a few-na nometres thick low-temperature GaN nucleation layers on c-plane sapphire. D espite extremely high densities of extended defects, the layers show a narr ow (002) x-ray diffraction peak, superimposed by broad diffuse scattering. Triple-axis transverse and radial scans were measured for (001) reflections of different orders and for various GaN layer thicknesses. The results can be described by an interfacial displacement-difference correlation functio n. Its microscopic origin is assigned to either inversion domain boundaries or edge-type threading dislocations in thp GaN layers, in agreement with f indings of transmission electron microscopy. These defects are associated w ith an only weak rotational disorder perpendicular to the growth plane as p roven by the x-ray scattering characteristics.