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

Abstract

GaN(0001) epitaxial layers were grown by molecular beam epitaxy on afew-nanometres thick low-temperature GaN nucleation layers on c-planesapphire. Despite extremely high densities of extended defects, the layersshow a narrow (002) x-ray diffraction peak, superimposed by broad diffusescattering. Triple-axis transverse and radial scans were measured for (00l)reflections of different orders and for various GaN layer thicknesses. Theresults can be described by an interfacial displacement-difference correlationfunction. Its microscopic origin is assigned to either inversion domainboundaries or edge-type threading dislocations in the GaN layers, in agreementwith findings of transmission electron microscopy. These defects areassociated with an only weak rotational disorder perpendicular to the growthplane as proven by the x-ray scattering characteristics.