Structural perfection of GaN epitaxial layers according to x-ray diffraction measurements

Two- and three-crystal diffractometric study of the structural perfection of GaN epitaxial films grown on sapphire, GaAs, and SiC substrates is reported. The diffraction intensity distributions around the reciprocal-lattice points are shown to be extended in the direction parallel to the surface, which is connected with the anisotropy of the local strain fields in the layers. A comprehensive analysis is made of the broadening for several reflection orders measured in three geometries, namely, Bragg, symmetric Laue, and grazing-angle diffraction. The five independent components of the microdistorsion tensor delta e(ij), as well as the average coherent-scattering lengths in two directions, tau(z) and tau(x), have been obtained. It is shown that for most samples the components responsible for reflection broadening along the surface are noticeably larger, i.e. delta e(xx)>delta e(zz), and delta e(zx)>delta e(xz), as well as tau(z)> tau(x). All tensor components are related to a specific dislocation type. Electron microscopy of the samples revealed a high density of pure edge and pure screw dislocations extending normal to the interface, and which provide a dominant contribution to e(xx) and e(zx), respectively. (C) 1999 American Institute of Physics. [S1063-7834(99)00801-1].