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

Abstract

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 δe _ij , as well as the average coherent-scattering lengths in two directions, τ _z and τ _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. δe _xx >δe _zz , and δe _zx >δe _xz , as well as τ _z >τ _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.