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

Two- and three-crystal diffractometric study of the structural perfection o f GaN epitaxial films grown on sapphire, GaAs, and SiC substrates is report ed. The diffraction intensity distributions around the reciprocal-lattice p oints are shown to be extended in the direction parallel to the surface, wh ich is connected with the anisotropy of the local strain fields in the laye rs. A comprehensive analysis is made of the broadening for several reflecti on orders measured in three geometries, namely, Bragg, symmetric Laue, and grazing-angle diffraction. The five independent components of the microdist orsion tensor delta e(ij), as well as the average coherent-scattering lengt hs in two directions, tau(z) and tau(x), have been obtained. It is shown th at for most samples the components responsible for reflection broadening al ong the surface are noticeably larger, i.e. delta e(xx)>delta e(zz), and de lta 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. [S1063-7 834(99)00801-1].