Intensity distribution of the three-wave diffraction from dislocation epitaxial layers in the reciprocal space

The three-wave X-ray diffraction in strongly disordered epitaxial layers of GaN and ZnO is experimentally investigated. The charts of the intensity distribution in the reciprocal space are plotted in coordinates $q _\theta$ and $q_\varphi$ for the most intensive three-wave combination (1010)/(1011) by means of subsequent $\theta$- and $q_\varphi$-scanning. A nontrivial shape of the $\theta$-sections of these contours at a distance from the $q_\varphi$ center of reflection is revealed; it is different for different samples. For the $\theta$-curves at the center of reflection, we observed a common peak that may be approximated by the Voigt function with a power-low decrease in the intensity at the wings; the decrease law (from -4.5 to -5.0) is found to be considerably greater than that for the similar curves of two-wave diffraction and not depending on the dislocation density and distribution in layers. In some films we observed a coarse-block structure; in addition, it follows from the distribution in the reciprocal space that these blocks are turned with respect to each other around a normal to the surface, which allows us to suggest the existence of low-angle boundaries between them, consisting exclusively of edge dislocations.