Abstract:
Three-wave diffraction of X-rays is measured using the Renninger scheme for a series of GaN epitaxial layers of various thicknesses and degrees of structural perfection. In each 30$^\circ$-angular interval of azimuthal rotation, all ten three-wave peaks determined by the geometry of diffraction with the 0001 first forbidden reflection and CuK$_{\alpha}$ radiation are observed. The $\varphi$- and $\theta$-scanned diffraction curves are measured for each three-wave combination. The angular FWHM of the diffraction peaks formed in experiments and its relation with the parameters of the two-wave diffraction pattern and the dislocation structure of the layers are analyzed. It is shown that the $\varphi$-scan peaks are less sensitive to the degree of structural perfection than the $\gamma$-mode peaks. The strongest dependence on the dislocation density for the latter peaks is observed for the $(1\bar{1}00)/(\bar{1}101)$ and $(3\bar{21}0)/(\bar{3}211)$ three-wave combinations with a pure Laue component of secondary radiation, while the $(01\bar{1}3)/(0\bar{1}1\bar{2})$ combination with a large Bragg component exhibits the weakest dependence. Splitting of three-wave Renninger peaks associated with the coarse-block structure of some of the layers with rotations of the blocks about the normal to the surface is detected. The total integrated intensity of all three-wave combinations is determined and their ratios are in qualitative agreement with the theory.
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