X-ray diffraction determination of the degree of ordering of a solid solution in epitaxial AlGaN layers

The three-wave diffraction in epitaxial layers of an Al$_x$Ga$_{1-x}$N solid solution has been measured using the Renninger scheme. From a comparison with similar diagrams measured earlier for GaN and AlN layers, it follows that the curves for the solid solution have a significantly higher intensity in the range of azimuthal angles outside the three-wave peaks. This indicates the existence of the two-wave Bragg reflection 0001 forbidden for wurtzite structures. It has been shown that the appearance of this reflection is due to a partial ordering of the solid solution in Al$_x$Ga$_{1-x}$N layers. From the measured structure factor F(0001) of the layers with various Al concentrations, it follows that the degree of ordering (an Al excess in one cation AlGa plane and an Al deficit in another plane) of the given series of samples is almost independent of the solid solution layer composition.