X-ray study of step induced lateral correlation lengths in thin AlGaN nucleation layers

A 15 nm thick high temperature AlGaN nucleation layer was grown on a slightly off-oriented c-plane sapphire substrate by metal-organic vapor phase epitaxy. The evaluation of this layer by high resolution X-ray diffraction yields a nucleation with a threefold distribution of the rotational disorder within about 4deg in the ω-scan at the in-plane AlGaN(11-bar00) reflection, whereas the ω-scan at the symmetric AlGaN(0002) reflection exhibits a very narrow correlation peak. In addition, two weaker signals could be determined in the symmetric ω-scan. The positions of these minor maxima with respect to the correlation peak depend on the azimuth angle of the incident X-ray beam. Based on the determined off-orientation angle and lateral coherence lengths the phenomenon is explained in terms of a stepped sapphire surface morphology. (author)