Influence of misoriented substrates and dislocations on the diffraction patterns of superlattices

The structures of superlattices on misoriented (001) substrates are described by means of two modified models. From both models, it can be concluded that there is an intensity distribution rod in the reciprocal space perpendicular to the surface and the symmetry of the diffraction pattern is reduced from tetragonal to monoclinic. However, in the first model the diffraction pattern near the substrate reflections and in the second model that near the so-called `average lattice' reflections remains tetragonal. Neglection of both of these facts can cause systematic errors in the determination of the lattice parameters of superlattices. These errors are estimated. Additionally, 60° misfit dislocations are discussed as a reason for the tilt of the layer system with respect to the substrate. The validity of the theoretical predictions was proved by X-ray diffraction studies on symmetric and asymmetric reflections on three GaAs/Ga_{1 - x}In_xAs superlattices grown on (001) GaAs. The first sample has a large misorientation and the diffraction behaviour corresponds to the above-mentioned models. The second sample shows a diffraction pattern that can be explained by a tilt of the layer system in relation to the substrate. The third sample has a large misorientation and a large tilt of the layer system and so its diffraction pattern shows an overlap of both effects.