Strain relaxation onset in In0.08Ga0.92 As/GaAs multiple-quantum wells investigated by high-resolution X-ray diffraction and atomic force microscopy

In this work we analyze the strain relaxation onset in In0.08Ga0.92As/GaAs multiple-quantum wells (MQWs) grown by low-pressure metal organic chemical vapour deposition. The strain field determination and the structural characterization of the epitaxial layers and heterointerfaces are performed by high-resolution X-ray diffraction and reciprocal space mapping, while the surface morphology is analyzed by atomic force microscopy (AFM). An accurate determination of the strain status of the MQWs indicates a pseudomorphic growth in the epilayers. Nevertheless, the reciprocal space measurements show a distinct intensity profile of the diffuse scattering around each satellite peak. The diffuse intensity is very much elongated along the in-plane direction and the elongation is found to increase with the well layer thickness. Furthermore, in some cases lateral lobes appear within the elongated peaks which indicate an enhanced lateral ordering of the interfaces. AFM images of the heterostructure surface reveal the presence of monolayer steps. The step front distance is found to be related to the in-plane ordering measured by the X-ray experiments. In addition, we show that AFM and X-ray measurements combined to each other can provide a deeper insight in the strain relaxation phenomenon as well as direct information on the vertical correlation of the heterointerfaces.