X-ray diffraction study of strain relaxation, spontaneous compositional gradient, and dislocation density in GeSn/Ge/Si(100) heterostructures
The strain relaxation, depth profiles of composition and density of dislocations in GeSn epilayers were studied by using the x-ray diffraction. Regions with uniform and graded composition, different levels of strain relaxation, and dislocation density were found in the GeSn layers grown at fixed growth conditions. At the initial stage of growth, the GeSn layer is under ~8 × 10–3 compressive strain, and the Sn composition is close to the target value of 6.6%. With increasing layer thickness, the Sn content is enhanced by 0.85 ± 0.12% due to the relief of compressive strain down to ~5.0 × 10–4. The plastic strain relaxation is dominated by 60° misfit dislocations at the GeSn/Ge interface, where their density is 3 × 105 cm−1. The relaxed GeSn layer serves as a buffer for the high-quality GeSn overlayer with uniform and enhanced Sn composition. The laboratory-based x-ray diffraction is shown as a non-destructive reliable technique for strain, composition, and defects characterization, as an alternative to secondary ion mass spectrometry and transmission electron microscope techniques.