We studied the role of composition grading in the strain relaxation process of SiGe films grown on Si(1 1 0) substrates by gas-source molecular beam epitaxy (MBE). The crystalline morphologies of the samples with and without a compositionally step-graded buffer layer were investigated using X-ray diffraction (XRD), a scanning transmission electron microscope (STEM), and an atomic force microscope (AFM). The composition grading was found to suppress the nucleation of growth twins. It was found that the strain relaxation mechanism depended on the growth method of the SiGe layers. in addition, the evolution of the crystalline morphology during step-graded buffer growth was investigated in detail. The evolution of defects during the growth of the graded layers and its relevancy to the strain relaxation process were clarified. (C) 2009 Elsevier Ltd. All rights reserved.