We examine the nanostructural properties of Si/Si1−x Ge x island superlattices with 0.37 < x < 0.56 grown at 620-640 °C by molecular beam epitaxy. Analytical transmission electron microscopy (TEM) shows that during growth Ge atoms migrate towards the center of the large islands to maintain epitaxial growth and that the most regular structures are obtained at higher Ge composition when the built-in strain is also higher. High-resolution x-ray reciprocal space mapping shows that these heterostructures remain pseudomorphic and that the undulations are aligned along  directions and exhibit a long-range coherence and vertical correlation as revealed by the presence of strong satellites in a wave vector direction parallel to the surface. Raman spectra of these samples exhibit the usual Ge-Ge, Si-Ge and Si-Si vibrational modes. When compared with planar Si/Si1−x Ge x superlattices the Ge-Ge and Si-Ge Raman peaks are shifted to lower frequencies indicating an average alloy composition that is approximately x = 0.1 less than the nominal values, which is in general agreement with analytical TEM.