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High-resolution x-ray diffraction has been performed on strained SiGe nanoscale islands grown coherently on Si͑001͒. Reciprocal space maps show a widely extended ''butterfly''-shaped island reflection and strong diffuse scattering around the substrate reflection. From such intensity maps the Ge content and its distribution inside the islands are evaluated. This is done by simulation of diffuse scattering for a variety of island models. The island shape is known from atomic force and scanning electron microscopy. The only free parameter was the Ge distribution, here approximated by a vertical concentration profile. With an abrupt increase of Ge content at about one third of the island height a rather good agreement with the experimental results is achieved. The strain distribution in the islands is then given by the finite element calculations, which are part of the simulation algorithm.

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Ex situ observed growth stages of LPE-SiGe/Si͑001͒ Stranski-Krastanow islands with a germanium content of 10% give clear evidence of a rapid shape transition at one third of the final island height. The island shape changes from a lenslike type without a top facet to truncated pyramids with ͕111͖ side facets and an ͑001͒ top facet. High-resolution x-ray diffraction has been applied to islands with higher germanium content of about 30%. Experimental results are compared with respective kinematical scattering simulations based on finite element calculations for the strain field. From these simulations the three-dimensional germanium composition profile inside the islands can be extracted and it substantiates a similar growth scenario with a distinct shape transition at one third of the final island height also for this germanium concentration range. We attribute the observed finite island size to a distinct nucleation problem at the island bottom caused by exceptional high strain energy around the island corners in combination with a strain driven wetting layer depression.