Probing the composition of Ge dots and Si∕Si1−xGex island superlattices

We use analytical transmission electron microscopy to map the composition of Ge dot and Si∕Si1−xGex island structures grown on (001) Si by molecular beam epitaxy or ultrahigh vacuum chemical vapor deposition. Energy-dispersive x-ray spectroscopy reveals that nominally pure Ge dots grown by molecular beam epitaxy at 650°C exhibit considerable intermixing with the average Ge composition typically increasing from nearly zero at the base to about 50% at the top of the dot. In pyramid shaped dots, the Ge composition increases linearly up to the top of the dot, while for dome dots, a saturation of the incorporation rate is seen beyond a distance of 7nm from the substrate interface. Probing of Si∕Si1−xGex island superlattices also reveals large Si∕Ge intermixing with a Ge accumulation at the crest and Ge depletion at the troughs of the islands. These results are corroborated by x-ray diffraction and Raman scattering measurements.

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