Compositional Redistribution in Coherent ${\rm Si}_{1\hbox{-}x}{\rm Ge}_{ x}$ Islands on Si(100)

Coherent ${\rm Si}_{ 1-x}{\rm Ge}_{ x}$ island growth by molecular beam epitaxy is studied for a fixed growth temperature but for different Ge concentrations in the range 0.37 $\leq{x}\leq$ 0.56. A combined transmission electron microscopy, X-ray diffraction, and Raman spectroscopy characterization of the samples showed that during growth the Ge migrates towards the center of the large islands to minimize the strain energy, while maintaining epitaxial growth, and that the most uniform structures are obtained at higher Ge composition when the built-in strain is also higher.