We report on Raman scattering measurements on Si-capped Ge quantum structures grown by molecular beam epitaxy on Si(001) at low temperatures. We find a double band structure in the Ge-Ge frequency range for nanoislands grown at substrate temperatures ranging in the interval 300-500 °C. Complementary information has been obtained from performing Raman scattering experiments on annealed samples. The results are interpreted in terms of a model that considers quantum structures (hut clusters) composed of a strained Ge core and a more relaxed SiGe shell. © 2005 IOP Publishing Ltd.