Alloying in Ge(Si)/Si(001) self-assembled islands during their growth and capping: XPS and AFM study

In this paper, we present a study on the Ge composition and shape evolution of self-assembled Ge/Si(001) islands during the island growth and the subsequent Si capping at 750 degrees C. By combining atomic force microscope images and x-ray photoemission spectroscopy data, we quantitatively determine the Ge distribution in the wetting layer and in the islands, separately. We found that in as-grown sample, the wetting layer is substantially Si-richer than the islands, its average composition being independent of the growth rate. Upon capping, the islands proceed to a reverse Stranski-Krastanov shape evolution, with a progressive Si enrichment of both the wetting layer and the islands. We demonstrate that this evolution occurs at constant island volume. The observed behavior indicates the suppression of the lateral diffusion of both Ge and Si atoms from the wetting layer to the surface of the enlarging islands, and vice versa.