It is well known that the physical properties of semiconductor nanostructures, which have been grown in most cases by the Stranski-Krastanow (SK) mechanism, depend on their size, shape, strain and composition. In the case of the growth of Ge on Si(001), where the 2D-3D transition is driven by the 4.16% lattice mismatch between Ge and Si, the increase of Ge coverage above a critical thickness of around 4 ML can make coherent islands. First, square pyramids appear, and then dome-shaped islands are formed. At about 9 ML, the misfit strain can no longer be accommodated coherently and larger islands called superdomes are present. This raises detailed questions as to dependence on the growth rate, temperature etc. To provide answers to such questions, in-situ X-ray studies are extremely important. Professor G. Bauer (Institute of Semiconductor and Solid State Physics, Johannes Kepler University in Linz, Austria) and his colleagues recently performed grazing-incidence small angle X-ray scattering (GISAXS) and diffraction (GID) experiments with a UHV-MBE chamber. They clarified the kinetics of the growth of Ge superdomes and their facets on Si(001) surfaces, as a function of deposited Ge thickness for different growth temperatures at a low growth rate, by in situ grazing-incidence small-angle x-ray scattering in combination with in situ grazing-incidence x-ray diffraction. At a low growth rate, intermixing is found to be enhanced and superdomes are formed already at lower coverages than previously reported. In addition, the research team observed that at the dome-to-superdome transition, a large amount of material is transferred into dislocated islands, either by dome coalescence or by anomalous coarsening. Once dislocated islands are formed, island coalescence is a rare event and introduction of dislocations is preferred. The superdome growth is thus stabilized by the insertion of dislocations during growth. For more information, see the paper, "In situ X-ray scattering study on the evolution of Ge island morphology and relaxation for low growth rate: Advanced transition to superdomes", M.-I. Richard et al., Phys. Rev. B 80, 045313 (2009).
