Local equilibrium and global relaxation of strained SiGe/Si(001) layers

We investigate the morphological evolution of islands obtained by epitaxial growth of Ge on Si(001) substrates. We are able to obtain highly uniform distributions of SiGe islands, which exhibit a barn shape. In addition to previously observed facets, we identify higher index facets, which are not observed in dome-shaped islands. The evolution of the island-related facet area provides evidence of a transition from domes to steeper barns, which continues the sequence of coherent island types before the onset of plastic relaxation. For higher Ge coverages, when plastically relaxed islands (superdomes) form, the island ensemble loses its homogeneity. This is essentially the result of anomalous coarsening, with material being transferred from coherent islands to larger superdomes.