Lateral ordering of coherent Ge islands on Si(001) studied by triple-crystal grazing incidence diffraction

We have applied triple-crystal grazing incidence diffraction to self-assembled islands on a Ge/Si(001) superlattice. Lateral ordering in the near-surface region is evaluated from reciprocal space mappings around different surface reflections. The observed intensities are explained by the short-range order strain modulation of the lattice parameter in the substrate induced by coherent Ge islands. We find the island-induced strain modulation to be arranged in a local square lattice. A nearest neighbor disorder parameter, the size distribution of the islands, and a correlation length are obtained from the presented model.