  X-ray diffuse scattering in the vicinity of a basis-forbidden (200) Bragg
reflection was measured for a sample with uncapped self-assembled Ge islands
epitaxially grown on Si(001). Our results provide evidence of atomically
ordered SiGe domains in both islands and wetting layer. The modeling of x-ray
profiles reveals the presence of antiphase boundaries separating the ordered
domains in a limited region of the islands where the stoichiometry is close to
Si0.5Ge0.5.
