  The model systems for self-organized quantum dots formed from elemental and
compound semiconductors, namely Ge grown on Si(001) and InAs on GaAs(001), are
comparatively studied by scanning tunneling microscopy. It is shown that in
both material combinations only two well-defined families of faceted and
defect-free nanocrystals exist (and coexist). These three-dimensional islands,
pyramids and domes, show common morphological characteristics, independently of
the specific material system. A universal behavior is further demonstrated in
the capping-passivation process that turns the nanocrystals in true quantum
dots.
