  The annealing of sub-critical Ge wetting layers (WL<3.5 ML) initiates the
formation of 3D nanostructures, whose shape and orientation is determined by
the WL thickness and thus directly related to the strain energy. The emergence
of these nanostructures, hillocks and pre-quantum dots, is studied by scanning
tunneling microscopy. A wetting layer deposited at 350 C is initially rough on
the nanometer length-scale and undergoes a progressive transformation and
smoothening until for T>460 C vacancy lines and the 2xn reconstruction are
observed. The metastable Ge wetting layer (WL) then collapses to form 3D
nanostructures whose morphology is controlled by the WL thickness: firstly,
hillocks, with a wedding cake-type structure where the step edges run parallel
to the <110> direction, are formed for thin wetting layers, while {105}-faceted
structures, so-called pre-quantum dots (p-QDs), are formed from thicker layers.
The wetting layer thickness and thus the misfit strain energy controls the type
of structure. The crossover thickness between hillock and p-QDs regime is
between 1.6 and 2.1 ML. The hillocks have larger lateral dimensions and volumes
than p-QDs, and the p-QDs are exceptionally small quantum dots with a lower
limit of 10 nm in width. Our work opens a new pathway to the control of
nanostructure morphology and size in the elastically strained Ge/Si system.
Keywords: Quantum dots, Stranski-Krastanov, epitaxy, strained layers, scanning
tunneling microscopy
