Obviously, the strain energy depends on the film thickness, and increases with increasing number of atomic layers. The interface energy, on the other hand, does not vary with film thickness. It depends on the misfit between substrate and deposit lattice, and is generally minimized for a pseudomorphic structure with no apparent misfit between substrate and actual deposit lattice. As a result, there is a driving force for ultrathin deposit layers to restructure with increasing thickness and finally approach and adopt the bulk structure of the deposit, e.g., by the formation of misfit dislocations . This is often discussed as one of the origins for the change from layer-by-layer growth to 3D island formation during Stranski-Krastanov growth . Excellent overviews on these aspects are given in ref. .