Surface electronic structure modifications due to buried quantum dots

Coherently strained Ge islands have been grown on Si(100) substrates by means of molecular beam epitaxy and subsequently covered by a 10-nm-thick Si cap layer. In situ scanning tunneling microscopy revealed surface protrusions (up to about 0.4 nm) whose height depends on the size of the buried Ge dots. From the surface deformation, the in-plane strain within the capping layer was calculated. Evidence for directed diffusion of Si adatoms away from the highly strained regions was found. On the protrusions, a lowering of the surface band gap was measured using locally resolved scanning tunneling spectroscopy. This can be explained by changes of the electronic structure of the silicon surface induced by the inhomogeneous strain around the buried dots.