Directed assembly and strain engineering of SiGe films and nanostructures

The lattice-mismatch-induced strain in growth of Ge on Si produces many consequences, both in 2D film growth and in 3D nanostructure formation. New phenomena occur if silicon-on-insulator (SOI) is used as a substrate. The top Si layer in SOI is not thermodynamically stable, and can dewet and form 3D nanocrystals that can serve as a template for further growth of nanostructures, such as carbon nanotubes. Coherent 3D Ge nanocrystals grown on the thin Si template of SOI can provide sufficient stress to distort the Si layer and cause the oxide underneath to flow, potentially influencing electrical properties of the Si. These measurements provide new insights on bending of very thin substrates during heteroepitaxial growth, something that may also impact 2D film growth and devices made from strained films, via defect generation mechanisms that are distinct from the behavior on bulk Si. One of these mechanisms, dislocation formation, can be uniquely investigated with low-energy electron microscopy. Combining growth with nanopatterning, we have been able to fabricate side-gated quantum dots in Si/SiGe two-dimensional electron gases that show excellent electrical behavior.