Beyond the heteroepitaxial quantum dot: Self-assembling complex nanostructures controlled by strain and growth kinetics

Heteroepitaxial growth of GeSi alloys on Si (001) under deposition conditions that partially limit surface mobility leads to an unusual form of strain-induced surface morphological evolution. We discuss a kinetic growth regime wherein pits form in a thick metastable wetting layer and, with additional deposition, evolve to a quantum dot molecule-a symmetric assembly of four quantum dots bound by the central pit. We discuss the size selection and scaling of quantum dot molecules. We then examine the key mechanism-preferred pit formation-in detail, using ex situ atomic force microscopy, in situ scanning tunneling microscopy, and kinetic Monte Carlo simulations. A picture emerges wherein localized pits appear to arise from a damped instability. When pits are annealed, they extend into an array of highly anisotropic surface grooves via a one-dimensional growth instability. Subsequent deposition on this grooved film results in a fascinating structure where compact quantum dots and molecules, as well as highly ramified quantum wires, are all simultaneously self-assembled.