Fabrication of self-assembled silicon germanium quantum nanostructures on silicon surfaces by molecular beam epitaxy

There is presently much interest in the fabrication of semiconductor quantum nanostructures (quantum wells, quantum wires and quantum dots) which are believed to have potential applications in Si-based opto-electronics and in the area of ultra-fast computing. Although many fabrication techniques exist now, most of them suffer from slow processing, poor interfaces and high defect densities. Heteroepitaxial growth, however, holds the promise to fabricate defect-free quantum nanostructures with high quality interfaces and excellent optical properties.; Heteroepitaxial fabrication of quantum nanostructures is based on the fact that strained thin films are unstable against modulation of the surface profile, such as strain-induced coherent islands or ledges. Current research efforts focus on understanding the mechanisms for strain relaxation in order to control the size and spatial distribution of such structures.; This dissertation discusses the strain relaxation mechanisms and the surface morphology evolution in SiGe/Si grown by Molecular Beam Epitaxy (MBE). Thin films have been extensively characterized by in-situ Reflection High Energy Electron Diffraction (RHEED) and ex-situ Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM). The islanding behavior of Si_0.75Ge_0.25 on Si(100) surfaces from 450°C to 800°C are described. There is no island formation at 450°C due to the limited kinetics. Rectangular-based islands elongated in elastically soft <100> directions at 500°C, 550°C and 650°C, and square-based prisms at 700°C and 800°C are observed. However, an anomalous suppression of island formation at 600°C is caused by the reduced driving force due to Ge redistribution in the thin film/substrate. For the SiGe/Si(110) system, SiGe ledges aligned along elastically hard <111> directions are formed. Island formation seems to be inhibited on Si(110) surface. This is believed to be caused by the Si(110) surface anisotropy and surface instability. In addition, a unique nanostructure termed “quantum-dot molecule,” which has potential use for quantum cellular automata, is fabricated. An initial description of the formation mechanisms of quantum-dot molecules is presented.