Fabrication of SiGe nanostructures for infrared devices

The molecular beam epitaxy (MBE) growth conditions for tensile strained silicon (Si) on germanium (Ge) substrates were investigated. The basic pre-conditions for the epitaxial growth were established with the development and optimization of a Ge cleaning procedure. Homoepitaxial growth of the Ge buffer layer was explored and resulted in a flat surface with monoand double-atomic steps only. The main focus, however, was put on the subsequent Stranski- Krastanow (SK) heteroepitaxial 3D-growth. Self-organized Si islands on Ge(001) substrates provide a confinement of Δ2-electrons, which is of special interest for conduction-electron spin manipulation or for photoluminescence in the near- or mid-infrared range. Under tensile strain, the wetting layer (WL) thickness is considerably increased to about 11 ML for the growth at 500°C. For comparison, under compressive strain 3D-growth commences already after 2-3 MLs. A further result of the experiments is the rather unusual coexistence of SK growth and plastic strain relaxation already at an early stage of 3D growth. SK growth is usually an equilibium process between the surface-, interface- and strain-energies with the driving force to minimize the total energy. Thus, modifying the surface energy in the total energy balance with the help of surfactants was analyzed. As surfactants, 0.1 ML of either carbon (C) or antimony (Sb) were deposited onto the Ge buffer layer just prior to Si deposition. Carbon or antimony pre-deposition significantly reduced the wetting layer thickness but could not prevent the whole system from plastic strain relaxation within the islands.