Virtual Ge-rich SiGe substrates as a technology for the integration of epitaxial heterostructures on Si(111) substrates

Abstract

In addition to improving components for integrated circuits, the integration of new technologies such as spintronics or optoelectronics on Si substrates is becoming increasingly important. One way of achieving this is, for example, the optimisation of virtual germaniumrich SiGe substrates on Si(111) substrates. For this purpose, carbon-mediated epitaxy was adapted for the growth of such virtual substrates in this work. It was shown that the layers produced in this way with a Ge content of over 50 % grow fully relaxed and smooth at ultra-thin layer thicknesses. In addition, a c(2×8) surface reconstruction for SiGe layers was demonstrated for the first time. However, the threading dislocation density is still too high for use in devices. Next to the structural properties, the thermal properties, as well as the surfaces and interfaces play a crucial role in the performance of the resulting components. For this reason, the SiGe layers were investigated under different annealing steps with regard to the surface properties, whereby an irreversible reconstruction transition from c(2×8) to reversible 7×7↔”1×1” changes could be demonstrated for the SiGe layers for the first time. The specific transition temperatures depend on the Ge content and the surface roughens when the 7×7↔”1×1” reconstruction change has occurred. It was hypothesised that the high adatom density of the ”1×1” reconstruction cannot be reduced via step incorporation when the layer cools rapidly, resulting in island formation. In contrast to the SiGe layers, the Ge layers remain smooth after the typical c(2×8)↔1×1 reconstruction transition, as in this case the adatom density for c(2×8) is higher than for 1×1. The virtual substrates characterised in detail were finally examined for their compatibility with the high-k material Nd2O3. In this work, monocrystalline hexagonal Nd2O3 was grown on Si(111), while the oxide on virtual Ge substrates has a cubic phase without an interlayer. In contrast, growth on SiGe layers produces a silicate interlayer and a mixed phase of Nd2O3. The oxide shows a reduction in the induced tensile stress in the cubic phase during growth on Ge-rich SiGe layers due to the formation of hexagonal components. In addition, only Nd2O3 layers with silicate bonds to the underlying layer and a 7×7 surface reconstruction before oxide growth show hexagonal parts, which seems to be crucial for the formation of the hexagonal phase.