Low-dimensional systems in silicon/silicon-germanium heterostructures

Recent advances in epitaxial growth technology have made the formation of high-quality, strained-layer heterostructures in the silicon-germanium material system possible. This thesis presents an overview of a range of low-temperature measurements of some of these structures. As the materials are relatively new, the processing techniques for making samples are not well-established, so the thesis discusses the methods used, and in particular, it describes a variety of attempts to fabricate gated devices. Transport measurements of a high-mobility two-dimensional hole gas at low temperatures are described and analysed. Effective mass, quantum lifetime and phase-coherence times are extracted, with the temperature-dependence of the latter following a power-law, the exponent of which indicates a relatively clean system. This exponent predicts the scaling exponents around quantum Hall effect to Hall insulator transitions which are also measured. Screening is shown to cause a strong temperature-dependence of the conductivity. Transport measurements of ungated and Schottky-gated samples of high-quality two-dimensional electron gases at low temperatures are also presented. General features are discussed, including a strong overshoot associated with odd-numbered Hall plateaux and an accompanying asymmetry in the valley-spilt Shubnikov-de Haas peaks. A possible explanation in terms of strong inter-valley scattering is put forward. A range of new behaviours is shown to arise when the carrier density is varied by means of a gate. An anomalous quenching of the valley splitting at a filling factor of 3, resistivity fluctuations at high fields and other effects are presented and discussed. Finally, the thesis describes far infrared measurements of a range of electron gas samples. Cyclotron resonance frequencies of ungated samples deviated from the expected proportionality to the magnetic field. This is explained as resulting from a disorder potential coupled with electron-electron interactions, leading to an apparent lateral confinement.