Electronic Conduction Processes in Low-Dimensional Semiconductor Systems.

Available from UMI in association with The British Library. The work reported in this thesis is an investigation into conduction processes in systems with reduced dimensionality. It is divided into two parts, the first dealing with transport of electron in a plane, and the second dealing with carrier transport perpendicular to thin layers of semiconductor material. A preparatory chapter gives the simple background to the devices used in the studies, a silicon MOSFET, and a GaAs-AlAs superlattice, which are both examples of low dimensional systems. In Part-I, the electrical conductivity of a two -dimensional sheet of electrons is investigated, with particular emphasis on the effects of quantum localisation and electron -electron interactions. Both of these effects give rise to corrections to the electrical conductivity, and can be probed by judicious use of varying magnetic fields and temperature changes. From our investigation we find that the theoretical predictions for the magnitude of the electron-electron interaction effect is smaller than that which we determine experimentally. Several different experiments including parallel field magnetoconductivity, temperature dependence, and temperature dependence with a small applied magnetic field, all point to the same conclusion, indicating that although the theory has many dramatic successes, it also has several shortcomings. In Part-II we have given an account of experiments designed to investigate the tunnelling of electrons through indirect-band-gap barrier systems. This is accomplished by measuring the photoluminescence associated with the superlattice as a function of hydrostatic pressure. Details of several calculations are also given, which help with the interpretation of the experimental data, and the combination of the calculations and the experiments leads us to conclude that the lowest conduction band state, i.e. the X-states in the AlAs, are important in tunnelling through such structures.