This thesis describes a study of reentrant metal-insulator transitions observed in the inversion layer of extremely high mobility Si-MOSFETs. Magneto-transport measurements were carried out in the temperature range 20mK-4.2 K in a $\sp3$He/$\sp4$He dilution refrigerator which was surrounded by a 15 Tesla superconducting magnet. Below a melting temperature $(T\sb{M}\sim500$ mK) and a critical electron density $(n\sb{s}\sim9\times10\sp $ cm$\sp{-2}),$ the Shubnikov-de Haas oscillations in the diagonal resistivity enormous maximum values at the half filled Landau levels while maintaining deep minima corresponding to the quantum Hall effect at filled Landau levels. At even lower electron densities the insulating regions began to spread and eventually a metal-insulator transition could be induced at zero magnetic field. The measurement of extremely large resistances in the milliKelvin temperature range required the use of very low currents (typically in the $10\sp{-12}$ A range) and in certain measurements minimizing the noise was also a consideration. The improvements achieved in these areas through the use of shielding, optical decouplers and battery operated instruments are described. The transport signatures of the insulating state are considered in terms of two basic mechanisms: single particle localization with transport by variable range hopping and the formation of a collective state such as a pinned Wigner crystal or electron solid with transport through the motion of bound dislocation pairs. The experimental data is best described by the latter model. Thus the two dimensional electron system in these high mobility Si-MOSFETs provides the first and only experimental demonstration to date of the formation of an electron solid at zero and low magnetic fields in the quantum limit where the Coulomb interaction energy dominates over the zero point oscillation energy. The role of disorder in favouring either single particle localization or the formation of a Wigner crystal is explored by considering a variety of samples with a wide range of mobilities and by varying the ratio of the carrier density (controlled by the applied gate voltage) to the impurity density (fixed during sample growth). A phase diagram showing the boundaries between the two dimensional electron gas, the Wigner solid, and the single particle localization induced insulator is established in terms of carrier density and sample mobility.
| Identifer | oai:union.ndltd.org:uottawa.ca/oai:ruor.uottawa.ca:10393/9768 |
| Date | January 1995 |
| Creators | Campbell, John William M. |
| Contributors | D'Iorio, Marie, |
| Publisher | University of Ottawa (Canada) |
| Source Sets | Université d’Ottawa |
| Detected Language | English |
| Type | Thesis |
| Format | 179 p. |
Page generated in 0.002 seconds