Experimental Study of Some Aspects of Localization in a Two-Dimensional Electron Gas
Abstract
In the first part of the thesis, the study of the variable-range hopping conduction in a wide, nominally two-dimensional sample is described. Due to the specific nature of the conduction process, the existence of reproducible conductance fluctuations is possible even in a sample of such large dimensions. This work presents the first experimental study of mesoscopic phenomena in a "macroscopic" sample, including the effect of the magnetic field. The analysis of the results shows very good qualitative agreement with theoretical predictions. The averaged conductance is found to obey the one-dimensional hopping law, consistent with the model employed to explain the origin of the fluctuations. The influence of the higher lying subbands on the transport of electrons in the ground 2D subband has been studied in the second part. In particular, the thermally activated conduction was considered. The substrate bias and perpendicular magnetic field have been used to vary the subband splitting. The evidence has been found for the existence of long band tails associated with higher subbands. In addition, the effect of the magnetic field on the activation energy shows the enhancement of strong localization by the field. Finally, several techniques for determining the sodium concentration N_{ox} from the threshold voltage in sodium-doped Si MOSFETs have been studied. While these methods give results that are in agreement with each other at low values of N _{ox}, the transconductance develops complicated structure near the threshold with the further increase in N_{ox} and the thresholds deduced from Shubnikov-de Haas oscillations do not shift at all with N_{ox} . The results of the Hall voltage measurement shift as expected but there is also an additional effect caused by electron-electron interactions in the weakly localized regime. Still, the Hall measurement is suggested as the only transport technique that can offer reliable information on N_{ox} in the case of high sodium concentrations.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- January 1990
- Bibcode:
- 1990PhDT........82P
- Keywords:
-
- ELECTRON GAS;
- Physics: Condensed Matter