Overview

This thesis investigates electron transport in low dimensional GaN/AlGaN mesoscopic systems. Nanofabrication techniques give researchers the power to confine electrons in semiconductors to low dimensional mesoscopic systems. The host material can be so clean and electronically simple that we are not limited by the foibles of a particular material. Almost all interesting experiments on mesoscopic semiconductor have been based on GaAs/AlGaAs heterostructure because of the high quality of the material, and the fact that the electrons behave similarly to free electrons in vacuum except for modified physical parameters (effective electron mass and g-factor, etc.). Nonetheless, puzzles remain even for the simplest mesoscopic structures in GaAs. By moving to a different material (GaN/AlGaN heterostructure), we can examine the universality of the observed behaviors of GaAs-based mesoscopic systems, and we can also probe how things change when we vary important parameters: GaN has a higher effective mass (3X) and lower dielectric constant (0.7X) than GaAs, making interactions more important relative to kinetic energy. GaN also has a higher g factor (4.5X), making it easier to control spin states by applying magnetic field. In this thesis I will present our results of transport measurement on two types of mesoscopic system based on GaN/AlGaN: A quasi-1D system: Quantum Point Contacts (QPC) in GaN were fabricated and measured at low temperature. We observed well-quantized conductance plateaus, and the plateaus split into spin-resolved plateaus at high perpendicular magnetic field. We also observed features of 0.7 structure, an unresolved puzzle in GaAs QPCs. A 0D system: Quantum Dots in GaN were fabricated and Coulomb blockade oscillations of conductance were observed at low temperature. The distribution of the spacing between consecutive Coulomb Blockade Peaks reveals the statistical properties of the level spacing of the confined electrons in the Quantum dot, which is predicted to have a variation close to mean level spacing. In previous experimental works on GaAs and Si dots, Gaussian distributions with a broad range of widths were observed. The observation of variation greater than mean level spacing in some GaAs and Si Quantum dot experiments has been attributed to the effect of strong electron-electron interactions. In the GaN dot we studied here, the electron-electron interactions are even stronger than in those previous experiments, yet we observed a Gaussian distribution of peak spacings with a width close to the mean level spacing, refuting the interpretation of broad variations in peak spacing in previous studies.

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