Quantum Interference and Electron Transport in Synthesized GaN Nanowires

Abstract

The advance of technology not only leads the trend of industry but also promotes the development of economy. Sometimes, it even affects the political situations. Nanotechnology applied in electronic industry is one of the representatives. The progress of advancement can be sensed from the pace of the renew of electronic products. Quantum mechanical phenomena has been emerging as a result of progressive downsize of the electronic devices. Studying these phenomena opens up a route to create devices with new concepts. Gallium nitride is one of the most popular semiconductor materials for electronic devices. In this thesis, we focus on the study of gallium nitride nanowires, including synthesis, characterization, and transport measurements especially for electron interference phenomena such as universal conductance fluctuations and weak localization. In chapter one, we will present an introductory overview to gallium nitride nanowires, including crystal structure, electronic properties, and its potential applications. In addition, the foundation transport theory in constrained matters and quantum interference effects are introduced. Chapter two introduces the synthesis of gallium nitride nanowire. Nanowire growth mechanism is explained first. Then we show the nanowire synthesis method and setup of the growth facilities. Those facilities we used and parameters we applied are also presented and discussed. Characterization of synthesized nanowires is shown in the chapter three. We used scanning electron microscopy and transmission electron microscopy to study the morphology of the nanowires. By analyzing the energy dispersive x-ray spectra, we identified the elemental constitution of the nanowires. Moreover, the crystal structure of the nanowires is determined by both electron and x-ray diffractions. We also measured the optical gap from photoluminescence spectra. We summed up those characteristics and confirmed that the nanowires were indeed gallium nitride nanowires. In order to perform transport measurements, gallium nitride nanowire devices were made. Chapter four shows these nanowire device fabrication processes. The main process includes lithography and metal deposition. Both are important for device fabrication and measurements. The contact issues, which should be carefully addressed, for the wide energy gap semiconductor and metal are also discussed. Furthermore, electron interference phenomena in nanowires are expected to be observed at low temperature. Thus we applied cryogenic system in our measurements. Principle of the cryogenic system and setup of our measurement are also introduced therein. Chapter five presents the results of measurements. First, field effect behaviors are analyzed. We calculate the carrier densities and mobilities for nanowires from those data. From those electronic properties, we know how to adjust the growth parameters that are related to electronic properties of the synthesized GaN nanowires. These devices show the best performance in terms of electrical conductance and were chosen for magnetoconductance measurement at low temperature. Universal conductance fluctuations and weak localization were measured. The phase coherence length of electrons in GaN nanowires thus estimated by analyzing these behaviors.