published_or_final_version / abstract / Physics / Doctoral / Doctor of Philosophy

In this project, nano-sized gallium oxide and indium oxide produced by ion implantation of nitrogen/carbon and subsequent rapid thermal annealing (RTA) have been investigated. The material synthesis technique is based on using implantation of different species, which include nitrogen, carbon, oxygen and argon, with variable implant dosage to form an amorphous surface layer on GaAs or InP substrates. RTA then provides the required thermal energy for the amorphous material to re-crystallize. We found that the type of implanted species play an important role in controlling the material for nation during the RTA stage. Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were conducted to provide detailed characterization on the samples. For the nitrogen/carbon implanted samples, RTA at 950°C resulted in the formation of single crystalline Ga2O3 nano-ribbons on the sample surface. These Ga2O3 ribbons possess strong visible photoluminescence and cathodoluminescence. For the carbon implanted InP samples, In2O3 nanowires were found on the InP sample surface when RTA was performed at 750°C. However, In2O 3 nanowires only occurred when gold was present. On the other hand, when the nitrogen implanted samples were annealed in pure nitrogen ambient, a Raman peak at 577cm-1 associated with GaN was observed. Cross-sectional TEM showed that the thickness of the region containing GaN was about 40nm. We also used the synthesized GaN as a buffer layer to grow ZnO film by using MOCVD. The possible formation mechanisms of these nanomaterials and the role of the implanted species are discussed. For the nanowires with gold nano-particles at the free end, we believe that they are synthesized by vapour-liquid-solid (VLS) mechanism. On the other hand, the growth of nano-wires in the cases where no gold nano-particles on the free end may be explained on the basis of a vapour-solid (VS) mechanism. For the case of carbon or nitrogen implantation, carbon works as a reduction agent and nitrogen favours the formation of group III nitride template, which may lead of the growth of nano-wires. / Lo, Kwong Chun. / "March 2008." / Adviser: Aaron H. P. Ho. / Source: Dissertation Abstracts International, Volume: 70-03, Section: B, page: 1887. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2008. / Includes bibliographical references (p. 112-119). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [200-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstracts in English and Chinese. / School code: 1307.

Thesis (Ph. D.)--University of Hong Kong, 2006. / Title proper from title frame. Also available in printed format.

Thesis (M. Phil.)--University of Hong Kong, 2002. / Includes bibliographical references (leaf 99).

A pulse-laser based time-resolved photocurrent (TRPC) and photoluminescence (TRPL) system with a programmable Boxcar integrator/averager system incorporated was implemented to investigate the optical properties and charge carrier dynamics in a GaInP/GaAs single-junction photovoltaic device for the purposes of understanding fundamental optoelectronic processes in the solar cell. The implementation of whole system was realized by integrating the instrument of a Boxcar averager system with a pulse laser source + spectroscopic facilities. The delay time control and data acquisition were organized by the software code. The effects of the hardware configurations and the software parameters on the performance of the system were particularly addressed for the optimization of measurement conditions and precisions. Two main functions of TRPC and TRPL with a wide time range were demonstrated for the system. The system was employed to measure temperature- and bias voltages-dependent TRPC and TRPL spectra of a GaInP/GaAs single-junction photovoltaic device. The spectral data show a lot of information about the transient dynamic behaviors of photogenerated charge carriers in the device, including both the rise and decay processes. Interestingly, the measured time-resolved photocurrent curves are characterized by a fast rising edge followed by a relatively slow decay process as the temperature increases. Relevant theoretical calculations and analysis to the experimental curves were also carried out to understand diffusion and transport processes of charge carriers inside the device. The results show that the variation in temperature and reverse biases results in the structural change in the space charge region of the P-N junction and therefore affects the rise and decay time constants of the time-resolved photocurrent. The TRPL spectral data give information of mid-way radiative recombination of charge carriers in the device. / published_or_final_version / Physics / Master / Master of Philosophy

published_or_final_version / abstract / toc / Physics / Master / Master of Philosophy

published_or_final_version / Physics / Master / Master of Philosophy

The conduction band of InAs lies lower in energy than the GaSb valence band. In order to preserve continuity of the Fermi level across the interface, charge transfer takes place resulting in a confined quasi two dimensional electron gas (2DEG) in the InAs and a confined quasi two dimensional hole gas (2DHG) in the GaSb. This study is an investigation into the vertical transport in an n-InAs/p-GaSb single heterojunction (SHET). Application of a forward bias (InAs negative with respect to GaSb) increases the 2DEG and 2DHG concentrations and, therefore, their confinement energies. Eventually a critical bias is reached where the electron confinement energy moves above the hole confinement energy (the theoretical voltage induced semimetal/semiconductor transition V_c). Any subsequent increase in voltage is expected to result in a current decrease, and a region of negative differential resistance (NDR) should occur. The SHET can be grown with two distinct interface types, 'InSb-like' and 'GaAs-like'. This in turn affects the vertical transport characteristics of each type. Experimental IV traces at various pressures are compared with the corresponding results from sophisticated self-consistent band profile calculations taking into account band mixing effects for the first time through a k.p theory framework. Experimental IV traces of the SHETs under a magnetic field parallel to the interface are also compared with results from calculations that take into account the coupling of the growth and in-plane electron and heavy hole motions. Both sets of analysis support earlier conclusions that NDR occurs after V_c for both interfaces, and that each interface supports a different conduction mechanism. Evidence of multiple phonon processes occurring in both sample types is observed for the first time and is proposed to reconcile the above experimental observations with theory. This data is found to offer explanations for a number of other observations. Field perpendicular to the interface leads to the observation of features beyond the NDR region in both sample types. In samples with an 'InSb-like' interface, applying additional hydrostatic pressure leads to very strong features beyond the main NDR. Through a complex self-consistent decoupled model taking into account electrons and heavy holes, all these features are proposed to be due to a filling of an integer number of Landau levels. The band profile is predicted to alter dramatically at this point. The same model explains the observation of weaker features at 1 bar at high fields (~ 40T). A variation of NDR position is found with a rotation of an-plane field.

Thesis (M.S.)--Ohio University, August, 2000. / Title from PDF t.p.

Thesis: Elec. E., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 1975 / Includes bibliographical references. / Elec. E. / Elec. E. Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science