Thesis (Ph. D.)--Hong Kong University of Science and Technology, 2003. / Includes bibliographical references (leaves 102-112). Also available in electronic version. Access restricted to campus users.

Wang Hui. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2004. / Includes bibliographical references (leaves 78-84). / Abstracts in English and Chinese. / ABSTRACT (ENGLISH) --- p.i / ABSTRACT (CHINESE) --- p.iii / ACKNOWLEDGEMENTS --- p.iv / CONTENTS --- p.v / Chapter CHAPTER 1: --- INTRODUCTION --- p.1 / Chapter 1.1 --- Motivation behind This Study --- p.1 / Chapter 1.2 --- Concept of Quantum Dots --- p.2 / Chapter 1.2.1 --- Introduction to Semiconductors / Chapter 1.2.2 --- From Bulk Semiconductor to Quantum Dots / Chapter 1.3 --- Device Applications of Quantum Dots --- p.10 / Chapter 1.3.1 --- Laser Diodes / Chapter 1.3.2 --- Infrared Photodetectors / Chapter CHAPTER 2: --- METAL-ORGANIC VAPOR PHASE EPITAXY --- p.14 / Chapter 2.1 --- Introduction in Epitaxial Growth --- p.14 / Chapter 2.1.1 --- What's Epitaxy / Chapter 2.1.2 --- Heteroepitaxy Techniques / Chapter 2.2 --- MOVPE Growth System and Principles --- p.15 / Chapter 2.2.1 --- What's MOVPE / Chapter 2.2.2 --- MOVPE Chemistry and Basic Concepts / Chapter 2.2.3 --- Growth Regimes in MOVPE Process / Chapter CHAPTER 3: --- SELF-ASSEMBLED QUANTUM DOT GROWTH THEORY --- p.23 / Chapter 3.1 --- Strained Layer Heteroepitaxy --- p.23 / Chapter 3.2 --- Epitaxial Growth Modes --- p.24 / Chapter 3.2.1 --- Introduction / Chapter 3.2.2 --- Frank-van-der-Merve Mode / Chapter 3.2.3 --- Stranski-Krastanow Mode / Chapter 3.2.4 --- Volmer-Weber Mode / Chapter 3.3 --- Self-assembly of Quantum Dots --- p.30 / Chapter 3.4 --- Current Issues and Problems --- p.30 / Chapter CHAPTER 4: --- EXPERIMENTAL METHODS --- p.34 / Chapter 4.1 --- Equipment --- p.34 / Chapter 4.2 --- Preparation of Sample --- p.34 / Chapter 4.3 --- Growth Rate and Composition Determination --- p.35 / Chapter 4.4 --- Characterization Techniques --- p.40 / Chapter 4.4.1 --- Atomic Force Microscopy (AFM) / Chapter 4.4.2 --- Photoluminescence (PL) / Chapter 4.4.3 --- Other Techniques / Chapter CHAPTER 5: --- RESULTS AND DISCUSSION --- p.44 / Chapter 5.1 --- Introduction --- p.44 / Chapter 5.2 --- Formation Trends of In As QDs --- p.45 / Chapter 5.2.1 --- Experimental Procedures / Chapter 5.2.2 --- Results and Discussion / Chapter 5.2.2.1 --- Effect of Growth Temperature / Chapter 5.2.2.2 --- Effect of Growth Rate / Chapter 5.2.2.3 --- Effect of In As Coverage / Chapter 5.2.2.4 --- Effect of Buffer Layer Material / Chapter 5.2.3 --- Summary / Chapter 5.3 --- Annealing of InAs QDs under Dissimilar Ambient Flux --- p.65 / Chapter 5.3.1 --- Experimental Procedures / Chapter 5.3.2 --- Results and Discussion / Chapter 5.3.3 --- Summary / Chapter CHAPTER 6: --- CONCLUSIONS --- p.75 / Chapter 6.1 --- Summary --- p.75 / Chapter 6.2 --- Future Work --- p.77 / BIBLIOGRAPHY --- p.78 / PUBLICATION LIST --- p.84 / APPENDIX: Abbreviations --- p.85

Spins localized in semiconductor nanostructures have been intensively investigated for quantum spintronics. These include the spin of single electron localized by quantum dots or impurities, and spins of the lattice nuclei. These localized spins can be exploited as carriers of quantum information, while in some circumstances they also play the role of deleterious noise sources for other quantum objects through their couplings. Quantum control of the spins in semiconductor nanostructures is therefore of central interest for quantum applications. In this thesis, we address several problems related to the quantum control of electron or hole spin and nuclear spins in semiconductor quantum dots and impurity centers. The first problem studied is the control of nuclear spin bath for a hole spin qubit in III-V semiconductor quantum dot. In quantum dots formed on III-V compounds, the direct band gap of the host material allows ultrafast optical addressability of a single electron or hole spin qubit. However, nonzero nuclear spins of group III and group V elements result in a large statistical fluctuation in the Zeeman splitting of the spin qubit which then dephases in nanosecond time scale. We present a novel feedback scheme to suppress the statistical fluctuation of the nuclear spin field for enhancing the coherence time of the hole spin qubit. We also find positive feedback control which can amplify the magnitude of the nuclear field, so that a bimodal distribution can develop, realizing a quantum environment that can not be described by a single temperature. The second problem addressed here is the control of donor spin qubits in silicon architecture which have ultra-long quantum coherence time. We developed the quantum control scheme to realize the quantum metrology of magnetic field gradient, based on the celebrated Kane’s architecture for quantum computation. The scheme can also be generalized to calibrate the locations of the donors. In the third part of the thesis, we investigate a novel type of quantum dot formed in a new class of two-dimensional semiconductors, monolayer transition metal dichalcogenides (TMDs), which exhibit interesting spin and pseudospin physics. This novel quantum dot system may offer new opportunity for quantum spintronics in the ultimate 2D limit, and we investigate here the valley pseudospin as a possible quantum bit carrier. A main finding is that, contrary to the intuition, the lateral confinement by the quantum dot potential does not lead to noticeable valley hybridization, and therefore the valley pseudospin in monolayer TMDs QD can well inherit the valley physics such as the valley optical selection rules from the 2D bulk which implies a variety of quantum control possibilities. / published_or_final_version / Physics / Doctoral / Doctor of Philosophy

Here I report on our studies of the electronic properties of elliptical quantum dots in a perpendicular external magnetic field, and in the presence of the Rashba spin-orbit interaction. Our work indicates that the Fock-Darwin spectra, corresponding to the non-interacting electrons in an elliptical quantum dot display a strong signature of the Rashba spin-orbit coupling even in a low magnetic field, as the anisotropy of the quantum dot is increased. An explanation of this pronounced effect with respect to the anisotropy is presented. The strong spin-orbit coupling effect manifests itself prominently in the corresponding dipole-allowed optical transitions and hence is susceptible to direct experimental observation.

In this work we study the effect of built in electrostatic fields in Quantum Dots. Built-in electrostatic fields in Zincblende quantum dots originate mainly from--(1) the fundamental crystal atomicity and the interfaces between two dissimilar materials, (2) the strain relaxation, and (3) the piezoelectric polarization. We also study the geometric dependence of built in fields on 3 shapes namely Box, Dome and Pyramid. The main objectives are 3 fold they are (1) Explore the nature and the role of crystal atomicity at the interfaces and built-in fields (strain-field, and piezoelectric polarization) in determining the energy spectrum and the wave functions. (2) To identify the shift in the one-particle energy states, symmetry-lowering and non-degeneracy in the first excited state and strong band-mixing in the overall conduction band electronic states. (3) Finally geometric dependence of the above-mentioned phenomena. We discuss the importance atomistic effects and the need for 3 dimensional atomistic simulator NEMO 3D. We also discuss the effect of built in fields in HEMT (High Electron Mobility Transistor).

Thesis (Ph. D.)--University of Oregon, 1999. / Typescript. Includes vita and abstract. Includes bibliographical references (leaves 150-156). Also available for download via the World Wide Web; free to University of Oregon users. Address: http://wwwlib.umi.com/cr/uoregon/fullcit?p9947989.

Colon cancer (CC) ranks high in morbidity and mortality amongst the most frequent occurring cancers worldwide. Mortality rates are mostly caused by mis-diagnosis and the poor efficacy of treatment. The aim of this study was to enhance our insights of quantum dots, for early detection and targeted drug delivery, thereby reducing toxicity to normal cells and reducing side effects that are caused by previous colon cancer medicine. The synthesis, characterization and cytotoxicity studies of CdSe/ZnSe quantum dots (QDs), nanocrystals are reported. Toxicological properties of the Cd2+ core are reduced by capping quantum dots with ZnSe, varying chain length and functional group ligands. Fluorescence wavelength and their size is improved by varying Cd2+ source and varying nanocrystal synthesis growth temperature. CdSe/ZnSe quantum dots are characterized with FT-IR to elucidate their structure. High-resolution transmission electron microscopy (HRTEM), X-Ray Diffraction (EDX), Photoluminescence spectroscopy (PL) and Ultraviolet-visible spectroscopy (UV-Vis) are used to measure their size and composition. Ligand exchange reactions are conducted with the use of 3-Mercaptopropanoic acid (3-MPA) to facilitate bio-compatibility and stability of CdSe/ZnSe QDs. Temperature stability of various ligand capped and stabilized CdSe/ZnSe QDs are measured by using thermogravimetric analysis (TGA). Caco-2 cell line is cultured from colon cancer, and cytotoxic studies are conducted to test for cell viability of various capped 3-Mercaptopropanoic acid (3-MPA) CdSe/ZnSe quantum dots at various concentrations. Myristic acid capped CdSe/ZnSe quantum dots produce high fluorescing mono-disperse quantum dots. The capping material, synthesis temperature and Cd2+ source of CdSe/ZnSe QDs affect fluorescence wavelength and thermal stability of quantum dots. Fluorescence wavelength is improved by using CdCl2.7H2O source of Cd2+. Cytotoxicity was found to be dependent on the concentration and the capping material of quantum dots. CdSe/ZnSe quantum dots toxicity is adjusted and reduced by varying the length, size and type of the capping ligand on the surface of quantum dots.

This thesis investigates the carrier and gain dynamics of semiconductor quantum dots in the strong quantum confinement regime (i.e. the dot radius is smaller than the bulk excitonic Bohr radius). The materials under investigation are InP and CdSe. We can summarize our findings as follows: For the first time, the quantum confined ground state in InP quantum dots has been observed at room temperature by femtosecond spectral holeburning. This is the first confirmation of the observation of a strongly confined quantum dot made of III-V semiconductor materials. In CdSe quantum dots with a radius of half the bulk exciton Bohr radius, the carrier and gain dynamics have been investigated. The predicted phonon bottleneck, which should slow down carrier relaxation up to nanoseconds, has not been found. The carrier relaxation rates are rather on the order of 1 eV/ps. Gain has been found for the first time in strongly confined quantum dots. The existence of gain was proven by spectral holeburning in the gain region. The gain buildup and decay dynamics have been studied on a femtosecond and picosecond timescale. A multi-level model including biexcitons accounts for the gain formation. The model has been confirmed by three-beam spectral holeburning experiments and femtosecond pump-probe experiments with circularly polarized light. Some quantum dots did not show gain under high optical excitation but instead exhibited photodarkening. The carrier separation and localization dynamics of this photodarkening process has been studied on a femtosecond timescale. For the first time, the shift of the bleaching towards lower energy during the localization process could be observed on a femtosecond timescale. Finally, pulse propagation in bulk CdSe at multiple Pi-pulses has been studied. For the first time, strong evidence for the observation of self induced transparency in semiconductors has been found. Also, optical precursors, probably of nonlinear nature, have been found.

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