published_or_final_version / abstract / Physics / Master / Master of Philosophy

He, Zikai. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2013. / Includes bibliographical references. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract also in Chinese.

In this thesis work, a novel low level birefringence detection (LLBD) system operating at 1150 nm was set up based on the photoelastic modulation techniques. The noise level of current LLBD system is about 0.03° and the maximum fluctuation of data in ten measurements is 0.05° and close to the noise level of system. With a slit confining the light, the spatial resolution of the system is 10 mum. Optical orientation and retardation can be simultaneously measured by this system, making the stress measurement more convenience than the traditional PE methods. These peculiar features make this system capable of investigating the details of stress distribution in semiconductor structures. / Our solution reveals that the nonlinear stress filed is responsible for the shift of the zero stress point. Further it is indicated that the classical Stoney formula can either overestimate or underestimate the actual film stress due to the process induced nonlinear stress in the substrate. / Stress and strain play an essential role in determining the structural, electrical, and optical properties of semiconductor materials, and, ultimately, the semiconductor device performance. Many methods have been utilized to measure the stress in semiconductors. Among them infrared photoelasticity method is a promising one, which can be used both in the industrial characterization and scientific research. This thesis is an endeavor in this subject matter and will present our research results of studying the stress problems in semiconductor structures by using infrared photoelasticity method. / The LLBD system was applied to measure the stress distributions in the substrates of SiO2/Si structure. Some deviations from the classical film theory were observed in our experiments. For example non-linear stress fields were observed in all samples whatever their process conditions are. Besides the locations of neutral axis (zero stress point) was not located at the depth of 2/3 thickness of the substrate from the interface as expected by the bi-metallic theory. To interpret these deviations, a theoretical analysis was given to investigate the problem of stress distribution in film/substrate structure. A series of solutions were deduced to modify the Stoney formula and bi-metallic strip theory with the consideration of the nonlinear stress in substrate. / by Liu Xianghua. / "Feb 2006." / Adviser: Sai Peng Wong. / Source: Dissertation Abstracts International, Volume: 67-11, Section: B, page: 6674. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2006. / Includes bibliographical references. / 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.

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published_or_final_version / abstract / Physics / Doctoral / Doctor of Philosophy

published_or_final_version / Physics / Master / Master of Philosophy

The Semiconductor Industry Association (SIA) has identified the integration of copper (Cu) with low-dielectric-constant (low-k) materials as a critical goal for future interconnect architectures. A fundamental understanding of the chemical interaction of Cu with various substrates, including diffusion barriers and adhesion promoters, is essential to achieve this goal. The objective of this research is to develop novel organic polymers as Cu/low-k interfacial layers and to investigate popular barrier candidates, such as clean and modified tantalum (Ta) substrates. Carbon-silicon (C-Si) polymeric films have been formed by electron beam bombardment or ultraviolet (UV) radiation of molecularly adsorbed vinyl silane precursors on metal substrates under ultra-high vacuum (UHV) conditions. Temperature programmed desorption (TPD) studies show that polymerization is via the vinyl groups, while Auger electron spectroscopy (AES) results show that the polymerized films have compositions similar to the precursors. Films derived from vinyltrimethyl silane (VTMS) are adherent and stable on Ta substrates until 1100 K. Diffusion of deposited Cu overlayers is not observed below 800 K, with dewetting occurred only above 400 K. Hexafluorobenzene moieties can also be incorporated into the growing film with good thermal stability. Studies on the Ta substrates demonstrate that even sub-monolayer coverages of oxygen or carbide on polycrystalline Ta significantly degrade the strength of Cu/Ta chemical interactions, and affect the kinetics of Cu diffusion into bulk Ta. On clean Ta, monolayer coverages of Cu will de-wet only above 600 K. A partial monolayer of adsorbed oxygen (3L O2 at 300 K) results in a lowering of the de-wetting temperature to 500 K, while saturation oxygen coverage (10 L O2, 300 K) results in de-wetting at 300 K. Carbide formation also lowers the de-wetting temperature to 300 K. Diffusion of Cu into the Ta substrate at 1100 K occurs only after a 5-minute induction period. This induction period increases to 10 min for partially oxidized Ta, 15 min for carbidic Ta and 20 min for fully oxidized Ta.

Semiconductor devices based on III-V materials have been the focus of intense research due to their superior electron mobility and favorable energy direct bandgap which are applicable in infrared wavelength range optoelectronics and high speed electronic systems. The thesis presented here consists of two thrusts; the first focusing on infrared applications, and the second focusing on InP-based heterojunction bipolar transistors (HBTs). In the first thrust, we investigate type-II InAs/GaSb superlattice IR detector devices and the effect of substrate orientation on InSb and InAs nanostructure morphology. In the second thrust, we study InP-based high frequency HBTs. A low resistance InAs ohmic contact is demonstrated, and we presented along with a study of the crystalline qualities in GaAs0.5Sb0.5 films grown on tilted- axis InP substrates. Chapter 2 presents fabrication and characterization of two type-II superlattice structures with 15 monolayer (ML) InAs/12ML GaSb and 17ML InAs/7ML GaSb grown on GaSb (100) substrates by solid-source molecular beam epitaxy (MBE). The X-ray diffraction (XRD) measurements of both the 15ML InAs/12ML GaSb and 17MLInAs/7ML GaSb superlattices indicated excellent material and interface qualities. The cutoff wavelengths of 15ML InAs/12ML GaSb and 17ML InAs/7ML GaSb superlattices photodetectors were measured to be 6.6μm and 10.2μm, respectively. These different spectral ranges were achieved by growing alternating layers of varying thicknesses which allowed for bandgap engineering of the superlattices of InAs and GaSb. Lastly, a mid-IR type-II superlattice photodiode was demonstrated at 80K with a cutoff wavelength at 6.6µm. The device exhibited a near background limited performance (BLIP) detectivity at 80K and higher temperature operation up to 280K. In Chapter 3, we show that the (411) orientation, though not a naturally occurring surface, is a favorable orientation to develop a buffer layer into a super flat surface at a certain high growth temperature. The (411) surface is a combination of localized (311) and (511) surfaces but at a high growth temperature, adatoms can obtain enough energy to overcome the energy barrier between these localized (311) and (511) surfaces and form a uniform (411) surface with potential minima. This results in a super flat surface which is promising for high-density nanostructure growth. In this work, this is the first time that the highest InSb and InAs nanostructures density can be achieved on the (411) surface which is in comparison with the (100), (311), and (511) surfaces. Chapter 4 of this thesis addresses the use of an InAs layer as a low-resistance ohmic contact to InP-based heterostructure devices. Selective area crystal growth of InAs on a dielectric (Benzocyclobutene, BCB polymer) covered InP (100) substrate and direct growth of InAs on InP substrate were performed by MBE. Heavy doping of InAs using Te was carried out to determine the lowest sheet resistance. Based on scanning electron microscope (SEM) and XRD measurements, increasing substrate temperature from 210 ℃ to 350 ℃, led to an improvement in crystallinity from a polycrystalline layer to a single crystal layer with a corresponding improvement of surface morphology. Moreover, a narrow X-ray diffraction peak indicated full-relaxation of the inherent 3.3% lattice-mismatch in InAs/InP layers. Furthermore, around 290 ℃ a tradeoff was reached between crystallinity and optimized dopant incorporation of Te into InAs for the lowest sheet resistance. Lastly, Chapter 5 discusses the effect of substrate tilting on the material properties of MBE grown GaAsSb alloys closely lattice-matched to an InP substrate. InP(100) substrates tilted 0°off-(on-axis), 2°off-, 3°off-, and 4°off-axis were used for MBE growth; then the material qualities of GaAsSb epitaxial layers were compared using various techniques, including high resolution XRD, photoluminescence (PL) and transmission-line measurements (TLM). Substrate tilting improved the crystalline quality of the GaAsSb alloys, as shown by a narrower XRD linewidth and enhanced optical quality as evidenced by a strong PL peak. The results of TLM show that the lowest sheet resistance was achieved at a 2° off-axis tilt.

The need for efficient, cheap, and durable semiconductors for photovoltaic and optoelectronic applications has spurred a number of dramatic recent developments in semiconductor quantum physics. Aided by advanced synthetic and characterization techniques, the development of high-quality, nano-structured, tunable materials has resulted in the observation of many novel phenomena. The goal of this thesis is to develop and apply methods in theoretical condensed matter science to the study of these promising materials. In Chapter 1 I explore methylammonium lead iodide (MAPbI3), a paradigmatic hybrid organic-inorganic perovskite system. To explain charge carrier dynamics in this material, I develop a microscopic tight-binding model. The average band structure is calculated and the magnitude of the temperature-dependent band gap opening and Urbach energy is quantified. The charge carrier mobility is calculated within a linear response formalism and its temperature dependence is characterized. Overall, the fully ab initio model is found to explain several non-trivial experimental phenomena while making minimal assumptions concerning the nature of the electron-phonon coupling and the character of the nuclear motion in these materials. In Chapters 2 and 3, I turn to the subject of atomically-thin transition metal dichalcogenides. I improve upon past variational calculations of exciton and trion binding energies in these materials by applying diffusion Monte Carlo to exactly calculate exciton, trion, and biexciton binding energies within an effective few-body Hamiltonian. Carriers are assumed to experience two-body interactions of the Keldysh type that have been parameterized previously from electronic structure calculations. The structures of the exact ground state wavefunctions are calculated and compared to those of the previous variational trial wavefunctions. Next, I calculate the doping dependence of the rate of exciton and trion elastic scattering with free electrons within first-order time-dependent perturbation theory. The calculation provides the first theoretical estimate of the intrinsic trion linewidth in these materials. Finally, in Chapter 4, I study variants of the GW approximation to the one-particle Green's function for calculating correlation energies and spectral weights for the three-dimensional homogeneous electron gas. By relating the cumulant generating function to the improper GW self-energy, I develop a new cumulant-based GW approximation. The approach is compared to existing methods first via solution of a simple linearly-coupled electron phonon model and later through application to the electron gas problem.

Biocompatible anatase TiO2 single-crystals with 27 % -- 50 % chemically reactive facets were obtained in 90 minutes by using a microwave-assisted method. The preparation involved an aqueous solution of titanium tetrafluoride and an ionic liquid (1-methyl-imidazolium tetrafluoroborate). The as-obtained TiO2 single-crystals exhibited a truncated tetragonal bipyramidal shape. By simply changing the concentration of the ionic liquid, the level of reactive facets can be continuously tuned from 27 % to 50 %. The use of microwave heating is critical as it allows rapid and uniform heating of the reaction mixture. The TiO2 single-crystals were characterized by XRD, TEM, XPS and FESEM. The products exhibited excellent photocatalytic efficiency for both oxidation of nitric oxide in air and degradation of organic compounds in aqueous solution under UV light irradiation. The relationship between the physicochemical properties and the photocatalytic performance of the samples was discussed. The TiO2 single-crystals were found to be nontoxic using Zebrafish (D. rerio) as a model. / Bismuth oxyhalide semiconductors (BiOBr, BiOCl) with marigold-like open architectures were also prepared by a solvothermal method involving imidazolium-based ionic liquids and ethylene glycol. The 3D self-assembled marigold-like materials were effective photocatalysts for degrading organic pollutants and generating hydrogen. The main advantages of the new materials were large surface area, high surface-to-bulk ratio, facile species transportation, and ease of recovery and regeneration. / By using a microwave-assisted hydrothermal method involving titanium tetrafluoride and a tetrafluoroborate-based ionic liquid (1-butyl-3-methyl-imidazolium-tetrafluoroborate), a micro-sheet anatase TiO2 single crystal photocatalyst with remarkable 80 % reactive facets was synthesized. The as-obtained TiO2 single-crystal exhibited a truncated tetragonal bipyramidal shape. The high reactivity of facets made these single crystals highly photocatalytically active. They were easily recyclable and thermally stable up to 800 °C. / Furthermore, a simple and environmentally benign approach for the synthesis of photocatalytically active rutile TiO2 mesocrystals was developed. It was a microwave-assisted hydrothermal method involving titanium(III) chloride as the only reactant. The resulting 1D rutile nanowires could easily assemble into 3D hierarchical architectures without the help of surfactants or additives. The average aspect ratio for the nanowires was 267. The BET specific surface area of the mesocrystal was 16 m2/g. / Part I: Size-tunable monodispersed hierarchical metallic Ni nanocrystals (58-190 nm in diameter) were prepared by the reduction of Ni2+ with hexadecylamine under atmospheric pressure. The diameter of the particles could be tuned by simply changing the reaction time. A reaction mechanism was proposed and the relationships between the size, hierarchical surfaces and the magnetic properties were investigated. The as synthesized Ni crystals exhibited higher coercivities than the bulk metallic material owing to the reduced size and the hierarchical surface structure. The saturation magnetization (Ms) and the ratio of remanence to saturation (Mr/Ms) increased with increasing particle size. / Part II: A facile microwave-assisted solvothermal method was developed for the controlled synthesis of novel 3D CdS structures. Dendrite-, star-, popcorn- and hollow sphere-like CdS structures could be obtained by changing the reaction conditions including the reaction temperature and the amounts of reagents and solvents. The results revealed that the final structures were related to the solvent properties such as surface tension and viscosity. The degree of supersaturation was also responsible for the morphology variation and it could be adjusted by the reaction temperature. The CdS products with different morphologies exhibited interesting shape-dependent optical properties and photocatalytic activities. / The optical band energy of the product exhibited an obvious red-shift of 0.2 eV with aspect to that of pure rutile TiO2. This red-shift effect may be ascribed to the high aspect ratio of the rutile nanowires. The products showed excellent photocatalytic activity for NO removal in air and the activity was well maintained after three cycles. Gold modification on the rutile TiO2 resulted in a 50 % improvement in the photocatalytic performance. / This thesis focuses primarily on the preparation of various functional materials with controllable structures and properties. The first part describes the synthesis of materials by solvothermal methods. The second part describes the rapid fabrication of novel semiconductor materials by microwave-assisted methods. / Zhang, Dieqing. / Source: Dissertation Abstracts International, Volume: 72-04, Section: B, page: . / Thesis (Ph.D.)--Chinese University of Hong Kong, 2010. / Includes bibliographical references (leaves 189-190). / 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 Company, [200-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract also in Chinese.