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The purpose of this thesis is to investigate the operation of GaAs field-effect transistors with particular attention to the existence of negative resistance regions in the current-voltage characteristics, velocity overshoot effects, the role of substrate, and the role of heterojunctions. The approach used is to solve the electron transport equation using the Monte Carlo method which accounts for non-local effects in electron transport. Arguments are presented to support the contention that the negative resistance regions in the current-voltage characteristics observed in some experimental devices and produced by other researchers' computer simulations are attributed, in part, to the negative differential mobility of GaAs. The main reason of the existence of this negative resistance is related to the active layer thickness and it will be explained in terms of the rotation of the velocity vector. Electron velocity overshoot, a consequence of non-local effects, is examined in terms of gate length. The velocity overshoot becomes significant for FET structures with gates less than a micron in length and has many significant effects on the device performance. It is found also that velocity overshoot accounts for the undesirable saturation characteristics of submicron gate length GaAs FET which are observed in practical devices. However, it was also found that the presence of a low-doped n-type GaAs substrate below the active layer removes the negative resistance regions in the current-voltage characteristics. This is attributed to the effect of carrier injection from the active layer into the substrate which leads to the decrease of the effective channel thickness. This then will decrease the transconductance of the device, increase the gate pinchoff voltage and lower the device frequency response. This degradation of device's performance depends entirely on the purity and properties of the substrate. The performance of substrated FETs can be improved by preventing electron penetration into the substrate. This situation can be reached by using AlGaAs substrate whose energy band gap is higher than that of GaAs which then leads to electron confinement in the active layer. The use of AlGaAs in FETs can be in different forms. These will also be demonstrated in this thesis.

Cheung, Wing Kin = 應變及間隔效應對La0.67Sr0.33MnO3摻雜Pr0.67Ca0.33MnO3的傳導特性的影響之研究 / 張榮健. / On t.p. "0.67", "0.33" and "3" are subscripts. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2007. / Includes bibliographical references (leaves 89-93). / Text in English; abstracts in English and Chinese. / Cheung, Wing Kin = Ying bian ji jian ge xiao ying dui La0.67Sr0.33MnO3 shan za Pr0.67Ca0.33MnO3 de chuan dao te xing de ying xiang zhi yan jiu / Zhang Rongjian. / Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- Development of magnetoresistance materials --- p.1 / Chapter 1.2 --- What is magnetoresistance? --- p.1 / Chapter 1.2.1 --- Anisotropic magnetoresistance --- p.2 / Chapter 1.2.2 --- Giant magnetoresistance (GMR) --- p.3 / Chapter 1.2.3 --- Colossal magnetoresistance (CMR) --- p.4 / Chapter 1.3 --- Possible origins of CMR in manganites --- p.6 / Chapter 1.3.1 --- Tolerance factor --- p.6 / Chapter 1.3.2 --- Double exchange mechanism --- p.8 / Chapter 1.3.3 --- Jahn-Teller distortion --- p.11 / Chapter 1.3.4 --- Charge-ordering and phase separation --- p.13 / Chapter 1.4 --- Movtiation --- p.17 / Chapter 1.5 --- Literature review --- p.18 / Chapter 1.5.1 --- Single-layer manganite thin films --- p.18 / Chapter 1.5.2 --- Multilayer system - manganites with insulating spacers --- p.19 / Chapter 1.6 --- Scope of thesis --- p.20 / Chapter 2 --- Instrumentation --- p.21 / Chapter 2.1 --- Sample preparation --- p.21 / Chapter 2.1.1 --- Facing-target sputtering technique (FTS) --- p.21 / Chapter 2.1.2 --- Deposition system --- p.24 / Chapter 2.2 --- Annealing system --- p.26 / Chapter 2.2.1 --- Oxygen annealing --- p.26 / Chapter 2.3 --- Sample characterization --- p.28 / Chapter 2.3.1 --- X-ray diffraction (XRD) --- p.28 / Chapter 2.3.2 --- Alpha - step profiler --- p.30 / Chapter 2.3.3 --- Transport property measurement --- p.30 / Chapter 3 --- Preparation and characterization of single-layer thin films --- p.32 / Chapter 3.1 --- Introduction --- p.32 / Chapter 3.2 --- Fabrication and characteristization of the sputtering targets --- p.32 / Chapter 3.3 --- Deposition procedure --- p.36 / Chapter 3.3.1 --- Preparation of substrate --- p.36 / Chapter 3.3.2 --- Deposition process --- p.36 / Chapter 3.4 --- Parameters related to epitaxial growth of LPSCMO thin films --- p.37 / Chapter 3.4.1 --- Substrate materials --- p.37 / Chapter 3.4.2 --- Substrate temperature --- p.38 / Chapter 3.4.3 --- Oxygen partial pressure --- p.38 / Chapter 4 --- Thickness and strain effects in epitaxial LPSCMO thin films --- p.43 / Chapter 4.1 --- Introduction --- p.43 / Chapter 4.2 --- Structural characterization of LPSCMO thin films --- p.45 / Chapter 4.2.1 --- Thickness effect --- p.45 / Chapter 4.2.2 --- Strain effect --- p.49 / Chapter 4.3 --- Transport properties and magnetoresistance measurement --- p.51 / Chapter 4.3.1 --- Thickness effect --- p.51 / Chapter 4.3.2 --- Strain effect --- p.60 / Chapter 4.4 --- Discussion --- p.63 / Chapter 5 --- Effect of spacer in [LPSCMO/SCuO] multilayer --- p.65 / Chapter 5.1 --- Sample preparation --- p.65 / Chapter 5.2 --- Characterization of as-deposited multilayer samples --- p.67 / Chapter 5.2.1 --- Structural anaylsis --- p.67 / Chapter 5.2.2 --- Resistance measurement --- p.72 / Chapter 5.3 --- Oxygen-annealing treatment --- p.76 / Chapter 5.3.1 --- Introduction --- p.76 / Chapter 5.3.2 --- Structural anaylsis --- p.77 / Chapter 5.3.3 --- Resistance measurement --- p.78 / Chapter 5.4 --- Discussion --- p.85 / Chapter 6 --- Conclusion --- p.87 / Chapter 6.1 --- Summary --- p.87 / Chapter 6.2 --- Future outlook --- p.88 / Bibliography --- p.89

A first approach to engineering problems in nanosized systems requires a thorough understanding of how physical properties change as size decreases from the macroscale. One important class of properties that can be severely affected by such a downward size shift are transport properties - classical mass, momentum and energy transport. Using atomistic simulation techniques, primarily molecular dynamics, and statistical expressions for diffusion, viscosity, and thermal conductivity formulated in terms of atomistic properties, three case studies of transport in important, nanosized systems are investigated, including confined water systems, silicon-germanium nanos- tructures, and carbon nanostructures. In the first study of confined water systems, diffusion and viscosity are of primary interest, as recent experimental studies have shown notably increased rates of diffusion through nano-confined carbon nanotube structures. In this work, a full treatment of the transport properties is provided in both water clusters and water thin films, both having characteristic size scales under 11 nm. The diffusion, viscosity, and thermal conductivity in the nanosized systems are all shown to be significantly different from bulk water systems, with diffusion and thermal transport increasing and viscosity decreasing. For silicon-germanium nanostructures, the thermal transport properties are exclusively considered, with the problem of interest concerning the control of thermal transport through a strict control on the nanostructure. Quantum dot superlattices are shown to be effective structures for controlling the thermal transport properties, the available range of thermal conductivity using these structures being 0.1-160 W/mK. The final study concerns graphene nanostructures, which in terms of thermal transport have some of the highest thermal conductivities of any available materials. Control of thermal transport properties is again of primary importance, with various physical aspects - defects, shape, and size - being probed in graphene, graphene nano ribbons, carbon nanotubes, and fullerenes to determine their influence on transport; overall, these structures yield a large range of thermal transport, 10-2500 W/mK.

published_or_final_version / abstract / Chemistry / Doctoral / Doctor of Philosophy

Kong, Yu Chau = La₂/₃Ca₁/₃MnO₃/Y₂O₃和La₂/₃Ca₁/₃MnO₃/Gd₂O多層薄膜及其三文治結構的傳導特性之研究 / 江如秋. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2009. / Includes bibliographical references. / Abstract also in Chinese. / Kong, Yu Chau = La₂/₃Ca₁/₃MnO₃/Y₂O₃ he La₂/₃Ca₁/₃MnO₃/Gd₂O duo ceng bo mo ji qi san wen zhi jie gou de chuan dao te xing zhi yan jiu / Jiang Ruqiu. / Abstract --- p.i / 論文摘要 --- p.ii / Acknowledgements --- p.iii / Table of Contents --- p.iv / List of Figures --- p.vii / List of Tables --- p.x / Chapter Chapter 1 --- Introduction / Chapter 1.1 --- Introduction to Magnetoresistance --- p.1 / Chapter 1.1.1 --- Anisotropic magnetoresistance (AMR) --- p.3 / Chapter 1.1.2 --- Giant magnetoresistance (GMR) --- p.3 / Chapter 1.1.3 --- Colossal magnetoresistance (CMR) --- p.5 / Chapter 1.1.4 --- Tunneling magnetoresistance (TMR) --- p.7 / Chapter 1.2 --- Possible origins of CMR in doped manganite --- p.8 / Chapter 1.2.1 --- Double exchange mechanism --- p.10 / Chapter 1.2.2 --- Jahn-Teller (JT) distortion --- p.14 / Chapter 1.2.3 --- Tolerance factor --- p.16 / Chapter 1.2.4 --- Charge Ordering and percolation theory --- p.18 / Chapter 1.3 --- Motivation --- p.21 / Chapter 1.4 --- Literature Review --- p.23 / Chapter 1.4.1 --- Multilayers --- p.23 / Chapter 1.4.2 --- Junction --- p.25 / Chapter 1.5 --- Scope of this thesis --- p.26 / References --- p.27 / Chapter Chapter 2 --- Instrumentation / Chapter 2.1 --- Thin film deposition --- p.30 / Chapter 2.1.1 --- Facing target sputtering (FTS) --- p.31 / Chapter 2.1.2 --- Vacuum system --- p.33 / Chapter 2.2 --- Oxygen annealing --- p.35 / Chapter 2.3 --- Characterization --- p.35 / Chapter 2.3.1 --- Alpha step profilometer --- p.35 / Chapter 2.3.2 --- X-ray diffraction (XRD) --- p.36 / Chapter 2.3.3 --- Transport measurement --- p.39 / Reference --- p.41 / Chapter Chapter 3 --- Epitaxial growth of La2/3Ca1/3MnO3 and rare earth oxide single layer thin films / Chapter 3.1 --- Introduction --- p.42 / Chapter 3.2 --- Preparation of sputtering targets of rare earth metal and La2/3Ca1/3MnO3 --- p.42 / Chapter 3.3 --- Epitaxial growth of LCMO and Re2O3 thin film --- p.44 / Chapter 3.3.1 --- Substrate materials --- p.44 / Chapter 3.3.2 --- Deposition conditions --- p.45 / Chapter 3.3.3 --- Deposition procedures --- p.48 / Chapter 3.4 --- Characterization of single layer thin films --- p.49 / Chapter 3.5 --- Discussion --- p.52 / References --- p.53 / Chapter Chapter 4 --- LCMO/Y2O3 and LCM0/Gd203 multilayers / Chapter 4.1 --- Sample preparation --- p.54 / Chapter 4.2 --- As-deposited multilayers --- p.56 / Chapter 4.2.1 --- Structural Characterization --- p.56 / Chapter 4.2.2 --- Transport properties --- p.62 / Chapter 4.3 --- Oxygen annealing --- p.67 / Chapter 4.3.1 --- Introduction --- p.67 / Chapter 4.3.2 --- Post-annealing condition --- p.68 / Chapter 4.4 --- Oxygen post-annealed multilayers --- p.69 / Chapter 4.4.1 --- Structural Characterization --- p.69 / Chapter 4.4.2 --- Transport properties --- p.76 / Chapter 4.5 --- Discussion --- p.85 / References --- p.86 / Chapter Chapter 5 --- LCMO/Y2O3/LCMO trilayer sandwiches / Chapter 5.1 --- Introduction --- p.88 / Chapter 5.2 --- Sample preparation --- p.89 / Chapter 5.3 --- Current-Voltage (I-V) measurement --- p.90 / References --- p.92 / Chapter Chapter 6 --- Conclusion / Chapter 6.1 --- Summary --- p.93 / Chapter 6.2 --- Further work --- p.94

Thermoelectric devices allow for direct conversion of heat into electricity as well as solid-state refrigeration. The skutterudite family of compounds continues to be of considerable interest both scientifically and technologically due to their unique physical properties, in particular as promising thermoelectric materials. In this thesis, the basic thermoelectric phenomena and some background history on skutterudites will be reviewed. Rhombohedral derivatives of the cubic skutterudite CoSb3, namely Co4-xFexGe6Se6 with x=0, 1, 1.5 (p-type) and rare-earth filled Ce0.13Co4Ge6Se6 and Yb0.14Co4Ge6Se6 (n-type), were synthesized and their synthesis and low temperature transport properties will be discussed. Reitveld refinement and elemental analysis were used to identify the structure and stoichiometry of these compositions. Both Fe substitution and rare-earth filling reduced the thermal conductivity compared with Co4Ge6Se6 skutterudite derivative. In addition the electrical and thermal properties of these compounds are greatly affected by doping. This fundamental investigation reveals new insight and is intended as part of the continuing effort to explore different skutterudite compositions and structure types for potential thermoelectric applications.

Thermoelectric power generators can convert a temperature gradient into electrical energy, serving as a new energy resource by utilizing solar energy or by utilizing more waste heat. Thermoelectric coolers have the advantage of no moving parts, are quiet and release no gases that are harmful to the atmosphere, in contrast to compression-based refrigeration. While the low efficiency of “classical” thermoelectric devices limits their wide applications, the exploration of better thermoelectric materials is of great importance to improve the efficiency of thermoelectric devices. Good thermoelectric materials are usually narrow band gap semiconductors with a large Seebeck coefficient, reasonably high electrical conductivity and low thermal conductivity. This thesis deals with the exploration of new thermoelectric materials based on transition metal tellurides and the optimization of bulk materials based on oxides of low toxicity and high stability in air. In the first project, seven new ternary or quaternary tellurides, crystallizing in three different structure types, were synthesized and characterized. Single crystal X-ray diffraction was used for crystal structure determination; powder X-ray diffraction and energy dispersive X-ray analysis (EDAX) were used for phase and composition analyses. Physical properties of these compounds were predicted by electronic structure calculations and confirmed by physical property measurements. In the second project, two series of n-type doped perovskite SrTiO3 were prepared in a high temperature tube furnace under dynamic high vacuum of the order of 10–6 mbar, namely SrTi1-x(Nb,Ta)xO3, and Sr1-xLaxTi1-x(Nb,Ta)xO3. The phase purity was characterized by means of powder X-ray diffraction and electron probe micro analysis (EPMA). Rietveld refinements were performed to check for purity and symmetry reduction. The physical properties, such as Seebeck coefficient, electrical conductivity, and thermal conductivity, were measured at high temperatures for all the samples. Of the series of Nb/Ta-doped strontium titanates SrTi1-x(Nb,Ta)xO3, SrTi0.90Ta0.10O3 exhibits the highest ZT value, namely 0.17 for at 752 K. Of the double substituted series, Sr0.99La0.01Ti0.99Ta0.01O3 was best with ZT = 0.13 at 660 K. The rapid increases imply that higher ZT values are likely to occur at higher temperatures.

Thermoelectric power generators can convert a temperature gradient into electrical energy, serving as a new energy resource by utilizing solar energy or by utilizing more waste heat. Thermoelectric coolers have the advantage of no moving parts, are quiet and release no gases that are harmful to the atmosphere, in contrast to compression-based refrigeration. While the low efficiency of “classical” thermoelectric devices limits their wide applications, the exploration of better thermoelectric materials is of great importance to improve the efficiency of thermoelectric devices. Good thermoelectric materials are usually narrow band gap semiconductors with a large Seebeck coefficient, reasonably high electrical conductivity and low thermal conductivity. This thesis deals with the exploration of new thermoelectric materials based on transition metal tellurides and the optimization of bulk materials based on oxides of low toxicity and high stability in air. In the first project, seven new ternary or quaternary tellurides, crystallizing in three different structure types, were synthesized and characterized. Single crystal X-ray diffraction was used for crystal structure determination; powder X-ray diffraction and energy dispersive X-ray analysis (EDAX) were used for phase and composition analyses. Physical properties of these compounds were predicted by electronic structure calculations and confirmed by physical property measurements. In the second project, two series of n-type doped perovskite SrTiO3 were prepared in a high temperature tube furnace under dynamic high vacuum of the order of 10–6 mbar, namely SrTi1-x(Nb,Ta)xO3, and Sr1-xLaxTi1-x(Nb,Ta)xO3. The phase purity was characterized by means of powder X-ray diffraction and electron probe micro analysis (EPMA). Rietveld refinements were performed to check for purity and symmetry reduction. The physical properties, such as Seebeck coefficient, electrical conductivity, and thermal conductivity, were measured at high temperatures for all the samples. Of the series of Nb/Ta-doped strontium titanates SrTi1-x(Nb,Ta)xO3, SrTi0.90Ta0.10O3 exhibits the highest ZT value, namely 0.17 for at 752 K. Of the double substituted series, Sr0.99La0.01Ti0.99Ta0.01O3 was best with ZT = 0.13 at 660 K. The rapid increases imply that higher ZT values are likely to occur at higher temperatures.