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The ability to manipulate material response to dynamical processes depends on the extent of understanding of transport properties and their variation with chemical and structural features in materials. In this perspective, current work focuses on the thermal and electronic transport behavior of technologically important bulk and nanomaterials. Strontium titanate is a potential thermoelectric material due to its large Seebeck coefficient. Here, first principles electronic band structure and Boltzmann transport calculations are employed in studying the thermoelectric properties of this material in doped and deformed states. The calculations verified that excessive carrier concentrations are needed for this material to be used in thermoelectric applications. Carbon- and boron nitride-based nanomaterials also offer new opportunities in many applications from thermoelectrics to fast heat removers. For these materials, molecular dynamics calculations are used to evaluate lattice thermal transport. To do this, first, an energy moment term is reformulated for periodic boundary conditions and tested to calculate thermal conductivity from Einstein relation in various systems. The influences of the structural details (size, dimensionality) and defects (vacancies, Stone-Wales defects, edge roughness, isotopic disorder) on the thermal conductivity of C and BN nanostructures are explored. It is observed that single vacancies scatter phonons stronger than other type of defects due to unsatisfied bonds in their structure. In pristine states, BN nanostructures have 4-6 times lower thermal conductivity compared to C counterparts. The reason of this observation is investigated on the basis of phonon group velocities, life times and heat capacities. The calculations show that both phonon group velocities and life times are smaller in BN systems. Quantum corrections are also discussed for these classical simulations. The chemical and structural diversity that could be attained by mixing hexagonal boron nitride and graphene provide further avenues for tuning thermal and electronic properties. In this work, the thermal conductivity of hybrid graphene/hexagonal-BN structures: stripe superlattices and BN (graphene) dots embedded in graphene (BN) are studied. The largest reduction in thermal conductivity is observed at 50% chemical mixture in dot superlattices. The dot radius appears to have little effect on the magnitude of reduction around large concentrations while smaller dots are more influential at dilute systems.

This dissertation covers the study of the thermoelectric properties of GeTe and SnTe. The goal of this research is to develop high-performance lead-free thermoelectric materials that can replace PbTe-based systems so that thermoelectric technology could be bring into real application. During the study, extensive investigations on the electrical and thermal transport behaviors were conducted both experimentally and theoretically. In Chapter 1 ~ 3, the origin of thermoelectricity, modelling and characterization methods are discussed in detail. In Chapter 4, study on the thermoelectric properties of Bi, Zn and In co-doped GeTe was presented. Initial doping with Bi enhanced the performance by tuning the electronic properties and bringing down the thermal conductivity. Subsequent Zn doping permitted to maintain the high power factor by increasing carrier mobility and reducing carrier concentration. Subsequent In doping boosted the density of state effective mass. A peak zT value of 2.06 and an average zT value of 1.30 have been achieved in (Ge0.97Zn0.02In0.01Te)0.97(Bi2Te3)0.03. In Chapter 5, we thoroughly investigated the transport properties of SnTe-Sb2Te3 alloying system, provided useful insight of the mechanism of the enhanced Seebeck coefficient. To also overcome the poor carrier mobility, Pb compensation was performed which effectively optimized the carrier mobility. Meanwhile, Pb compensation broke the charge balance, allowing Sb to precipitate out of the structure. These second-phase particles provided additional source of phonon scattering, effectively suppressing the lattice thermal conductivity. As a result, a peak zT of 1.1 at 778K and an average zT of 0.56 from 300K to 778K was achieved in (Sn0.98Ge0.05Te)0.91 (Sb2Pb0.5Te)0.09, which is one of the best SnTe-based thermoelectric systems. / Thesis / Master of Science (MSc) / Thermoelectric materials can generate energy from temperature gradient, making them potential solutions for the escalating energy crisis. The state-of-the-art thermoelectric material is PbTe which shows outstanding performance and high stability. However, the toxicity of Pb element limits its practical application. It is the purpose of this work to develop high-performance GeTe and SnTe-based thermoelectrics to reduce the usage of PbTe. Combining theoretical calculations and experimental characterizations, detailed investigation on the transport properties, crystal structure and microstructure were performed on both GeTe and SnTe. Relations between their thermoelectric properties and their composition, synthesis method and microstructure were revealed. This work paves the path for the development of environmentally friendly and high-performance thermoelectric systems.

<p> The work described in this thesis is devoted to the application of defect engineering in the development of silicon photonic devices. The thesis is divided into simulation and experimental portions, each focusing on a different form of defect engineered silicon: ion implantation induced amorphous silicon and solid-phase epitaxial regrowth suppressed polycrystalline silicon.</p> <p> The simulations are directed at silicon rib waveguide Raman laser applications. It is shown that a uniform, divacancy defect concentration will not enhance Raman gain. The excess optical loss and free carrier lifetime of rib waveguides with remote amorphous silicon volumes were simulated. Net gain was demonstrated depending on the geometry of the structure. For a waveguide structure with rib width, rib height and slab height of W = 1.5, H = 1.5 and h = 0.8 μm respectively, the optimal separation between the edge of the rib and the amorphous region is ~2.0 μm. Surface recombination velocity modification was shown to be an effective means to reduce free carrier lifetime.</p> <p> Experimental work was devoted to the characterization of a novel form of polycrystalline silicon created by amorphizing the entire silicon overlayer of a silicon-on-insulator wafer. Solid-phase epitaxial regrowth of the amorphous silicon is suppressed upon annealing due to the lack of a crystal seed and results in polycrystalline silicon. This material was characterized with ellipsometry, positron annihilation spectroscopy and x-ray diffraction. The material properties are shown to be heavily dependent on the annealing conditions. Ellipsometry showed that the refractive index at 1550 nm is comparable to crystalline silicon. Positron annihilation spectroscopy showed that the polycrystalline material exhibits a high concentration of vacancy-type defects while vertically regrown crystalline silicon does not. X-ray diffraction showed that the polycrystalline silicon is non-textured, strained in tension and is characterized by grain sizes less than 300 nm.</p> <p> Defect etching and optical measurements using a waveguide geometry were performed in order to characterize the lateral regrowth and the optical loss of the polycrystalline material. Lateral regrowth in the [011] direction was 1.53 and 0.96 μm for 10 minute anneals at 750 and 900 °C respectively, and at least 2.5 μm at 650 °C. Waveguide optical loss measurements with adjacent polycrystalline regions separated from the rib by at least 5.5 μm showed no separation dependence. The intrinsic optical loss of the polycrystalline material was estimated to be 1.05 and 1.57 dB/cm for TM and TE polarizations after a 900 °C anneal. Vertically regrown c-Si was shown to exhibit less than 3.0 dB/cm optical loss after annealing at 550 °C .</p> / Thesis / Master of Applied Science (MASc)

The rise of organometal halide perovskite materials with extremely intriguing properties have opened a new horizon in the design of high speed and low price optoelectronic devices. The bandgap in the crystalline structure of these materials can be easily tuned for various applications and their dominant non-excitonic dynamics eliminate the requirement of a bulk or heterostructure for charge carrier separation. These unique properties increase the photo-sensitivity of perovskite-based optoelectronics and provide them with a low time constant, resulting in high precision fast devices. Realization of perovskite-based devices translates directly to inexpensive and simplified architectures of optoelectronic systems. In perovskite-based devices, costly silicon or wide bandgap semiconductor fabrication technology is largely replaced by solution processable methods. Their bandgap tunability allows the reduction of the required optical accessories and interconnects in optoelectronic components. For instance, a tuned perovskite-based detector can substitute a narrowband detecting system consisting of a conventional detector and its required optical accessories such as lenses and color filters. These properties of perovskite-based devices lead to the realization of inexpensive, low power and high-performance optoelectronic systems. In this work, the design of a narrowband, low noise, high performance and stable photodetector based on organic-inorganic hybrid perovskite structure is proposed. The full width at half maximum (FWHM) of the device would be in the nanometer range. The response of the device can be tuned using either different ratios of the lead salts or synthetic dyes (macromolecules) in the crystalline structure for color discrimination in machine vision and imaging applications. Non-excitonic photocarrier generation, tunability of the optical bandgap and low voltage requirements for charge carrier generation are the keys to the utility of this optoelectronic device. The goals of this project were to identify the required functional materials (lead salts and synthetic dyes based on their molecular structures) and optimize their performance; the study of their effect on the charge collection narrowing mechanism and bandwidth specifications defined for detectivity, linear dynamic range (LDR) and photoresponse speed. To achieve these goals, it was proposed to study the light detection properties as well as spectroscopic and semiconductor parameter characteristics of fabricated devices. The design considerations of such devices are versatile and may be modulated for different applications.

In the first study, superconducting 2D NbSe₂ was examined towards its prototypical demonstration as a transition-edge sensor, where photoexcitation caused a thermodynamic phase transition in NbSe₂ from the superconducting state to the normal state. The efficacy of the optical absorption was found to depend on the wavelength of the incoming radiation used, which ranged from the ultra-violet (405 nm), visible (660 nm), to the infrared (1060 nm). In the second case involving WSe₂, the UV-ozone treatment revealed the presence of localized excitonic emission in 1L WSe₂ that was robust and long-lived. Our third material platform dealt with hybrid 0D-2D ensembles based on graphene and WSe₂, specifically graphene–endohedral, WSe₂–fullerene (C₆₀), and WSe₂–Au nanoparticles, and exhibited exceptional performance gains achieved with both types of hybrid structures. Next, we investigated WSe₂ based mixed dimensional hybrids. Temperature T-dependent and wavelength λ-dependent optoelectronic transport measurements showed a shift in the spectral response of 1L WSe₂ towards the SPR peak locations of Au-Sp and Au-BP, fostered through the plexciton interactions. Models for the plexcitonic interactions are proposed that provide a framework for explaining the photoexcited hot charge carrier injection from AuNPs to WSe₂ and its influence on the carrier dynamics in these hybrid systems. Last, we studied interactions of vdWs hybrid structures composed of WSe₂ with 0D buckminsterfullerene (C₆₀) spheres. Our results indicate that the C₆₀-WSe₂ vdWs hybrid heterostructure appears to be an attractive architecture for enabling charge transfer and high performance photodetection capabilities. T-dependent electrical transport measurements after C₆₀ deposition revealed a dominant p-type conduction behavior and a significant ×10³ increase in WSe₂ field-effect mobility, with a maximum field-effect mobility of 281 cm²V⁻¹s⁻¹ achieved at 350 K and room-T mobility of 119.9 cm²V⁻¹s⁻¹ for the C₆₀-WSe₂ hybrid.

No description available.

A redistribuição e ativação elétrica dos dopantes tipo n (As e Sb) e tipo p (Ga e In) em Si com excesso de vacâncias foram analisadas. As vacâncias foram geradas por implantação iônica de altas doses de oxigênio ou nitrogênio em alta temperatura, de acordo com procedimentos já estudados. Em seguida foram implantados os dopantes com dose de 5x1014 cm-2 a 20 keV na região rica em vacâncias. Dopagens idênticas foram realizadas em amostras de Si sem vacâncias e em SIMOX. Em seguida foram feitos recozimentos a 1000ºC por 10 s ou 15 min. Os perfis atômicos dos dopantes foram medidos com Medium Energy Ion Scattering e os perfis dos dopantes ativados, com Hall diferencial. A redistribuição e as propriedades elétricas de cada um dos dopantes no Si sem vacâncias foram bastante similares às observadas no SIMOX, porém várias diferenças foram observadas em relação às amostras com excesso de vacâncias. As vacâncias reduziram a ativação elétrica do As e do Sb, mas proporcionaram maior estabilidade da ativação após recozimentos longos. A redistribuição destes dopantes foi infuenciada pelo íon usado na geração das vacâncias, ou seja, nitrogênio ou oxigênio. O oxigênio proporcionou maior dose retida de As e o nitrogênio, maior dose retida de Sb. Já para o Ga e o In, as vacâncias tiveram papel fundamental na sua redistribuição, diminuindo a difusão para fora das amostras e garantindo maior dose retida. A ativação elétrica do Ga e especialmente a do In foram baixas, onde observamos forte influência do íon pré-implantado, principalmente o oxigênio. / The redistribution and electrical activation of n type (As and Sb) and p type (Ga and In) dopants in Si with excess vacancy concentration were analyzed. The vacancies were formed by high dose ion implantation of oxygen or nitrogen at high temperature, following previously studied procedures. Dopants were implanted to a dose of 5x1014 cm-2 at 20 keV in the vacancy rich regions of the samples. Identical doping implantations were performed in bulk Si and SIMOX. Samples were then submitted to thermal annealing at 1000ºC for 10 s or 15 min. The dopants atomic profiles were obtained by Medium Energy Ion Scattering and the active dopant profiles, by differential Hall measurements. The redistribution and the electrical properties of each dopant in bulk Si were similar to those observed in SIMOX, but several differences were observed in the vacancy-rich samples. Vacancies reduced the electrical activation of As and Sb, although the activation was maintained stable after long annealing times. The redistribution of these dopants was, otherwise, dominated by the ion used in the vacancy generation, i.e., nitrogen or oxygen. The presence of oxygen resulted in larger As retained dose, while the presence of nitrogen, in larger Sb retained dose. Regarding the p type dopants, Ga and In, the vacancies played an important role in their redistribution, reducing their out-diffusion and allowing larger retained doses. Ga and especially In electrical activation was low, where strong influence of the pre-implanted ions was observed, especially oxygen.

Die Kenntnisse über die zur Passivierung führenden Wechselwirkungen des Verunreinigungselements Cu mit EL2 und EL6 wurden dahingehend erweitert, dass ein Gültigkeitsbereich für die Messungen zur Bestimmung der konzentrationsproportionalen Messgröße der Defekte festgelegt wurde. Der Defekt EL6 ist das die 0.8 eV-PL-Emission bedingende und die Ladungsträgerlebensdauer determinierende Rekombinationszentrum. Die Lebensdauer wird durch mindestens ein weiteres Zentrum beeinflusst. Die atomare Struktur des EL6 wurde mit AsGa VAs und die des weiteren Zentrums mit Asi bestimmt. Mittels Wärmebehandlung kann die Ladungsträgerlebensdauer gezielt beeinflusst werden. Bei der Bildung und Annihilation sowie der Verteilung der Defekte EL2, EL6, VGa und der As-Ausscheidungen besteht eine wechselseitige Korrelation. Sowohl strukturelle Defekte als auch die Inkorporation von Dotierelementen, deren Atomradien deutlich kleiner sind als Ga und As, stellen beeinflussende Faktoren dar. Das Defekt-Transformations-Modell erklärt die Bildung wachstumsfähiger Keime einer As-Ausscheidung durch EL2 bzw. EL6 mit den sich in der ersten Koordingationssphäre befindenden As-Atomen. Das Gitterrelaxations-Modell ermöglicht die Interpretation der katalytischen Wirkung des Dotierelements C. Die Umsetzung dieser Ergebnisse bedeutet die Anwendung des Defect Engineerings im SI-LEC-GaAs.

A redistribuição e ativação elétrica dos dopantes tipo n (As e Sb) e tipo p (Ga e In) em Si com excesso de vacâncias foram analisadas. As vacâncias foram geradas por implantação iônica de altas doses de oxigênio ou nitrogênio em alta temperatura, de acordo com procedimentos já estudados. Em seguida foram implantados os dopantes com dose de 5x1014 cm-2 a 20 keV na região rica em vacâncias. Dopagens idênticas foram realizadas em amostras de Si sem vacâncias e em SIMOX. Em seguida foram feitos recozimentos a 1000ºC por 10 s ou 15 min. Os perfis atômicos dos dopantes foram medidos com Medium Energy Ion Scattering e os perfis dos dopantes ativados, com Hall diferencial. A redistribuição e as propriedades elétricas de cada um dos dopantes no Si sem vacâncias foram bastante similares às observadas no SIMOX, porém várias diferenças foram observadas em relação às amostras com excesso de vacâncias. As vacâncias reduziram a ativação elétrica do As e do Sb, mas proporcionaram maior estabilidade da ativação após recozimentos longos. A redistribuição destes dopantes foi infuenciada pelo íon usado na geração das vacâncias, ou seja, nitrogênio ou oxigênio. O oxigênio proporcionou maior dose retida de As e o nitrogênio, maior dose retida de Sb. Já para o Ga e o In, as vacâncias tiveram papel fundamental na sua redistribuição, diminuindo a difusão para fora das amostras e garantindo maior dose retida. A ativação elétrica do Ga e especialmente a do In foram baixas, onde observamos forte influência do íon pré-implantado, principalmente o oxigênio. / The redistribution and electrical activation of n type (As and Sb) and p type (Ga and In) dopants in Si with excess vacancy concentration were analyzed. The vacancies were formed by high dose ion implantation of oxygen or nitrogen at high temperature, following previously studied procedures. Dopants were implanted to a dose of 5x1014 cm-2 at 20 keV in the vacancy rich regions of the samples. Identical doping implantations were performed in bulk Si and SIMOX. Samples were then submitted to thermal annealing at 1000ºC for 10 s or 15 min. The dopants atomic profiles were obtained by Medium Energy Ion Scattering and the active dopant profiles, by differential Hall measurements. The redistribution and the electrical properties of each dopant in bulk Si were similar to those observed in SIMOX, but several differences were observed in the vacancy-rich samples. Vacancies reduced the electrical activation of As and Sb, although the activation was maintained stable after long annealing times. The redistribution of these dopants was, otherwise, dominated by the ion used in the vacancy generation, i.e., nitrogen or oxygen. The presence of oxygen resulted in larger As retained dose, while the presence of nitrogen, in larger Sb retained dose. Regarding the p type dopants, Ga and In, the vacancies played an important role in their redistribution, reducing their out-diffusion and allowing larger retained doses. Ga and especially In electrical activation was low, where strong influence of the pre-implanted ions was observed, especially oxygen.