Indiana University-Purdue University Indianapolis (IUPUI) / Fabrication and analysis of Copper Indium Gallium di-Selenide (CIGS) nanoparticles-based thin film solar cells are presented and discussed. This work explores non-traditional fabrication processes, such as spray-coating for the low-cost and highly-scalable production of CIGS-based solar cells. CIGS nanoparticles were synthesized and analyzed, thin CIGS films were spray-deposited using nanoparticle inks, and resulting films were used in low-cost fabrication of a set of CIGS solar cell devices. This synthesis method utilizes a chemical colloidal process resulting in the formation of nanoparticles with tunable band gap and size. Based on theoretical and experimental studies, 100 nm nanoparticles with an associated band gap of 1.33 eV were selected to achieve the desired film characteristics and device performances. Scanning electron microcopy (SEM) and size measurement instruments (Zetasizer) were used to study the size and shape of the nanoparticles. Electron dispersive spectroscopy (EDS) results confirmed the presence of the four elements, Copper (Cu), Indium (In), Gallium (Ga), and Selenium (Se) in the synthesized nanoparticles, while X-ray diffraction (XRD) results confirmed the tetragonal chalcopyrite crystal structure. The ultraviolet-visible-near infra-red (UV-Vis-NIR) spectrophotometry results of the nanoparticles depicted light absorbance characteristics with good overlap against the solar irradiance spectrum. The depositions of the nanoparticles were performed using spray-coating techniques. Nanoparticle ink dispersed in ethanol was sprayed using a simple airbrush tool. The thicknesses of the deposited films were controlled through variations in the deposition steps, substrate to spray-nozzle distance, size of the nozzle, and air pressure. Surface features and topology of the spray-deposited films were analyzed using atomic force microscopy (AFM). The deposited films were observed to be relatively uniform with a minimum thickness of 400 nm. Post-annealing of the films at various temperatures was studied for the photoelectric performance of the deposited films. Current density and voltage (J/V) characteristics were measured under light illumination after annealing at different temperatures. It was observed that the highest photoelectric effect resulted in annealing temperatures of 150-250 degree centigrade under air atmosphere. The developed CIGS films were implemented in solar cell devices that included Cadmium Sulfide (CdS) and Zinc Oxide (ZnO) layers. The CdS film served as the n-type layer to form a pn junction with the p-type CIGS layer. In a typical device, a 300 nm CdS layer was deposited through chemical bath deposition on a 1 $mu$m thick CIGS film. A thin layer of intrinsic ZnO was spray coated on the CdS film to prevent shorting with the top conductor layer, 1.5 μm spray-deposited aluminum doped ZnO layer. A set of fabricated devices were tested using a Keithley semiconductor characterization instrument and micromanipulator probe station. The highest measured device efficiency was 1.49%. The considered solar cell devices were simulated in ADEPT 2.0 solar cell simulator based on the given fabrication and experimental parameters. The simulation module developed was successfully calibrated with the experimental results. This module can be used for future development of the given work.

<div> <div> <div> <p>Organic-inorganic halide perovskite solar cells have increased efficiencies substantially (from 3% to > 22%), within a few years. However, these solar cells degrade very rapidly due to humidity and no longer are capable of converting photons into electrons. Methylammonium Lead Triiodide (CH3NH3PbI3 or MAPbI3) is the most common type of halide perovskite solar cell and is the crystal studied in this thesis. Graphene is an effective encapsulation method of MAPbI3 perovskite to reduce degradation, while also being advantageous because of its excellent optical and conductive properties. Using a PMMA transfer method graphene was chemical vapor depostion (CVD) grown graphene was transferred onto MAPbI3 and reduced the MAPbI3 degradation rate by over 400%. The PMMA transfer method in this study is scalable for roll-to- roll manufacturing with fewer cracks, impurites, and folds improving upon dry transfer methods. To characterize degradation a fluorescent microscope was used to capture photoluminescence data at initial creation of the samples up to 528 hours of 80% humidity exposure. Atomic force microscopy was used to characterize topographical changes during degradation. The study proves that CVD graphene is an effective encapsulation method for reducing degradation of MAPbI3 due to humidity and retained 95.3% of its initial PL intensity after 384 hours of 80% humidity exposure. Furthermore, after 216 hours of 80% humidity exposure CVD graphene encapsulated MAPbI3 retained 80.2% of its initial number of peaks, and only saw a 35.1% increase in surface height. Comparatively, pristine MAPbI3 only retained 16% of its initial number of peaks and saw a 159% increase in surface height. </p> </div> </div> </div>

The change of the spontaneous polarization due to a change of temperature is known as the pyroelectric effect and is restricted to crystalline, non-centrosymmetric and polar matter. Its main application is the utilization in infrared radiation sensors, but usage for waste heat energy harvesting or chemical catalysis is also possible. A precise quantification, i.e. the measurement of the pyroelectric coefficient p, is inevitable to assess the performance of a material. Hence, a comprehensive overview is provided in this work, which summarizes and evaluates the available techniques to characterize p. A setup allowing the fully automated measurement of p by utilizing the Sharp-Garn method and the measurement of ferroelectric hysteresis loops is described. It was used to characterize and discuss the behavior of p with respect to the temperature of the doped bulk III-V compound semiconductors gallium nitride and aluminum nitride and thin films of doped hafnium oxide, as reliable data for these materials is still missing in the literature. Here, the nitride-based semiconductors show a comparable small p and temperature dependency, which is only slightly affected by the incorporated dopant, compared to traditional ferroelectric oxides. In contrast, p of HfO2 thin films is about an order of magnitude larger and seems to be affected by the present dopant and its concentrations, as it is considered to be responsible for the formation of the polar orthorhombic phase.:1. Motivation and Introduction 2. Fundamentals 2.1. Dielectrics and their Classification 2.2. Polarization 2.3. Pyroelectricity 2.4. Ferroelectricty 2.5. Phase Transitions 2.6. Applications and Figures of Merit 3. Measurement Methods for the Pyroelectric Coefficient 3.1. General Considerations 3.1.1. Heating Concepts 3.1.2. Thermal Equilibrium 3.1.3. Electric Contact 3.1.4. Separation of Contributions 3.1.5. Thermally Stimulated Currents 3.2. Static Methods 3.2.1. Charge Compensation Method 3.2.2. Hysteresis Measurement Method 3.2.3. Direct Electrocaloric Measurement 3.2.4. Flatband Voltage Shift 3.2.5. X-ray Photoelectron Spectroscopy Method 3.2.6. X-ray Diffraction and Density Functional Theory 3.3. Dynamic Methods 3.3.1. Temperature Ramping Methods 3.3.2. Optical Methods 3.3.3. Periodic Pulse Technique 3.3.4. Laser Intensity Modulation Methods 3.3.5. Harmonic Waveform Techniques 4. Pyroelectric and Ferroelectric Characterization Setup 4.1. Pyroelectric Measurement Setup 4.1.1. Setup and Instrumentation 4.1.2. Automated Sharp-Garn Evaluation of Pyroelectric Coefficients 4.1.3. Further Examples 4.2. Hysteresis Loop Measurements 4.2.1. Instrumentation 4.2.2. Measurement and Evaluation 4.2.3. Examples 5. Investigated Material Systems 5.1. III-Nitride Bulk Semiconductors GaN and AlN 5.1.1. General Structure and Spontaneous Polarization 5.1.2. Applications 5.1.3. Crystal Growth and Doping 5.1.4. Pyroelectricity 5.2. Hafnium Oxide Thin Films 5.2.1. General Structure and Applications 5.2.2. Polar Properties in Thin Films 5.2.3. Doping Effects 5.2.4. Pyro- and Piezoelectricity 6. Results 6.1. The Pyroelectric Coefficient of Free-standing GaN and AlN 6.1.1. Sample Preparation 6.1.2. Pyroelectric Measurements 6.1.3. Lattice Influence 6.1.4. Slope Differences 6.2. Pyroelectricity of Doped Hafnium Oxide 6.2.1. Sharp-Garn Measurement on Thin Films 6.2.2. Effects of Silicon Doping 6.2.3. Dopant Comparison 7. Summary and Outlook A. Pyroelectric Current and Phase under Periodic Thermal Excitation B. Loss Current Correction for Shunt Method C. Conductivity Correction D. Comparison of Pyroelectric Figures of Merit Bibliography Publication List Acknowledgments / Die Änderung der spontanen Polarisation durch eine Änderung der Temperatur ist bekannt als der pyroelektrische Effekt, welcher auf kristalline, nicht-zentrosymmetrische und polare Materie beschränkt ist. Er findet vor allem Anwendung in Infrarot-Strahlungsdetektoren, bietet aber weitere Anwendungsfelder wie die Niedertemperatur-Abwärmenutzung oder die chemische Katalyse. Eine präzise Quantifizierung, d. h. die Messung des pyroelektrischen Koeffizienten p, ist unabdingbar, um die Leistungsfähigkeit eines Materials zu bewerten. Daher bietet diese Arbeit u.a. einen umfassenden Überblick und eine Bewertung der verfügbaren Messmethoden zur Charakterisierung von p. Weiterhin wird ein Messaufbau beschrieben, welcher die voll automatisierte Messung von p mit Hilfe der Sharp-Garn Methode und auch die Charakterisierung der ferroelektrischen Hystereseschleife ermöglicht. Aufgrund fehlerender Literaturdaten wurde dieser Aufbau anschließend genutzt, um den temperaturabhängigen pyroelektrischen Koeffizienten der dotierten III-V-Verbindungshalbleiter Gallium- und Aluminiumnitrid sowie dünner Schichten bestehend aus dotiertem Hafniumoxid zu messen und zu diskutieren. Im Vergleich zu klassichen ferroelektrischen Oxiden zeigen dabei die nitridbasierten Halbleiter einen geringen pyroelektrischen Koeffizienten und eine kleine Temperaturabhängigkeit, welche auch nur leicht durch den vorhandenen Dotanden beeinflusst werden kann. Dagegen zeigen dünne Hafniumoxidschichten einen um eine Größenordnung größeren pyroelektrischen Koeffizienten, welcher durch den anwesenden Dotanden und seine Konzentration beeinflusst wird, da dieser verantwortlich für die Ausbildung der polaren, orthorhombischen Phase gemacht wird.:1. Motivation and Introduction 2. Fundamentals 2.1. Dielectrics and their Classification 2.2. Polarization 2.3. Pyroelectricity 2.4. Ferroelectricty 2.5. Phase Transitions 2.6. Applications and Figures of Merit 3. Measurement Methods for the Pyroelectric Coefficient 3.1. General Considerations 3.1.1. Heating Concepts 3.1.2. Thermal Equilibrium 3.1.3. Electric Contact 3.1.4. Separation of Contributions 3.1.5. Thermally Stimulated Currents 3.2. Static Methods 3.2.1. Charge Compensation Method 3.2.2. Hysteresis Measurement Method 3.2.3. Direct Electrocaloric Measurement 3.2.4. Flatband Voltage Shift 3.2.5. X-ray Photoelectron Spectroscopy Method 3.2.6. X-ray Diffraction and Density Functional Theory 3.3. Dynamic Methods 3.3.1. Temperature Ramping Methods 3.3.2. Optical Methods 3.3.3. Periodic Pulse Technique 3.3.4. Laser Intensity Modulation Methods 3.3.5. Harmonic Waveform Techniques 4. Pyroelectric and Ferroelectric Characterization Setup 4.1. Pyroelectric Measurement Setup 4.1.1. Setup and Instrumentation 4.1.2. Automated Sharp-Garn Evaluation of Pyroelectric Coefficients 4.1.3. Further Examples 4.2. Hysteresis Loop Measurements 4.2.1. Instrumentation 4.2.2. Measurement and Evaluation 4.2.3. Examples 5. Investigated Material Systems 5.1. III-Nitride Bulk Semiconductors GaN and AlN 5.1.1. General Structure and Spontaneous Polarization 5.1.2. Applications 5.1.3. Crystal Growth and Doping 5.1.4. Pyroelectricity 5.2. Hafnium Oxide Thin Films 5.2.1. General Structure and Applications 5.2.2. Polar Properties in Thin Films 5.2.3. Doping Effects 5.2.4. Pyro- and Piezoelectricity 6. Results 6.1. The Pyroelectric Coefficient of Free-standing GaN and AlN 6.1.1. Sample Preparation 6.1.2. Pyroelectric Measurements 6.1.3. Lattice Influence 6.1.4. Slope Differences 6.2. Pyroelectricity of Doped Hafnium Oxide 6.2.1. Sharp-Garn Measurement on Thin Films 6.2.2. Effects of Silicon Doping 6.2.3. Dopant Comparison 7. Summary and Outlook A. Pyroelectric Current and Phase under Periodic Thermal Excitation B. Loss Current Correction for Shunt Method C. Conductivity Correction D. Comparison of Pyroelectric Figures of Merit Bibliography Publication List Acknowledgments

<p>Developing the next generation soft optoelectronic materials is of great importance for achieving high-performance, low-cost electronics. These novel material systems bring about new chemistry, physical phenomena, and exciting properties. Organic inorganic hybrid two-dimensional perovskites and organic stable radical molecules are two exciting material systems that bear high expectation and await extensive exploration.</p> <p>Organic inorganic hybrid two-dimensional perovskites are considered one of the solutions to the pressing instability issue of halide perovskites toward commercialization. Moreover, dimension reduction of perovskites creates new opportunities for using two-dimensional perovskites as thermoelectric applications due to the ultralow thermal conductivity. However, two-dimensional perovskite thermoelectric is still at its’ incipient stage of development, therefore a timely proof of potential is required to draw further research interests.</p> <p>In earlier part of this work, the two-dimensional perovskites featuring π-conjugated ligands are synthesized and optimized for high thermoelectric performance. With material design, device engineering, intensive measurements, and careful data analysis, we successfully showed that two-dimensional perovskites are competitive candidate for the emerging thermoelectric materials. Furthermore, temperature and carrier concentration dependencies on thermoelectric properties were also established, giving future researchers a generalized optimization strategy. </p> <p>Organic stable radical molecules are promising for organic electronics as stable radicals don’t require high conjugation for efficient solids-state charge transport. Thanks to their unique redox capability and the unpaired electrons, organic radicals have many unique electronic and magnetic properties that could be useful in spin-related applications. However, the understanding in charge transport mechanisms as well as structure-to-properties correlation remain shallow.</p> <p>In later part of this work, we achieved the highest recorded long channel electrical conductivity of non-conjugated radicals. Meanwhile, the important role of close packing between radical sites was demonstrated by slightly changing chemical design that resulted in drastic change in electrical conductivity. Finally, we concluded that the solid-state charge transport in non-conjugated species is governed by variable range hopping mechanisms. </p>

<p dir="ltr">Transitioning to a carbon-neutral future will require a broad portfolio of green energy generation and storage solutions. With the abundant availability of solar radiation across the Earth’s surface, energy generation from photovoltaics (PVs) will be an important part of this green energy portfolio. While silicon-based solar cells currently dominate the PV market, temperatures exceeding 1000 °C are needed for purification of silicon, and batch processing of silicon wafers limits how rapidly Si-based PV can be deployed. Furthermore, silicon’s indirect band gap necessitates absorber layers to exceed 100 µm thick, limiting its applications to rigid substrates.</p><p dir="ltr">Solution processed thin-film solar cells may allow for the realization of continuous, high-throughput manufacturing of PV modules. Thin-film absorber materials have direct band gaps, allowing them to absorb light more efficiently, and thus, they can be as thin as a few hundred nanometers and can be deposited on flexible substrates. Solution deposition of these absorber materials utilizing molecular precursor-based inks could be done in a roll-to-roll format, drastically increasing the throughput of PV manufacturing, and reducing installation costs. In this dissertation, solution processed synthesis and the characterization of two emerging direct band gap absorber materials consisting of earth abundant elements is discussed: the enargite phase of Cu₃AsS₄ and the distorted perovskite phase of BaZrS₃.</p><p dir="ltr">The enargite phase of Cu₃AsS₄ (ENG) is an emerging PV material with a 1.42 eV band gap, making it an ideal single-junction absorber material for photovoltaic applications. Unfortunately, ENG-based PV devices have historically been shown to have low power conversion efficiencies, potentially due to defects in the material. A combined computational and experimental study was completed where DFT-based calculations from collaborators were used inform synthesis strategies to improve the defect properties of ENG utilizing new synthesis techniques, including silver alloying, to reduce the density of harmful defects.</p><p dir="ltr">Chalcogenide perovskites are viewed as a stable alternative to halide perovskites, with BaZrS₃ being the most widely studied. With a band gap of 1.8 eV, BaZrS₃ could be an excellent wide-bandgap partner for a silicon-based tandem solar cell.Historically, sputtering, and solid-state approaches have been used to synthesize chalcogenide perovskites, but these methods require synthesis temperatures exceeding 800 °C, making them incompatible with the glass substrates and rear-contact layers required to create a PV device. In this dissertation, these high synthesis temperatures are bypassed through the development of a solution-processed deposition technique.A unique chemistry was developed to create fully soluble molecular precursor inks consisting of alkaline earth metal dithiocarboxylates and transition metal dithiocarbamates for direct-to-substrate synthesis of BaZrS₃ and BaHfS₃ at temperatures below 600 °C.</p><p dir="ltr">However, many challenges must be overcome before chalcogenide perovskites can be used for the creation of photovoltaic devices including oxide and Ruddlesden-Popper secondary phases, isolated grain growth, and deep level defects. Nevertheless, the development of a moderate temperature solution-based synthesis route makes chalcogenide perovskite research accessible to labs which do not have high temperature furnaces or sputtering equipment, further increasing research interest in this quickly developing absorber material.</p>

Die Effizienz von Verbindungshalbleitern hängt von ihrer lokalen Zusammensetzung und ihrer räumlichen Elementverteilung ab. Um die opto-elektronischen Eigenschaften solcher Bauelemente zu optimieren, ist ein detailliertes Verständnis und die Kontrolle der Zusammensetzungsgradienten entscheidend. Industriell wichtige Bauelemente sind Absorberschichten für Dünnschichtsolarzellen, die eine hohe Effizienz in Kombination mit einem geringen Materialbedarf und einer hohen elastischen Flexibilität bieten. Ein gängiges Herstellungsverfahren für Dünnschicht-Solarzellenabsorber ist das Annealen bei hohen Temperaturen. Im Gegensatz zu dem, was bei Fick'schen Diffusionsprozessen zu erwarten wäre – führt dieses regelmäßig zur Bildung steiler und stabiler Zusammensetzungsgradienten, die oft von den optimalen Profilen für hocheffiziente Absorber abweichen. In dieser Arbeit liegt das Hauptaugenmerk auf den mechanischen Spannungen, die sich im Inneren von Dünnschichten entwickeln, und auf deren Auswirkungen auf Diffusionsprozesse und die mikrostrukturelle Entwicklung des Materials. Es wird gezeigt, dass die Bildung von elastischen Spannungen die endgültigen Elementverteilungen stark beeinflusst und sogar zur Bildung von starken und stabilen Zusammensetzungsgradienten führt. In dieser Arbeit wird weiterhin argumentiert, dass die Wirkung der Spannungen auf die Gleichgewichts-Zusammensetzungsprofile von den mikrostrukturellen Eigenschaften des Materials abhängen kann, insbesondere vom Vorhandensein von Leerstellenquellen. Ein Vergleich numerischer Berechnungen mit Echtzeitdaten der energiedispersiven Röntgenbeugung, die während der Dünnschichtsynthese in-situ erfasst wurden, hilft zu zeigen, dass die so entwickelten Interdiffusionsmodelle die experimentell beobachteten Beugungsspektren und insbesondere die Stagnation der Interdiffusion vor Erreichen der vollständigen Durchmischung teilweise reproduzieren können. / The operational efficiency of compound semiconductors regularly depends on their local elemental composition and on the spatial distribution of contained elements. To optimize the opto-electronic properties of such devices, a detailed understanding and control of compositional gradients is crucial. Industrially important devices are thin-film solar cell absorber layers which deliver high photo-conversion efficiencies in combination with a low demand of material and high elastic flexibility. These materials use local variations in composition to tune their opto-electronic properties. A common fabrication process for thin-film solar cell absorbers involves annealing at high temperatures to achieve specific compositional gradients, which – contrary to what could be expected from simple Fickian diffusion processes – regularly results in the formation of steep and stable compositional gradients, often deviating from the optimal profiles for high-efficiency absorbers. In this work attention is focused especially on mechanical stresses developing inside thin-films and on their effects on diffusion processes and on the material’s micro-structural evolution. It is shown that the formation of elastic stresses strongly influences the final elemental distributions, even leading to the formation of strong and stable final compositional gradients. However, this thesis also argues that their exact effect on equilibrium composition profiles may depend on the detailed micro-structural properties of the material, especially on the presence of vacancy sources and sinks. A comparison of numerical calculations with real-time synchroton-based energy-dispersive X-ray diffraction data acquired in-situ during thin-film synthesis helps to demonstrate that the such developed interdiffusion models can partly reproduce the experimentally observed diffraction spectra and, especially, the stagnation of interdiffusion before total intermixing is achieved.

<p dir="ltr">Advancements in material science are accelerating technological evolution, driven by initiatives like the Materials Genome Project, which integrates computational and experi- mental strategies to expedite material discovery. In this work, we focus on the reliability of advanced materials under extreme conditions, a critical area for enhancing their technological applications.</p><p dir="ltr">Multi-principal component alloys (MPEAs) exhibit remarkable properties under extreme conditions. However, their vast compositional space makes a brute-force exploration of potential alloys prohibitive. We address this challenge by employing a Bayesian approach to explore the oxidation resistance of hundreds of alloys, applying computational techniques to accurately calculate and quantify errors in the melting temperatures of MPEAs, and investigating the compositional biases and short-range order in their nucleation behaviors.</p><p dir="ltr">Furthermore, we scrutinize the role of wide bandgap semiconductors, which are essential in high-power applications due to their superior breakdown voltage, drift velocity, and sheet charge density. The lack of lattice-matched substrates often results in strained films, which enhances piezoelectric effects crucial for device reliability. Our research advances the pre- diction of piezoelectric and dielectric responses as influenced by biaxial strain and doping in gallium nitride (GaN). Additionally, we delve into how various common defects affect the formation of trap states, significantly impacting the electronic properties of these materials. These studies offer significant advancements in understanding MPEAs and wide bandgap semiconductors under extreme conditions. We also provide foundational insights for developing robust and efficient materials essential for next-generation applications.</p>

<p dir="ltr">For reliable, facile, and user-friendly, solution-based synthesis of materials, the colloidal nanocrystal route has proven to be the method of choice for so many. The tunability that this process renders its users---from choice of precursors, solvent systems, and reaction conditions including temperature, pressure, and precursor addition order---is truly second to none. In their simplest form, these nanomaterials are usually comprised of an inorganic core of the desired material and an outer layer of surface-stabilizing molecules called ligands. These ligands provide colloidal stability and allow for the solution-processing of these materials for downstream usage in devices such as light-emitting diodes and photovoltaics, for example. In this thesis, the study and use of colloidal nanomaterials of Cu(In,Ga)(S,Se)₂ (CIGSSe), IIA-IVB-S₃ (including BaZrS₃ and SrZrS₃), alkaline earth polysulfides (IIAS_x; IIA = Sr, Ba; x = 2, 3), and other materials like Cu₂GeS₃ and Cu₂BaSnS₄, for studies into the formation, colloidal stability, and fabrication into solar cells was performed.</p><p dir="ltr">More specifically, an experimental protocol was developed to fabricate high-quality CIGSSe nanoparticles with carbonaceous residues that are substantially reduced from traditional pathways. Traditional methods for synthesizing colloidal CIGS NPs often utilize heavy, long-chain organic species to serve as surface ligands which, during annealing in a Se/Ar atmosphere, leave behind an undesirable carbonaceous residue in the film. In an effort to minimize these residues, N-methyl-2-pyrrolidone (NMP) was used as an alternative surface ligand. Through the use of the NMP-based synthesis, a substantial reduction in the number of carbonaceous residues was observed in selenized films. Additionally, the fine-grain layer at the bottom of the film, a common observation of solution-processed films from organic media, was observed to exhibit a larger average grain size and increased chalcopyrite character over those of traditionally prepared films, presumably as a result of the reduced carbon content, allowing for superior growth. As a result, a gallium-free CuIn(S,Se)₂ device was shown to achieve power-conversion efficiencies of over 11% as well as possessing exceptional carrier generation capabilities with a short-circuit current density (J_SC) of 41.6 mA/cm², which is among the highest for the CIGSSe family of devices fabricated from solution-processed methods. It was shown that pre-selenized films of sulfide nanoparticles instead of selenide nanoparticles performed better as solar cells. While the exact mechanism is still under debate, it appears that the growth phase during selenization, which varies depending on the chalcogen present in the starting material plays an important role.</p><p dir="ltr">The IIA-IVB-S₃ system is just beginning to emerge as a material system shown to be capable of solution-based synthesis methods. This is primarily due to the extremely high oxophilicity of the IVB elements, Ti, Zr, and Hf, necessitating that extreme care and judicial use of inert environments be used to synthesize these materials via solution-based methods. In the IIA-IVB-S₃ system exists some of the chalcogenide perovskites, including BaZrS₃, which are expected to have similar electronic properties to the well-known, high-performing halide perovskites, albeit much more stable, making them attractive prospects as novel semiconductor materials for optoelectronic applications. This work builds upon recent studies to show a general synthesis protocol, involving the use of carbon disulfide insertion chemistry to generate highly reactive precursors, that can be used towards the colloidal synthesis of numerous nanomaterials in the IIA-IVB-S₃ system, including BaTiS₃, BaZrS₃, BaHfS₃, α-SrZrS₃ and α-SrHfS₃. Additionally, we establish a method to reliably control the formation of the BaZrS₃ perovskite, a complication seen in previous literature where BaZrS₃ appears to exist as two different phases when synthesized via colloidal methods. The utility of these nanomaterials is also assessed via the measurement of their absorption properties and in the form of highly stable colloidal inks for the fabrication of homogenous, crack-free thin films of BaZrS₃. In addition to the chalcogenide perovskites, the IIA-S system was also explored to better understand the solution-based formation of these materials and how the control of IIA polysulfides can be achieved. We show that the synthesis of these materials is strongly correlated to the reaction temperature and that the length of the S_n^2- oligomer chain is the dependent variable. We also report on the synthesis of a previously unreported polymorph of SrS₂ which appears to take on the <i>C2/c</i> space group, the same as BaS₂.</p><p dir="ltr">Finally, some discussion is also provided on the use of transmission electron microscopy (TEM) to analyze the crystal structure of materials. Some tips and techniques used throughout this thesis are summarized in this section.</p>

The semiconducting compounds CdTe and CdZnTe have important applications in high-energy radiation detectors and as substrates for infrared devices. The materials offer large band gaps, high resistivity, and excellent charge transport properties; however all of these properties rely on very precise control of the material composition. Growing bulk crystals by the travelling heater method (THM) offers excellent compositional control and fewer defects compared to gradient freezing, but it is also much slower and more expensive. A particular challenge is the current need to grow new crystals onto existing seeds of similar size and quality. Simulations and experiments are used in this work to investigate the feasibility of growing these materials by THM without the use of large seed crystals. A new fixed-grid, multiphase finite element model was developed based on the level set method and used to calculate the mass transport regime and interface shapes inside the growth ampoule. The diffusivity of CdTe in liquid tellurium was measured through dissolution experiments, which also served to validate the model. Simulations of tapered THM growth find conditions similar to untapered growth with interface shapes that are sensitive to strong thermosolutal convection. Favourable growth conditions are achievable only if convection can be controlled. In preliminary experiments, tapered GaSb crystals were successfully grown by THM and large CdTe grains were produced by gradient freezing. Beginning with this seed material, 25 mm diameter CdTe and CdZnTe crystals were grown on 10 mm diameter seeds, and 65 mm diameter CdTe on 25 mm seeds. Unseeded THM growth was also investigated, as well as ampoule rotation and a range of thermal conditions and ampoule surface coatings. Outward growth beyond one or two centimeters was achieved only at small diameters and included secondary grains and twin defects; however, limited outward growth of larger seeds and agreement between experimental and numerical results suggest that tapered growth may be achievable in the future. This would require active temperature control at the base of the crystal and reduction of convection through thermal design or by rotation of the ampoule or applied magnetic fields. / Graduate / 0346 / 0794 / 0548 / jordan.roszmann@gmail.com

<p>Beta-gallium oxide (<i>β</i>-Ga₂O₃) is an emerging wide bandgap semiconductor for next generation power devices which offers the potential to replace GaN and SiC. It has an ultra-wide bandgap (UWBG) of 4.8 eV and a corresponding <i>E</i>_brof 8 MV/cm. <i>β</i>-Ga₂O₃also possesses a decent intrinsic electron mobility limit of 250 cm²/V<i>·</i>s, yielding high Baliga’s figure of merit of 3444. In addition, the large bandgap of <i>β</i>-Ga₂O₃gives stability in harsh environment operation at high temperatures. </p> <p>Although low-cost large-size <i>β</i>-Ga₂O₃native bulk substrates can be realized by melt growth methods, the unique property that (100) surface of <i>β</i>-Ga₂O₃has a large lattice constant of 12.23 Å allows it to be cleaved easily into thin and long nano-membranes. Therefore, <i>β</i>-Ga₂O₃FETs on foreign substrates by transferring can be fabricated and investigated before <i>β</i>-Ga₂O₃epitaxy technology becomes mature and economical viable. Moreover, integrating <i>β</i>-Ga₂O₃on high thermal conductivity materials has an advantage in terms of suppressing self-heating effects. </p><p>In this dissertation, structural and material innovations to overcome and improve critical challenges are summarized as follows: 1) Top-gate nano-membrane <i>β</i>-Ga₂O₃FETs on a high thermal conductivity diamond substrate with record high maximum drain current densities are demonstrated. The reduced self-heating effect due to high thermal conductivity of the substrate was verified by thermoreflectance measurement. 2) Local electro-thermal effect by electrical bias was applied to enhance the electrical performance of devices and improvements of electrical properties were shown after the annealing. 3) Thin thermal bridge materials such as HfO₂and ZrO₂were inserted between <i>β</i>-Ga₂O₃and a sapphire substrate to reduce self heating effects without using a diamond substrate. The improved thermal performance of the device was analyzed by phonon density of states plots of <i>β</i>-Ga₂O₃and the thin film materials. 4) Nano-membrane tri-gate <i>β</i>-Ga₂O₃FETs on SiO₂/Si substrate fabricated via exfoliation have been demonstrated for the first time. 5) Using the robustness of <i>β</i>-Ga₂O₃in harsh environments, <i>β</i>-Ga₂O₃ferroelectric FETs operating as synaptic devices up to 400 °C were demonstrated. The result offers the potential to use the novel device for ultra-wide bandgap logic applications, specifically neuromorphic computing exposed to harsh environments.<br></p>