Browsing by Subject "Doping"
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Item Impurities in silicon nanocrystals: the intentional and the inherent(2013-04) Rowe, David JSilicon nanocrystals (SiNCs) have become an important class of materials in the fields of photovoltaics, thermoelectrics, lighting, and medicine. Impurities within SiNCs dramatically alter the electrical and optical properties of the host material, whether the impurity is intentionally added in an attempt to manipulate properties, or is inherent to the material and its natural state. Despite such remarkable changes, impurity incorporation within SiNCs remains poorly understood, since concepts applied to understanding impurities in bulk materials may not completely translate to nanomaterials. Understanding the effect of SiNC impurities requires new technologies to produce materials suitable for study combined with new insights to expound the differences in the nanoscale physics. Nonthermal plasma-assisted gas-phase synthesis provides an excellent route to producing and investigating impurities within SiNCs due to the unique chemical reaction environment of the plasma. The robustness of such a technique allows for the production of very pure SiNCs or SiNCs with added impurities simply by adding different chemicals to the plasma. The chapters in this document focus on the effect that different impurities have on the properties of SiNCs. Chapter 2 focuses on heavily P-doped SiNCs exhibiting the first known observation of a unique electrical and optical property known as localized surface plasmon resonance (LSPR) within free-standing SiNCs. Chapter 3 explains the synthesis of B- and P-doped SiGeNC alloys and their deposition into thin films for thermoelectric applications. Chapter 4 highlights research which uses P-doped SiNCs to form emitter layers for pn-junction type solar cells, including device fabrication and optical characterization. Chapter 5 examines inherent impurities in the form of dangling bond defects which may be responsible for the quenching of SiNC photoluminescence, and their evolution during the process of air-ambient oxidation. Several appendices at the end of the document detail some of the fabrication processes used throughout this work, as well as brief reports of some side projects that may be of interest to researchers intent on studying SiNC synthesis and deposition technologies.Item Structure and Transport in Epitaxial BaSnO3: Doping, Mobility and the Insulator-Metal Transition(2018-08) Ganguly, KoustavThe recent discovery of high room temperature electron mobility in wide band gap BaSnO3 (BSO) has generated exceptional interest in this perovskite oxide for electronic devices. Outstanding issues with regards to epitaxial films include understanding transport mechanisms, determining the optimal dopant, and understanding the role of structural defects (like dislocations) in limiting mobility. Here, we discuss detailed temperature and field-dependent electronic transport in both oxygen vacancy and La-doped BSO films grown via high pressure oxygen sputter deposition. High-resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM), and scanning transmission electron microscopy (STEM) confirm phase-pure, close to stoichiometric, smooth, epitaxial BSO(001). Film thickness, growth rate, deposition temperature, and substrate (i.e., lattice mismatch) have all been systematically varied and related to mobility. Detailed transport accompanied with STEM has been used to understand the structure-electronic property relationships and reveal the correlation between misfit and threading dislocations in BSO thin films. As-grown undoped, insulating films can be made conductive with controllable n-type doping by vacuum reduction, resulting in 300 K Hall mobilities up to 35 cm2V-1s-1 (on LaAlO3(001)) at 5×1019 cm-3. The mobility-electron density relation has been probed in this manner, down to 2×1017 cm-3, the lowest electron density probed in BSO till date. 2% La-doped BSO films, on the other hand, demonstrate 300 K electron mobilities up to 70 cm2V-1s-1 at ~2 ×1020 electrons per cm3. With increasing film thickness a clear insulator-metal transition is observed with both dopants, likely related to defect density near the substrate. The low temperature upturn in resistivity observed in metallic-like BSO has been analyzed using out-of-plane and in-plane magnetoresistance (MR) measurements. Two-dimensional weak localization (WL) has been identified as the underlying mechanism behind this low temperature quantum correction. Overall, the results not only validate the technique of high-pressure oxygen sputtering as a viable approach to produce high quality BSO films, but also provide insight into the mobility-electron density relation, and mobility-limiting factors in these films. The mobility values reported in this thesis are record values for sputtered films and are comparable to that obtained via pulsed laser deposition (PLD) in previous studies.Item Synthesis and Doping of Silicon Nanocrystals for Versatile Nanocrystal Inks(2015-05) Kramer, NicolaasThe impact of nanotechnology on our society is getting larger every year. Electronics are becoming smaller and more powerful, the “Internet of Things” is all around us, and data generation is increasing exponentially. None of this would have been possible without the developments in nanotechnology. Crystalline semiconductor nanoparticles (nanocrystals) are one of the latest developments in the field of nanotechnology. This thesis addresses three important challenges for the transition of silicon nanocrys- tals from the lab bench to the marketplace: A better understanding of the nanocrystal synthesis was obtained, the electronic properties of the nanocrystals were characterized and tuned, and novel silicon nanocrystal inks were formed and applied using simple coating technologies. Plasma synthesis of nanocrystals has numerous advantages over traditional solution-based synthesis methods. While the formation of nanoparticles in low pressure nonthermal plasmas is well known, the heating mechanism leading to their crystallization is poorly understood. A combination of comprehensive plasma characterization with a nanoparticle heating model presented here reveals the underlying plasma physics leading to crystallization. The model predicts that the nanoparticles reach temperatures as high as 900 K in the plasma as a result of heating reactions on the nanoparticle sur- face. These temperatures are well above the gas temperature and sufficient for complete nanoparticle crystallization. Moving the field of plasma nanoparticle synthesis to atmospheric pressures is impor- tant for lowering its cost and making the process attractive for industrial applications. The heating and charging model for silicon nanoparticles was adapted in Chapter 3 to study plasmas maintained over a wide range of pressures (10 − 10^5 Pa). The model considers three collisionality regimes and determines the dominant contribution of each regime under various plasma conditions. Strong nanoparticle cooling at atmospheric pressures necessitates high plasma densities to reach temperatures required for crystallization of nanoparticles. Using experimentally determined plasma properties from the literature, the model estimates the nanoparticle temperature that is achieved during synthesis at atmospheric pressures. It was found that temperatures well above those required for crystallization can be achieved. Now that the synthesis of nanocrystals is understood, the second half of this thesis will focus on doping of the nanocrystals. The doping of semiconductor nanocrystals, which is vital for the optimization of nanocrystal-based devices, remains a challenge. Gas phase plasma approaches have been very successful in incorporating dopant atoms into nanocrystals by simply adding a dopant precursor during synthesis. However, little is known about the electronic activation of these dopants. This was investigated with field-effect transistor measurements using doped silicon nanocrystal films. It was found that, analogous to bulk silicon, boron and phosphorous electronically dope silicon nanocrystals. However, the dopant activation efficiency remains low as a result of self-purification of the dopants to the nanocrystal surface. Next the plasmonic properties of heavily doped silicon nanocrystals was explored. While the synthesis method was identical, the plasmonic behavior of phosphorus-doped and boron-doped nanocrystals was found the be significantly different. Phosphorus-doped nanocrystals exhibit a plasmon resonance immediately after synthesis, while boron-doped nanocrystals require a post-synthesis annealing or oxidation treatment. This is a result of the difference in dopant location. Phosphorus is more likely to be incorporated into the core of the nanocrystal, while the majority of boron is placed on the surface of the nanocrystal. The oxidized boron-doped particles exhibit stable plasmonic properties, and therefore this allows for the production of air-stable silicon-based plasmonic materials which is very interesting for certain applications. Finally the boron atoms were used to form a Lewis acidic nanocrystal surface chemistry allowing for the creation of ligand-less silicon nanocrystal solutions. This represents an immense step towards an abundant, non-toxic alternative to Pb and Cd-based nanocrystal technologies. The lack of long ligand chains enables the production of dense films with excellent electrical conductivity. This was demonstrated by forming uniform nanocrystal thin-films using simple and inexpensive spray coating techniques.Item Zinc oxide nanoparticles: doping, Inkjet printing, and electron accepting from photoexcited porphyrin dyes(2013-06) Bierbaum, Andrew JosephThis research attempted to extend the useful applications of ZnO by investigating ZnO nanoparticles, doping ZnO nanoparticles, characterizing electron injection from dye molecules into ZnO nanoparticles, and depositing thin films of doped ZnO nanoparticles using inkjet printing. Chapter 1 describes research that produced particles ranging from 2.7 nm to 1 μm of undoped and doped ZnO. These particles were made using solution methods with zinc acetate and aluminum and gallium nitrate salts as dopants, and the particles were characterized by ultraviolet visible absorption, photoluminescence, infrared absorption, and transmission or scanning electron microscopy. The doped ZnO nanoparticles displayed optical signatures of doping in particles larger than 10 nm. This is significant because doping of nanoparticles is still not fully understood, and there are few examples of successfully doping nanoparticles. Chapter 2 describes the research done toward inkjet printing of ZnO films for potential use in a fully inkjet printed solar cell. The research aim was to produce a TCO film of ZnO using inkjet printing that had a bulk resistivity between 10-2 and10-3 Ω cm, a thickness between 0.1 and 1 μm, the highest transparency possible, and processed using conditions under 250 ºC. Film produced using solution methods including inkjet printing were characterized by four point probe ohmmeter, x-ray diffraction, ultraviolet visible absorption, visible microscopy, profilometry, and scanning electron microscopy. Inkjet printed films produced using nanoparticles did not meet the production requirements, but ii progress towards these goals are presented along with the successes and shortcoming of the methods used. Chapter 3 describes the research done on charge transfer from photoexcited porphyrin dyes into ZnO nanoparticles dispersions in methanol. The goal of this research was to further the understanding of the dye-semiconductor interaction and important electron transfer characteristics. Using a series of three porphyrin dyes and a range of particle sizes, the rate of electron transfer was investigated.