Browsing by Subject "Sputtering"
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Item Epitaxial Growth of thin film strontium cobaltite: a feasibility study.(2012-06) Gulden, TobiasIn this work we present a feasibility study of epitaxial growth of thin films of strontium cobaltite, SrCoO3-\delta. The properties of strontium doped lanthanum cobaltite, La1-xSrxCoO3, have been widely studied for dopant concentration x<0.5, but little work has been performed on the x=1 member of the series. The main issue is that this is not a thermodynamically preferable state and close to stochiometric SrCoO3 in polycrystalline samples can only be obtained under high pressures of oxygen (>10kbar) or by electrochemical oxidation. However, theoretical calculations predict a phase change with respect to strain in epitaxially grown samples, from ferromagnetic-metallic behaviour in the bulk state to insulating-ferroelectric-antiferromagnetic behaviour for strongly strained films. This provides strong motivation for epitaxial growth of SrCoO3-d films. In this work we will present a feasibility study by using the methods of high-pressure oxygen sputtering (typically 1.0-4.0mbar) on SrTiO3(001) and LaAlO3(001) substrates. As anticipated, the presence of oxygen vacancies is a severe problem, but also epitaxial stabilization of non-cubic phases, an unexpected issue, arises. These are found to grow in multiple orientations. Overall, the samples exhibit only weak or no ferromagnetism, even though bulk SrCoO3 is known to be a strong ferromagnet. Based on the results, we present an outline for suggested further research on this topic.Item Mobility Optimization in LaxBa1-xSnO3 Thin Films Deposited via High Pressure Oxygen Sputtering(2017-08) Postiglione, WilliamBaSnO3 (BSO) is one of the most promising semiconducting oxides currently being explored for use in future electronic applications. BSO possesses a unique combination of high room temperature mobility (even at very high carrier concentrations, >1019 cm-3), wide band gap, and high temperature stability, making it a potentially useful material for myriad applications. Significant challenges remain however in optimizing the properties and processing of epitaxial BSO, a critical step towards industrial applications. In this study we investigate the viability of using high pressure oxygen sputtering to produce high mobility La-doped BSO thin films. In the first part of our investigation we synthesized, using solid state reaction, phase-pure stoichiometric polycrystalline 2% La-doped BaSnO3 for use as a target material in our sputtering system. We verified the experimental bulk lattice constant, 4.117 Å, to be in good agreement with literature values. Next, we set out to optimize the growth conditions for DC sputtering of La doped BaSnO3. We found that mobility for all our films increased monotonically with deposition temperature, suggesting the optimum temperature for deposition is >900°C and implicating a likely improvement in transport properties with post-growth thermal anneal. We then preformed systematic studies aimed at probing the effects of varying thickness and deposition rate to optimize the structural and electronic transport properties in unbuffered BSO films. In this report we demonstrate the ability to grow 2% La BSO thin films with an effective dopant activation of essentially 100%. Our films showed fully relaxed (bulk), out-of-plane lattice parameter values when deposited on LaAlO3, MgO, and (LaAlO3)0.3(Sr2TaAlO6)0.7 substrates, and slightly expanded out-of-plane lattice parameters for films deposited on SrTiO3, GdScO3, and PrScO3 substrates. The surface roughness’s of our films were measured via AFM, and determined to be on the nm scale or better. Specular XRD measurements confirmed highly crystalline films with narrow rocking curve FWHMs on the order of 0.05°. The optimum thickness found to maximize mobility was around 100 nm for films deposited at ~8 Å/min. These films exhibited room temperature mobilities in excess of 50 cm2V-1s 1 at carrier concentrations ~3 x 1020 cm-3 across 4 different substrate materials (LaAlO3, SrTiO3, GdScO3, and PrScO3). Contrary to expectations, our findings showed no dependence of mobility on substrate mismatch, indicating that threading dislocations are either not the dominant scattering source, or that threading dislocation density in the films was constant regardless of the substrate. The highest mobility film achieved in this study, 70 cm2V 1s 1, was measured for a film grown at a considerably slower rate (~2 Å/min) and lower thickness (~380 Å). Said film was deposited on a PrScO3 (110) substrate, the most closely lattice matched substrate commercially available for BSO (–2.2% pseudo-cubic). This film showed a high out-of-plane lattice parameter from X-ray diffraction (aop = 4.158 Å), suggesting a significantly strained film. This result highlights the possibility of sputtering coherent, fully strained, BSO films, far exceeding the theoretical critical thickness for misfit dislocation formation, on closely lattice matched substrates. Overall, this work validates the concept of high pressure oxygen sputtering to produce high mobility La-doped BSO films. The mobility values reported in this thesis are comparable to those found for films deposited via pulsed laser deposition in previous studies, and represent record values for sputter deposited BSO thin films.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.