Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
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Item ATOMIC LAYER DEPOSITION OF CADMIUM TELLURIDE FOR THE PASSIVATION OF MERCURY CADMIUM TELLURIDE(2021) Pattison, James William; Salamanca-Riba, Lourdes G; VanMil, Brenda; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Mercury cadmium telluride (MCT) is an important infrared (IR) detector material due to high quantum efficiency and the ability to tune the bandgap, covering important IR wavelengths from near-infrared (~1 m) to very-long wavelength infrared (>12 m) detection. Focal Plane Arrays (FPAs) are used to image in the infrared and consist of photodiodes that absorb IR photons, generating charge carriers that create an electric signal used to form an image by combining the signals from all of the photodiodes. Decreasing photodiode size increases the resolution of optical systems incorporating MCT FPAs, but challenges current state-of-the-art passivation processes. Passivation is needed to increase the signal-to-noise of a system by rendering benign the charge carrier transport. Physical-vapor-deposited (PVD) CdTe is the incumbent passivation material for MCT, but fails when applied to the next generation of MCT photodiodes because of non-conformal deposition. Atomic layer deposition (ALD) is a superior deposition technique in this regard because the vapor-phase chemicals enable conformal exposure of the surface as opposed to line-of-sight deposition in PVD. ALD of CdTe requires deposition temperature lowering to suppress out-gassing of Hg at elevated temperature, which leads to mercury vacancy formation, reducing signal-to-noise of any eventual detector. Previous demonstration of CdTe ALD was spontaneous above ~200 °C for chemisorbed dimethylcadmium (DMCd) to react with diethyltellurium (DETe). However, this temperature is incompatible with MCT devices, because of the loss of Hg from the material. This dissertation attempts to overcome the low temperature requirement of current CdTe ALD using a novel approach in which argon plasma successfully decomposes the chemisorbed DMCd, replacing temperature induced thermal decomposition, and induced CdTe growth using either DETe or bis(trimethylsilyl)telluride as the tellurium precursors at low temperatures. Film deposition conditions were developed through deposition on silicon substrates, and the process was transferred to MCT samples, demonstrating low temperature deposition, conformal deposition, and passivation of the MCT surface. The films were characterized by in situ spectroscopic ellipsometry (SE), x-ray photoelectron spectroscopy (XPS), x ray diffraction (XRD), and transmission electron microscopy (TEM). Photoconductive decay (PCD) measurements were made of MCT material passivated by CdTe ALD, demonstrating effective passivation through enhanced minority carrier lifetime.Item Remote Sensing of Clouds and Precipitation: Event-based Characterization, Life Cycle Evolution, and Aerosol Influences(2016) Esmaili, Rebekah; Zeng, Ning; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Global climate models, numerical weather prediction, and flood models rely on accurate satellite precipitation products, which are the only datasets that are continuous in time and space across the globe. While there are more earth observing satellites than ever before, gaps in precipitation retrievals exist due to sensor and orbital limitations of low-earth (LEO) satellites, which are overcome by merging data from different sensors in satellite precipitation products (SPPs). Using cloud tracking at higher resolutions than the spatio-temporal scales of LEO satellites, this thesis examines how clouds typically form in the atmosphere, the rate that cloud size and temperature evolve over the life cycle, and the time of day that cloud development take place. This thesis found that cloud evolution was non-linear, which disagrees with the linear interpolation schemes used in SPPs. Longer lasting clouds tended to achieve their temperature and size maturity milestones at different times, while these stages often occurred simultaneously in shorter lasting clouds. Over the ocean, longer lasting clouds were found to occur more frequently at night, while shorter lasting clouds were more common during the daytime. This thesis also examines whether large-scale Saharan dust outbreaks can impact the trajectories and intensity of cloud clusters in the tropical Atlantic, which is predicted by modeling studies. The presented results show that proximity to Saharan dust outbreaks shifts Atlantic cloud development northward and intense storms becoming more common, whereas on days with low dust loading small-scale, warmer clouds are more common. A simplified view of cloud evolution in merged rainfall retrievals is a possible source of errors, which can propagate into higher level analysis. This thesis investigates the difference in the intensity, duration, and frequency of precipitation in IMERG, a next-generation satellite precipitation product with ground radar observations over the contiguous United States. There was agreement on seasonal totals, but closer examination shows that the average intensity and duration of events is too high, and too infrequent compared to events detected on the ground. Awareness of the strengths and limitations, particularly in context of high-resolution cloud development, can enhance SPPs and can complement climate model simulations.Item The Star-Forming Properties of an Ultra-Hard X-ray Selected Sample of AGN(2016) Shimizu, Thomas Taro; Mushotzky, Richard; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis provides a comprehensive examination of star formation in the host galaxies of active galactic nuclei or AGN. AGN are bright, central regions of galaxies that are powered through accretion onto a supermassive black hole (SMBH). Through accretion and the loss of gravitational potential energy, AGN emit powerful radiation over all wavelengths of the electromagnetic spectrum. This radiation can influence the AGN's host galaxy through what is known as AGN ``feedback'' and is thought to suppress star formation as well as stop accretion onto the SMBH leading to a co-evolution between the SMBH and its host galaxy. Theoretical models have long invoked AGN feedback to be able reproduce the galaxy population we see today but observations have been unclear as to whether AGN actually have an effect on star formation. To address this question, we selected a large sample of local ($z < 0.05$) AGN based on their detection at ultra-hard X-ray energies (14--195 keV) with the \textit{Swift} Burst Alert Telescope (BAT). Ultra-hard X-ray selection frees our sample from selection effects and biases due to obscuration and host galaxy contamination that can hinder other AGN samples. With these 313 BAT AGN we conducted a far-infrared survey using the \herschel \textit{Space Observatory}. We use the far-infrared imaging to probe the cold dust that traces recent star formation in the galaxy and construct spectral energy distributions (SEDs) from 12--500 \micron. We decompose the SEDs to remove the AGN contribution and measure infrared luminosity which provides us with robust estimates of the star formation rate (SFR). Through a comparison with a stellar-mass matched non-AGN sample, we find that AGN host galaxies have larger dust masses, dust temperatures, and SFRs, confirming the results of previous studies that showed the optical colors of the BAT AGN are bluer than non-AGN. We find that the AGN luminosity as probed by the 14--195 keV luminosity is not related to the SFR of the host galaxy suggesting global, large scale star formation on an individual basis is not affected by the AGN. However, after a thorough analysis comparing our AGN to star-forming main sequence, a tight relationship between the SFR and stellar mass of a galaxy, we discover that our AGN as a whole show systematically lower specific SFRs (SFR/stellar mass). We confirm that AGN host galaxies, as a population, are transitioning between the star-forming and quiescent populations. This result supports the theory that AGN feedback has suppressed star formation, but we also consider other models that could reproduce our observations. Finally we conclude with a summary of this thesis and describe several ongoing and future projects that will push forward the exciting field of AGN research.Item Forced Convective Boiling via Infrared Thermography(2012) Kommer, Eric; Kim, Jungho; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Multiphase heat transfer is an important mechanism across wide variety of engineering disciplines. The prediction of the heat transfer rate as a function of flow conditions and temperature has been based almost exclusively on experimentally derived correlations. The quality of these correlations depends on the accuracy and resolution of the measurement technique. In addition to the complexities of flow boiling phenomenon in earth gravity, engineering design of space systems requires knowledge of any gravity dependence for heat transfer characteristics. Current research has shown significant variation in the heat transfer characteristics during pool boiling as a function of gravity magnitude. Research into flow boiling in variable gravity environments is extremely limited at this time, but necessary before multiphase systems can be designed for space. The objective of this study is to develop, validate, and use a unique infrared thermometry method to quantify the heat transfer characteristics of flow boiling in earth gravity, prior to use of the apparatus in variable gravity environments. This new method allows high spatial and temporal resolution measurements, while simultaneously visualizing the flow phenomenon. Validation of this technique will be demonstrated by comparison to accepted correlations for single and multiphase heat transfer in earth gravity environments.