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 PLD GROWTH OF HIGH QUALITY ZINC OXIDE THIN FILMS ON SI SUBSTRATES AND DEVICE DEVELOPMENT(2012) Esmaili Sardari, Saeed; Iliadis, Agis A.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Growth of high quality zinc oxide thin films on silicon substrates is particularly important because it combines the unique features of zinc oxide (ZnO) with mature CMOS technology and paves the way for device developments. However, this is a challenging task due to several technical and material-related fundamental issues which exist with the growth of this highly sought after compound semiconductor. In general, metal-oxide semiconductors suffer from non-stoichiometric growth which leads to unipolar doping properties, such as ZnO grows nominally n-type while NiO grows p-type. Thus, these materials can be doped easily either n or p-type while the other polarity is hard to achieve, if not impossible. Although methods for the growth of p-type films, using extrinsic doping or exotic precursors with post growth treatments have been reported on different substrates, the problem of controlled and stable extrinsic p-type doping of ZnO films remains an open subject for research. In the present work, we have achieved the growth of undoped p-type ZnO films on Si (100) substrates by pulsed laser deposition through the optimization of growth conditions and adjustments of growth dynamics. Currently no other reports of undoped p-type ZnO on Si substrates by PLD growth exist, showing stable p-type conductivity in a repeatable process. The structural, optical, and electrical properties of the grown films were examined using techniques such as X-ray photoelectron spectroscopy (XPS), X-ray diffraction spectroscopy (XRD), Photoluminescence spectroscopy (PL), Hall Effect four-point probe Van Der Pauw measurements, and Current-Voltage (I-V) measurements for the p-n heterojunctions. Electrical behavior of ZnO is generally attributed to intrinsic defects which include vacancies, interstitials, and anti-sites of Zn and O in addition to external contamination related defects such as hydrogen complexes. The effects of growth conditions and intrinsic defects including hydrogen contamination on the properties of the grown layers are studied. Moreover, the growth dynamics of ZnO polar planes, i.e. the stacking of O-2 and Zn+2 planes, on n and p-type Si substrates are discussed. Once material studies and growth optimizations are completed, high quality ZnO films are used in device fabrication. Two types of optoelectronic devices, a photoresistor and a Schottky diode are fabricated on Si substrates, and the electrical behavior of the devices are investigated. The high quality ZnO films also contributed to the development of a surface acoustic wave (SAW) biosensor.Item Formation and Piezoelectricity of Self-Assembled PbTiO3-CoFe2O4 Nanostructural Films(2008-06-13) tan, zhuopeng; Roytburd, Alexander L; Levin, Igor; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Main tasks of our research include: (1) exploring optimum growth conditions for PLD deposition of self-assembled nanophase PbTiO3-CoFe2O4 films with different compositions and orientations; (2) analyzing morphologies and nanostructures of the two-phase films to clarify relative effects of elastic energy and interface energy on the self-assembled film formation; (3) investigating stress state and relaxation of stresses arising as a result of a paraelectric-ferroelectric transformation in PbTiO3; (4) exploring ferroelectric state in the confined PbTiO3 nanophase in the films with {110} and {111} orientations. Principal results of the research are: (1). Optimum PLD growth conditions to obtain high quality films with distinct separation of epitaxial PbTiO3 and CoFe2O4 nanophases are found after systematic studies. (2). Nano-facets along {111} plane between PbTiO3 and CoFe2O4 phases are found to be generic in addition to orientation dependent macroscopic interfaces. We have concluded that accounting of interface and surface energies is important for description of nano-faceting of interfaces and the near substrate zone of the films while the two-phase morphologies are determined by the elastic interactions; (3). The investigation of the stress state of the {001} film arising due to paraelectric-ferroelectric transition of PbTiO3 have discovered the polydomain nanostructure of the ferroelectric phase with ~50-60% c-domains. Piezoresponse of PbTiO3 should be reduced dramatically by combined effects of dissolution of Fe in PbTiO3, a domains and constraints. The relative large dzz from previous research must contain large extrinsic contribution due to movement of nano-domain walls. (4). Switching spectroscopy piezoresponse force microscopy (SS-PFM) is used to characterize local piezo- ferroelectric property of confined ferroelectrics in {110} and {111} films with composition of 1/3PbTiO3-2/3CoFe2O4. It is proved that PbTiO3 nano-inclusions exhibit ferroelectricity in both films. 180o domain switching is observed under measurement condition (<10V) for the {110} films but not for the {111} film. Quantitatively, both films yield a piezoresponse of about 15% compared to bulk single crystal PbTiO3. It is a reasonable value of intrinsic piezoeffect taking into account mechanical and electrical constraints (depolarizing field) as well as the effect of Fe dissolution and possible in-plane domainsItem Development of a ZnO/SiOsub2/Si High Sensitivity Interleukin-6 Biosensor(2007-04-05) Krishnamoorthy, Soumya; Iliadis, Agis; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Biosensors offer the opportunity to sense biological material providing valuable information for medical diagnostics and monitoring of pathogens in the environment. Thus the development of high sensitivity, cost effective, real-time and portable biosensors is of primary importance. This thesis presents the development of a ZnO/SiOsub2/Si based CMOS compatible biosensor, for the real-time detection of interleukin-6 (IL-6). In this work, high quality ZnO films were grown on SiOsub2/Si substrates by pulsed laser deposition. A protein immobilization procedure for binding the IL-6 protein to the ZnO active area was developed and the morphology of the bio-molecules was studied using SEM and AFM techniques for the first time. A modified solid-phase Enzyme linked Immunosorbent assay (ELISA) technique was developed to measure the mass of protein bound onto the oxide surface. The study resulted in a mass of 0.364 pg/ml of IL-6 bound onto ZnO for an applied mass of 0.5 ng/ml. A guided shear mode surface acoustic wave (SAW) device in the ZnO/SiOsub2/Si system, with ZnO as the guiding layer on a SiOsub2/Si substrate, was modeled and fabricated. Two devices, operating at 708 MHz (device A) and 1.5 GHz (device B) were developed. The mass sensitivities of these devices were calibrated by applying a known copolymer mass in a window area opened in the SAW devices and by measuring the frequency shift due the application of the mass. The maximum mass sensitivity of devices A and B was 4.162 μm²/pg and 8.687 (μm²/pg) for ZnO guiding layer thickness of 340 nm and 160 nm respectively. A technique to apply IL-6 directly onto the SAW sensor surface was developed. For an applied IL-6 mass ranging from 20ng/ml - 2 μg/ml, applied in a 20x20 μm2 sensing area, the device measures IL-6 masses in the range of 1.2 fg-76.45 fg. A proof-of-concept experiment for the biosensor was setup with normal human serum to detect the presence of IL-6 in trace amounts. The device predicts three times as much IL-6 mass for normal human serum derived from pooled donors under the age of 55 as compared to that from a donor over the age of 55. This is understood to be the result of age related increased IL-6 levels and was independently confirmed through ELISA measurements. The ZnO/SiOsub2/Si sensor system therefore enables highly sensitive mass detection of the IL-6 protein to be realized.Item Combinatorial Exploration of Artificial Multiferroic Thin Films(2004-05-12) Lin, Chuan-Lan; Takeuchi, Ichiro; Material Science and EngineeringCombinatorial synthesis consists of high throughput fabrication and rapid characterization of compositionally varying samples to speed up the process of materials development. In this thesis, synthesis of composition spreads of artificial multiferroic thin film heterostructures consisting of alternating layers of (PTO) - (CFO) has been demonstrated using our combinatorial pulsed laser deposition (PLD) system. In the spread samples, the average composition changes continuously from pure PTO to pure CFO so that we can observe the changes in physical properties as a function of average composition. The coexistence of ferromagnetic and ferroelectric properties has been observed in a large extended region between pure CFO and PTO. An unexpected peak in the dielectric property has been observed in the middle of the spread, and it was identified as the composition where the ferroelectric phase transition takes place in PTO doped with CFO. The of ferroelectricity is found to be tunable from 500 ℃ to room temperature by controlling the average volume ratio of CFO and PTO. We have also found that the magnetic anisotropy in the materials changes by introducing PTO to CFO.