Theses and Dissertations from UMD
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Item ATOMIC LAYER DEPOSITION OF LEAD ZIRCONATE-TITANATE AND OTHER LEAD-BASED PEROVSKITES(2019) Strnad, Nicholas Anthony; Phaneuf, Raymond J; Polcawich, Ronald G; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Lead-based perovskites, especially lead zirconate-titanate (PbZrxTi1-xO3, or PZT), have been of great technological interest since they were discovered in the early 1950s to exhibit large electronic polarization. Atomic layer deposition (ALD) is a thin-film growth technique capable of uniformly coating high aspect-ratio structures due to the self-limited nature of the precursor chemisorption steps in the deposition sequence. In this thesis, a suite of related processes to grow lead-based perovskites by ALD are presented. First, a new process to grow ferroelectric lead titanate (PbTiO3, or PTO) by ALD using lead bis(3-N,N-dimethyl-2-methyl-2-propanoxide) [Pb(DMAMP)2] and tetrakis dimethylamino titanium [TDMAT] as the lead and titanium cation precursors, respectively, is discussed. A 360-nm thick PTO film grown by ALD displayed a maximum polarization of 48 µC/cm2 and remanent polarization of ±30 µC/cm2. Second, a new process (similar to the ALD PTO process) to grow PZT by ALD is demonstrated by partial substitution of TDMAT with either tetrakis dimethylamino zirconium or zirconium tert-butoxide. The 200 nm-thick ALD PZT films exhibited a maximum polarization of 50 µC/cm2 and zero-field dielectric constant of 545 with leakage current density < 0.7 µA/cm2. Third, a new ALD process for antiferroelectric lead hafnate (PbHfO3, or PHO) is presented along with electrical characterization showing a field-induced antiferroelectric to ferroelectric phase transition with applications for capacitive energy storage. Finally, ALD lead hafnate-titanate (PbHfxTi1-xO3, or PHT), considered to be an isomorph of PZT, is demonstrated by combining the process for PTO and PHO. The thin-film PHT grown by ALD is shown to have electronic properties that rival PZT grown at compositions near the morphotropic phase boundary (MPB). The processes for both ALD PZT and PHT are shown to yield films with promising properties for microelectromechanical systems (MEMS) actuators and may help to dramatically increase the areal work density of such devices.Item Bottom-Up Multiferroic Nanostructures(2009) Ren, Shenqiang; Wuttig, Manfred; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Multiferroic and especially magnetoelectric (ME) nanocomposites have received extensive attention due to their potential applications in spintronics, information storage and logic devices. The extrinsic ME coupling in composites is strain mediated via the interface between the piezoelectric and magnetostrictive components. However, the design and synthesis of controlled nanostructures with engineering enhanced coupling remain a significant challenge. The purpose of this thesis is to create nanostructures with very large interface densities and unique connectivities of the two phases in a controlled manner. Using inorganic solid state phase transformations and organic block copolymer self assembly methodologies, we present novel self assembly "bottom-up" techniques as a general protocol for the nanofabrication of multifunctional devices. First, Lead-Zirconium-Titanate/Nickel-Ferrite (PZT/NFO) vertical multilamellar nanostructures have been produced by crystallizing and decomposing a gel in a magnetic field below the Curie temperature of NFO. The ensuing microstructure is nanoscopically periodic and anisotropic. The wavelength of the PZT/NFO alternation, 25 nm, agrees within a factor of two with the theoretically estimated value. The macroscopic ferromagnetic and magnetoelectric responses correspond qualitatively and semi-quantitatively to the features of the nanostructure. The maximum of the field dependent magnetoelectric susceptibility equals 1.8 V/cm Oe. Second, a magnetoelectric composite with controlled nanostructures is synthesized using co-assembly of two inorganic precursors with a block copolymer. This solution processed material consists of hexagonally arranged ferromagnetic cobalt ferrite (CFO) nano-cylinders within a matrix of ferroelectric Lead-Zirconium-Titanate (PZT). The initial magnetic permeability of the self-assembled CFO/PZT nanocomposite changes by a factor of 5 through the application of 2.5 V. This work may have significant impact on the development of novel memory or logic devices through self assembly techniques. It also demonstrates a universal two-phase hard template application. Last, solid-state self assembly had been used recently to form pseudoperiodic chessboard-like nanoscale morphologies in a series of chemically homogeneous complex oxide systems. We improved on this approach by synthesizing a spontaneously phase separated nanolamellar BaTiO3-CoFe2O4 bi-crystal. The superlattice is magnetoelectric with a frequency dependent coupling. The BaTiO3 component is a ferroelectric relaxor with a Vogel-Fulcher temperature of 311 K. Since the material can be produced by standard ceramic processing methods, the discovery represents great potential for magnetoelectric devices.Item COMBINATORIAL DISCOVERY OF A MORPHOTROPIC PHASE BOUNDARY IN A LEAD-FREE PIEZOELECTRIC MATERIAL(2008-08-07) Fujino, Shigehiro; Takeuchi, Ichiro; Wuttig, Manfred; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)BiFeO3 (BFO) is known to display rich and intricate multiferroic and chemical properties. It has high switchable polarization and piezoelectric properties similar to PbTiO3, but it also suffers from high coercive fields and high leakage currents. This has prompted investigations of doped-BFO as novel solid solutions which may emulate the performance of PbZr1-xTixO3 (PZT) in the composition range of 0.48 < x < 0.52, where a morphotropic phase boundary (MPB) resides and displays substantially enhanced piezoelectric and ferroelectric properties. However neither an increase in electromechanical constants as a function of dopant concentration or domain structures indicative of a MPB piezoceramic had been reported in lead-free BFO-based systems prior to this work. There are some guidelines which predict the presence of MPBs, and one can explore novel compositions by systematically searching for similar structural transitions, Yet comprehensive mapping of compositions requires synthesis of an enormously large number of individual samples. We report on the discovery of a lead-free morphotropic (composition dependent, temperature independent) phase boundary with a simple perovskite structure. The combinatorial thin film strategy was used to identify a rhombohedral to pseudo-orthorhombic structural transition which exhibits a ferroelectric to antiferroelectric transition at approximately Bi0.86Sm0.14FeO3. At the morphtropic phase boundary, there is substantial enhancement in the dielectric and piezoelectric coefficient of Bi0.86Sm0.14FeO3 film is comparable to that of Pb Zr0.52Ti0.48O3 thin film. The discovered compound is a strong candidate if a lead-free piezoelectric material.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 domains