Defect-Structure-Property Relations in Complex-Oxide Ferroelectric Thin Films

Author(s): Sareminaeini, Sahar | Advisor(s): Martin, Lane W | Abstract: One role for modern materials science is to provide a foundation upon which scientists and engineers in diverse fields can address the needs of current and future societal challenges through the realization of next-generation technologies. Key to such advances is not only the development of advanced materials with novel or enhanced properties and performance, but also the know-how to synthesize and process such materials in a deterministic manner so that their properties can be effectively and efficiently utilized. Materials science is founded upon the concept that structure, processing, properties, and, ultimately, performance of materials are intimately interconnected. And, as the field has evolved, materials scientists and engineers have increasingly realized that even our best efforts to control these tenets can be remarkably hampered if we do not account for and address the role of material imperfections. Underlying all this is the fact that defects are unavoidable. Even in the most “perfect” materials, there are always finite concentrations of various structural and compositional defects. Although in some material systems defects have been extensively studied and used to engineer and improve properties, the general opinion of defects in ferroelectric community is not a good one – defects are regarded as “bad guys” and thought to be (uniformly) deleterious to material performance. But, armed with advances in our ability to synthesize, characterize, and model these materials, this negative connotation stands poised to be redefined. So can defects really be “good guys” in the ferroelectric world? In this Thesis I aim to view defects in a new light – a positive one – that casts them as another tool to design better ferroelectric materials with emergent properties and functionalities.In the present work, I demonstrate strong defect-structure-property couplings in thin film versions of various complex-oxide ferroelectrics (including BaTiO3, BiFeO3, PbTiO3, PbZrxTi1 xO3) and relaxor ferroelectrics (such as 0.68PbMg1/3Nb2/3O3-0.32PbTiO3) grown via pulsed-laser deposition, and show that such defect-structure-property interplays can be manipulated with deliberate introduction of certain defect types at controlled concentrations and locations which can provide new pathways to enhanced properties and novel functions. Among all defect types that can be present, this work only focuses on point defects as they are the most abundant defects in ionically-bonded solids such as complex-oxide ferroelectrics and play a particularly important role in impacting the properties of these materials. Nevertheless, in surprisingly few cases does one have a detailed understanding of point defects and their impact on the properties due to difficulties in their control and characterization. In the present work, I introduce various in situ and ex situ approaches for on-demand defect creation. The in situ approach relies on the variations of growth parameters (such as laser fluence, laser-repetition rate, target composition, and growth pressure) in order to control defects during the synthesis process of the thin films, while the ex situ approach focuses on the use of energetic ion beams (both defocused high-energy, and focused low-energy ion beams) to introduce defects in already-grown films. This controlled defect production is then used to perform systematic experimental studies on the evolution of various material properties (including transport, dielectric, and ferroelectric properties) as a function of defect type and across many orders of magnitude of defect concentration, which provides valuable understanding regarding the physics of defects in these complex systems. The nature of the induced defects and their impact on the properties are studied using a combination of conventional and advanced characterization techniques including X-ray diffraction, Rutherford backscattering spectrometry, scanning transmission electron microscopy, scanning probe microscopy, and electrical measurements such as traditional dielectric, ferroelectric, and transport measurements, switching kinetics studies, first-order reversal curve analysis, impedance spectroscopy, and deep-level transient spectroscopy. Ultimately, I show that establishing routes to achieve such control and understanding over defects is the key if we desire to use defects as “good guys” and as tools to our advantage for material control and design rather than being limited by them.