Exploring Ferroelectricity in Morphologically Engineered Complex Oxide Thin Films

Ferroelectrics are functional materials that possess mechanically and thermally coupled, spontaneous, reversible electric polarization. However, ferroelectrics exhibit challenges related to longevity and stability during utilization, which limit their potential in technologically promising commercial applications such as clean energy generation/conversion and high-density solid-state memory. To these challenges and deliver on their promise, we consider a strategy of “morphological engineering”: harnessing the structure-property-processing-performance paradigm of materials science and engineering to empower the enhanced design and deepened understanding of ferroelectric materials. To illustrate this strategy, I detail four studies involving the examination of morphologically engineered complex oxide thin films. First, perovskite BaTiO3 thin films morphologically engineered to prevent phase transformations by way of strain are used as the experimental basis for attempting to enhance the pyroelectric effects of a ferroelectric thin film outside the proximity of its ferroelectric-paraelectric phase transition. Variable-temperature properties are influenced by the presence of an asymmetric electrode-induced built in field, which are characterized using a novel pseudo-indirect method. Next, in fluorite HfO2 films morphologically engineered into a ferroelectric phase by ZrO2 alloying, endurance is extensively characterized across a thermal energy range spanning from near room temperature to cryogenic regimes. By studying the variable-temperature properties, a greater understanding of the thermally activated defect dynamics responsible for wake-up and fatigue is achieved. To support efforts to generate evidence for novel ferroelectricity in rhombohedrally distorted La0.33Sr0.66FeO3 thin films, variable-temperature and variable-frequency electrical measurements were conducted. Though ferroelectric-like hysteresis was observed, it is difficult to distinguish from charge emission-based origins of hysteretic electronic effects, and represents an open area of inquiry. Finally, attempts to morphologically engineer ferroelectricity in ZnO through structural distortions and insulation enhancement with Mg-doping and Li-doping are discussed. Single-phase, highly distorted wurtzite specimens were fabricated using both single-target and multi-target pulsed laser deposition strategies, but were not electrically insulating enough to observe polarization reversal. However, co-doped samples exhibit memristive properties that, when characterized using traditional ferroelectric measurement techniques, produce hysteresis loops that can be misinterpreted as evidence of ferroelectricity. In doing so, a potential explanation for the widely controversial reports of ferroelectricity in ZnO-based thin films is presented.