Electrode Effects on the Ferroelectric Properties of Hf_1-xZr_xO₂

Research constituting this dissertation began in the middle of December, in 2017. At its outset, a number of projects on various thin film research topics presented potential candidates for its subject. However, a clear need for immediate work on electrode effects on the ferroelectric properties of hafnium zirconium oxide (HZO), through collaboration with Sandia National Laboratories, provided an initial deterministic push, which precipitated this study. As of 2017, the ferroelectric properties of hafnium oxide-based ferroelectric thin films, including HZO, had been readily published upon in open literature since their first reporting in 2011 by Böske et al.1 and Müller et al.2,3 Led mostly by electrical and device engineering research groups, much of the early work was devoted to understanding how polarization wake-up and fatigue in this silicon-compatible ferroelectric material could be optimized for scalable memory applications. Such research mostly utilized TiN electrode materials, given their promising behavior when incorporated into HfO2-based devices and ready use in silicon technology. This work had identified that a favorable surface energy facilitated the stabilization of the ferroelectric orthorhombic phase in polycrystalline films grown to a thickness of < 40 nm, showed that oxygen vacancy content in the HfO2-based layer played an important role in the stabilization of this ferroelectric phase, demonstrated superior thickness and lateral scalability in the ferroelectric properties of HfO2-based films compared to conventional perovskites, and revealed that ferroelectric domain pinning and depinning, at least partially, drives the polarization instabilities that accompany field cycling of HfO2-based devices. Research produced by materials science-focused research groups had also shown important effects of stress on the orthorhombic phase content and ferroelectric domain structure, and had observed localized phase transformations occurring during field cycling.
However, by 2017, a number of knowledge gaps pertaining to the interactions between electrodes and HfO2-based layers remained. While the role of electrodes on the orthorhombic phase stabilization had been investigated, such findings mostly comprised observations on different behavior based upon which electrodes were utilized, and the various potential interactions that drive these different behaviors were largely unexplored. Comparisons between TaN and TiN electrodes revealed different ferroelectric orthorhombic phase compositions in processed HfO2-based films and strongly different polarization wake-up and fatigue behavior. Separately, the field-cycling behavior of devices processed with metallic electrodes had been sparsely investigated compared to binary nitrides. Electrode reactivity had been suggested as the cause of such different behaviors, however examinations involving specifically varying the electrode material to induce changes in the neighboring HfO2-based layers had not yet been widely undertaken, and a device-scale understanding of the effects of field cycling on the phase constitutions of HfO2-based devices remained elusive. Further, the field cycling behaviors of common binary nitride and metallic electrodes had only been briefly compared, and oxide electrodes, which aided in mitigating deleterious polarization fatigue during field cycling of conventional ferroelectric oxides, had not been explicitly investigated.
Separately, while stress effects on the ferroelectric properties of HfO2-based films had been experimentally revealed, the electrode contributions to these effects had not been investigated. For example, nearly every investigation of HfO2-based ferroelectrics observed that the presence of the top electrode during thermal processing resulted in enhanced contents of the orthorhombic phase. This enhancement was regularly attributed to mechanical stress imparted by the top electrode. However, the exact physical mechanism by which this stress was imparted by the top electrode, and the quantities of such stresses, had not been directly investigated. Such a gap stemmed, partially, from a dearth of knowledge about the elastic properties of HfO2-based ferroelectrics, and alluded to the need for adaptation and application of diffraction-based stress analysis techniques for these systems.
Within this dissertation, the interactions between binary nitride, metallic, and conductive oxide electrodes and HZO will be investigated. These interactions will be shown to be both chemical and physical in nature. Different field-cycling behaviors of HZO will be shown to be related to the manner in which the electrode and ferroelectric layers exchange oxygen, oxygen vacancies, and injected electrons. It will also be shown that the specific interface with the top electrode is responsible for influencing the stress of the HZO layer and stabilizing the orthorhombic phase. Such observations will be made through application of adapted diffraction and electrical characterization techniques, and will facilitate the future engineering of HfO2-based ferroelectric devices through tailoring their interactions with their neighboring electrode layers.