Review of defect chemistry in fluorite-structure ferroelectrics for future electronic devices

Ferroelectricity in fluorite-structure oxides, such as (doped) HfO2 and ZrO2, and their solid solution, nanolaminates, and superlattices has attracted increasing interest for future memory devices. The scalability of film thickness down to only several nm and the availability of matured deposition techniques render these materials highly practical for future high-density memory applications. Significant effort is being made in the community to identify the possible reasons for establishing solid physical backgrounds for the emergence of the (unexpected) ferroelectricity in these materials. Nonetheless, several technical obstacles must be overcome before adopting ferroelectric HfO2 for practical device applications. Among them, understanding the role of various forms of defects in ferroelectric phase formation and evolution of the device performance is crucial. The forms of defects include point defects such as oxygen vacancies, extrinsic defects such as residual carbon, nitrogen, and hydrogen, and two-dimensional defects such as an interfacial layer at the electrode interface and grain boundaries. Numerous previous studies on non-ferroelectric HfO2 or non-fluorite-structure ferroelectrics could form the foundation for the understanding of such thematics. In this review, the influences of various kinds of defects on the formation mechanism of various polymorphisms, and the accompanying ferroelectric and electrical properties are comprehensively studied based on previous literature. This review is expected to contribute to a better understanding of these emerging ferroelectric materials and to provide the community with new insights motivated by the previous studies on non-ferroelectric HfO2 and non-fluorite-structure ferroelectrics.