A perspective on the physical scaling down of hafnia-based ferroelectrics.

HfO-based ferroelectric thin films have attracted significant interest for semiconductor device applications due to their compatibility with complementary metal oxide semiconductor (CMOS) technology. One of the benefits of HfO-based ferroelectric thin films is their ability to be scaled to thicknesses as low as 10 nm while retaining their ferroelectric properties; a feat that has been difficult to accomplish with conventional perovskite-based ferroelectrics using CMOS-compatible processes. However, reducing the thickness limit of HfO-based ferroelectric thin films below the sub 5 nm thickness regime while preserving their ferroelectric property remains a formidable challenge. This is because both the structural factors of HfO, including polymorphism and orientation, and the electrical factors of HfO-based devices, such as the depolarization field, are known to be highly dependent on the HfOthickness. Accordingly, when the thickness of HfOdrops below 5 nm, these factors will become even more crucial. In this regard, the size effect of HfO-based ferroelectric thin films is thoroughly discussed in the present review. The impact of thickness on the ferroelectric property of HfO-based thin films and the electrical performance of HfO-based ferroelectric semiconductor devices, such as ferroelectric random-access-memory, ferroelectric field-effect-transistor, and ferroelectric tunnel junction, is extensively discussed from the perspective of fundamental theory and experimental results. Finally, recent developments and reports on achieving ferroelectric HfOat sub-5 nm thickness regime and their applications are discussed.