| Summary: | Abstract Ferroelectric thin films have recently received unprecedented attention due to the need to miniaturize electronic circuit devices. Synthesis and deposition processes along with theoretical calculations are improved remarkably to realize stable ferroelectric thin films up to nanometer thickness. However, even with technological advances, it is still difficult to overcome the size effect of ferroelectrics, so research is being conducted to achieve stable ferroelectricity in unit‐cell thicknesses thinner than the typical critical thicknesses. In this review, the size effects in ferroelectric thin films are described, and their importance and fundamental limitations are discussed. First, intrinsic and extrinsic factors affecting ferroelectricity are introduced based on the theoretical background of the size effects in ferroelectricity. Then, on understanding the size effects by considering complex interacting factors, the recent works showing ferroelectricity below the commonly known critical thicknesses in perovskite, fluorite oxides, and two‐dimensional (2D) ferroelectrics are introduced. Finally, the results of research efforts in scaling ferroelectric thin films with a future perspective are summarized.
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