Compared to traditional perovskite ferroelectric materials, HfO₂ has emerged as a prominent research focus due to its ability to retain significant ferroelectricity at the nanoscale. However, systematic studies on its performance in thicker films remain limited, leaving the intrinsic relationship between thickness variation and ferroelectric properties poorly understood. In this work, we successfully fabricated doped HfO₂-based ferroelectric thin films with thicknesses spanning tens to hundreds of nanometers. All these films exhibit robust ferroelectric characteristics, and their ferroelectric properties demonstrate a non-monotonic evolution with increasing thickness. Macroscopic electrical measurements and mesoscale domain switching analysis confirmed that the ferroelectric properties of Ce:HfO₂ films first diminish and then recover with the increase of film thickness. By further characterizing the evolution of microscopic structures, we elucidate the thickness effects on the grain size distribution and domain structure evolution. This framework clarifies the physical mechanism underlying the thickness-dependent ferroelectric behavior. Our findings provide critical experimental evidence for developing large-scale HfO₂-based ferroelectric devices and lay a theoretical foundation for optimizing thick-film ferroelectric materials for practical applications.

Flexible and transparent hafnium oxide-based ferroelectric films are attracting widespread attention because of the increasing demand for wearable electronic devices. However, the ultra-low voltage operation with robust and stable ferroelectricity, which is a prerequisite for portable device applications, has not been realized simultaneously. Here, we report flexible Hf_0.5Zr_0.5O₂ ferroelectric films with a saturation voltage of only 1.3/3 V and remanent polarization (2P_r) of 38/60 µC·cm². Negligible wake-up effect and superior stability resistance to compressive/tensile stress and high temperature up to 150 °C are also demonstrated. The polarization switching dynamics under bending are investigated based on the switching current measurement, suggesting that the intrinsic switching speeds keep almost constant at different bending radii. In addition, there is a negative correlation between the activation field and the compressive or tensile stress, which is due to the lowered energy barrier induced by the in-plane strain applied to the [111]-oriented hexagonal cell. Our work sheds light on the application of flexible, stable, and HfO₂-based ferroelectric thin films with ultra-low consumption.