The recent progress in ferroelectricity and antiferroelectricity in HfO2-based thin films is reported. Most ferroelectric thin film research focuses on perovskite structure materials, such as Pb(Zr,Ti)O3, BaTiO3, and SrBi2Ta2O9, which are considered to be feasible candidate materials for non-volatile semiconductor memory devices. However, these conventional ferroelectrics suffer from various problems including poor Si-compatibility, environmental issues related to Pb, large physical thickness, low resistance to hydrogen, and small bandgap. In 2011, ferroelectricity in Si-doped HfO2 thin films was first reported. Various dopants, such as Si, Zr, Al, Y, Gd, Sr, and La can induce ferro-electricity or antiferroelectricity in thin HfO2 films. They have large remanent polarization of up to 45 μC cm(-2), and their coercive field (≈1-2 MV cm(-1)) is larger than conventional ferroelectric films by approximately one order of magnitude. Furthermore, they can be extremely thin (<10 nm) and have a large bandgap (>5 eV). These differences are believed to overcome the barriers of conventional ferroelectrics in memory applications, including ferroelectric field-effect-transistors and three-dimensional capacitors. Moreover, the coupling of electric and thermal properties of the antiferroelectric thin films is expected to be useful for various applications, including energy harvesting/storage, solid-state-cooling, and infrared sensors.

still smaller than that of electrochemical capacitors due mainly to the use of the low dielectric constant ( ε r ) Al 2 O 3 thin fi lm as the dielectric layer ( ε r ≈ 9). The concurrent high-power capacitors are made mainly of linear dielectric polymers with an ESD of ≈1-2 J cm −3 . [ 3 ] Although the ε r values of linear dielectric polymers are generally small (≈2-5), their electric breakdown fi elds are quite high, thereby allowing the application of high voltages, which result in a relatively high ESD. [ 3 ] Nonetheless, their ESD values are not high enough, so many other materials have been studied for the purpose. Most previous research on this topic focused on AFE Pb(Zr,Ti)O 3 (PZT)-based fi lms because their ESD values are as large as ≈1 and ≈10-15 J cm −3 for bulk and thin fi lms, respectively. [ 3 ] An ESD value as high as ≈50 J cm −3 has been reported for thin PZT-based fi lms when a large ≈3.5 MV cm −1 electric fi eld was applied. [ 7,8 ] However, such a high electric fi eld may induce a signifi cant reliability concern for PZT fi lms. [ 9 ] In addition, the commercial use of PZT is restricted in many countries due to its environmental impact. Another signifi cant problem with PZT thin fi lms is the decrease in their ESD value by ≈20-40% when their operating temperature increases to 150 °C. [ 10,11 ] Other candidate materials are FE poly(vinylidenefl uoride) (PVDF)-based materials, which can have a large breakdown fi eld and maximum polarization. The highest ESD value of FE PVDF-based fi lms has been reported to be as high as ≈20 J cm −3 when a ≈8 MV cm −1 electric fi eld was applied. [ 3 ] However, the problem with such fi lms is that their ESD is only ≈40% of their total stored energy because PVDF is an FE material meaning that ≈60% of the total stored energy is retained in the material. The presence of remanent polarization ( P r ) prohibits the full discharge of the stored charges in any FE material. [ 3 ] In the case of AFE PVDF-based fi lms, an ESD value of ≈14 J cm −3 has been reported with a higher effi ciency of ≈70%. [ 12 ] 3D nanostructures, such as nanoholes or nanotrenches, would be needed to eventually contain an even higher ESD for practical use in electric vehicles. [ 5 ] However, both PZT-and PVDF-based materials are inappropriate for such nanostructures because they are too thick (thickness ( t f ) ≈ 10 2 -10 4 nm) to be incorporated into nanoscale structures. Therefore, a dielectric material that has an ESD value as high as that of PZT and PVDF with a high breakdown fi eld at a fi lm thickness of less than ≈10 nm must be found. The fi lm must also be well grown using the facile atomic layer deposition (ALD) technique to ensure fl uent step coverage with atomic accuracy in thickness control.FE and AFE HfO 2 -based thin fi lms with various dopants, such as Si, Al, and Zr, were recently reported, where the conventional Useful energy sources and ways to effi ciently distribute energy have been extensively studied. With the development of new energy generation and handling technologies, h...

The effects of annealing temperature (Tanneal) and film thickness (tf) on the crystal structure and ferroelectric properties of Hf0.5Zr0.5O2 films were examined. The Hf0.5Zr0.5O2 films consist of tetragonal, orthorhombic, and monoclinic phases. The orthorhombic phase content, which is responsible for the ferroelectricity in this material, is almost independent of Tanneal, but decreases with increasing tf. In contrast, increasing Tanneal and tf monotonically increases (decreases) the amount of monoclinic (tetragonal) phase, which coincides with the variations in the dielectric constant. The remanant polarization was determined by the content of orthorhombic phase as well as the spatial distribution of other phases.

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The appearance of ferroelectric (FE) and anti-ferroelectric (AFE) properties in HfO2-based thin films is highly intriguing in terms of both the scientific context and practical application in various electronic and energy-related devices. Interestingly, these materials showed a "wake-up effect", which refers to the increase in remanent polarization with increasing electric field cycling number before the occurrence of the fatigue effect. In this work, the wake-up effect from Hf0.5Zr0.5O2 was carefully examined by the pulse-switching experiment. In the pristine state, the Hf0.5Zr0.5O2 film mostly showed FE-like behavior with a small contribution from AFE-like distortion, which could be ascribed to the involvement of the AFE phase. The field cycling of only 100 cycles almost completely transformed the AFE phase into the FE phase by depinning the pinned domains. The influence of field cycling on the interfacial layer was also examined through the pulse-switching experiments.