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 ferroelectricity 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.
Recent progress in ferroelectricity and antiferroelectricity in HfO2‐based thin films is comprehensively reviewed. The properties of ferroelectric HfO2‐based films, different from those of conventional ferroelectrics, are believed to solve the problems of conventional ferroelectrics in non‐volatile memory. Moreover, the pyroelectricity of antiferroelectric films is expected to be useful for various applications, including energy harvesting and storage, solid‐state cooling, and infrared sensors.
In this study, the changes in the structural and electrical properties of ferroelectric Hf1-xZrxO2 films with various Zr contents (0.26-0.70) were systematically examined during electric field ...cycling, resulting in a "wake-up" effect. To quantify the degree of wake-up effect, a "variable" polarization as the difference between remanent and saturation polarization was suggested as a new parameter, which could be calculated by excluding the linear dielectric contribution from the total electric displacement. Here, the variable polarization value could be minimized for an optimized Zr content of 0.43, which was slightly lower than the value for the largest remanent polarization. The polymorphism in Hf1-xZrxO2 thin films is known to be complicated due to the relatively small energy differences between various phases, such as the monoclinic, tetragonal, and orthorhombic phases. The variations in the polarization-electric field characteristics and dielectric constant values could be qualitatively and quantitatively understood based on the competition of various polymorphs that are dependent on the Zr content. Furthermore, a schematic model for the spatial distribution of mixed phases was suggested for Hf1-xZrxO2 films with various Zr contents based on the experimental observations.
Hafnia (HfO2)‐zirconia (ZrO2) solid solution films show giant positive (ΔT = 13.4 K) and negative (ΔT = −10.8 K) electrocaloric effects that can be simply controlled by tuning the Hf/Zr ratio. It is ...expected that the combination of the electrocaloric effects with opposite signs in this lead‐free, simple, binary oxide can significantly improve the efficiency of electrocaloric cooling.
The chemical, physical, and electrical properties of the atomic layer deposited Hf
0.5
Zr
0.5
O
2
thin films using tetrakis(ethylmethylamino) (TEMA) and tetrakis(dimethylamino) (TDMA) precursors are ...compared. The ligand of the metal-organic precursors strongly affects the residual C concentration, grain size, and the resulting ferroelectric properties. Depositing Hf
0.5
Zr
0.5
O
2
films with the TDMA precursors results in lower C concentration and slightly larger grain size. These findings are beneficial to grow more ferroelectric-phase-dominant film, which mitigates its wake-up effect. From the wake-up test of the TDMA-Hf
0.5
Zr
0.5
O
2
film with a 2.8 MV/cm cycling field, the adverse wake-up effect was well suppressed up to 10
5
cycles, with a reasonably high double remanent polarization value of ~40 μC/cm
2
. The film also showed reliable switching up to 10
9
cycles with the 2.5 MV/cm cycling field without involving the wake-up effect but with the typical fatigue behavior.
Ferroelectric (FE) capacitor is a critical electric component in microelectronic devices. Among many of its intriguing properties, the recent finding of voltage drop (V-drop) across the FE capacitor ...while the positive charges flow in is especially eye-catching. This finding was claimed to be direct evidence that the FE capacitor is in negative capacitance (NC) state, which must be useful for (infinitely) high capacitance and ultralow voltage operation of field-effect transistors. Nonetheless, the NC state corresponds to the maximum energy state of the FE material, so it has been widely accepted in the community that the material alleviates that state by forming ferroelectric domains. This work reports a similar V-drop effect from the 150 nm thick epitaxial BaTiO3 ferroelectric thin film, but the interpretation was completely disparate; the V-drop can be precisely simulated by the reverse domain nucleation and propagation of which charge effect cannot be fully compensated for by the supplied charge from the external charge source. The disappearance of the V-drop effect was also observed by repeated FE switching only up to 10 cycles, which can hardly be explained by the involvement of the NC effect. The retained reverse domain nuclei even after the subsequent poling can explain such behavior.
Interests in nanoscale integrated ferroelectric devices using doped HfO2-based thin films are actively reviving in academia and industry. The main driving force for the formation of the metastable ...non-centrosymmetric ferroelectric phase is considered to be the interface/grain boundary energy effect of the small grains in polycrystalline configuration. These small grains, however, can invoke unfavorable material properties, such as nonuniform switching performance. This study provides an in-depth understanding of such aspects of this material through careful measurement and modeling of the ferroelectric switching kinetics. Various previous switching models developed for conventional ferroelectric thin-film capacitors cannot fully account for the observed time- and voltage-dependent switching current evolution. The accurate fitting of the experimental results required careful consideration of the inhomogeneous field distribution across the electrode area, which could be acquired by an appropriate mathematical formulation of polarization as a function of electric field and time. Compared with the conventional polycrystalline Pb(Zr,Ti)O3 film, the statistical distribution of the local field was found to be three times wider. The activation field and characteristic time for domain switching were larger by more than 1 order of magnitude. It indicates that doped HfO2 is inhomogeneous and “hard” ferroelectric material compared with conventional perovskite-based ferroelectrics.
To date, the high energy storage performances observed in the field-induced ferroelectric HfO2- or ZrO2-based films have had an obstacle to scale-up due to the involvement of low-k monoclinic phase ...at the large thickness (> ~ 10nm). Considering that the monoclinic phase formation is closely related with the in-situ (partial) crystallization during the atomic layer deposition (ALD) process, in this work, the ALD temperature of Hf0.5Zr0.5O2 thin films was lowered, and its influence on the energy storage performances was systematically examined. Carbon and nitrogen dopants incorporated at a low deposition temperature in combination with grain size decrease change the polymorphism of Hf0.5Zr0.5O2 thin film from the genuine ferroelectric to field-induced (incipient) ferroelectric crystal structure. The Hf0.5Zr0.5O2 thin film deposited at 210°C shows improved resistance to degradation by monoclinic phase involvement up to ~ 40nm compared to the previously-reported Hf0.3Zr0.7O2 thin films. By investigating Hf0.5Zr0.5O2 thin films with wide ALD temperature and thickness ranges, energy storage density of ~ 55Jcm−3 with an efficiency of ~ 57% can be achieved at the ~ 7.1nm Hf0.5Zr0.5O2 thin films deposited at 215°C. The performance can be retained even after 1010 bipolar switching cycles, and the film endures thermal stress up to 175°C without severe degradation, demonstrating notable reliability.
The crystallization in the as-deposited state and the resulting low-k monoclinic phase evolution were suppressed by changing the Zr content and the deposition temperature of the electrostatic supercapacitors based on the HfO2-ZrO2 solid solution. While the energy storage density of the Hf0.3Zr0.7O2 thin films deposited at 280°C drastically decreased at the large thickness due to the monoclinic phase formation (upper panel), the Hf0.5Zr0.5O2 thin films deposited at 210°C retained the large energy storage density even up to 40nm (bottom panel), enabling the scaling-up pathway of the HfO2-ZrO2 solid solution. Display omitted
•The field-induced ferroelectricity was acquired from the Hf0.5Zr0.5O2 thin film.•Chemical/structural changes by the decreased deposition temperature were elucidated.•The crystallization in the as-deposited state was minimized by controlling Zr content and Tdep.•The electrostatic HfO2-ZrO2 capacitor were scaled-up for the large energy storage.•The FFE Hf0.5Zr0.5O2 capacitor does not degrade even after 1010 switching cycles and up to 175°C.
This paper evaluates the effect of construction quality defects on the seismic vulnerability of reinforced concrete (RC) frames. The variability in the construction quality of material properties and ...structural detailing is considered to assess the effect on the seismic behavior of RC frames. Concrete strength and yield strength of the reinforcement are selected as uncertain variables for the material properties, while the variabilities in the longitudinal reinforcement ratio and the volumetric ratio of transverse reinforcement are employed for structural detailing. Taking into account the selected construction quality uncertainties, the sensitivity analysis of the seismic vulnerability of the RC frames is performed and the impact of significant parameters is assessed at the global and local levels. This extensive analytical study reveals that the seismic vulnerability of the selected RC frame is particularly sensitive to concrete strength and the volumetric ratio of transverse reinforcement.
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