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 promising energy storage properties of new lead‐free antiferroelectric HfxZr1‐xO2 (x = 0.1–0.4) films with high energy storage density are reported. The energy storage density of the Hf0.3Zr0.7O2 ...capacitor does not decrease with the increase in temperature up to 175 °C, and it decreases by only ≈4.5% after field cycling 109 times.
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.
Enhancement of capacitance by negative capacitance (NC) effect in a dielectric/ferroelectric (DE/FE) stacked film is gaining a greater interest. While the previous theory on NC effect was based on ...the Landau-Ginzburg-Devonshire theory, this work adopted a modified formalism to incorporate the depolarization effect to describe the energy of the general DE/FE system. The model predicted that the SrTiO3/BaTiO3 system will show a capacitance boost effect. It was also predicted that the 5 nm-thick Al2O3/150 nm-thick BaTiO3 system shows the capacitance boost effect with no FE-like hysteresis behavior, which was inconsistent with the experimental results; the amorphous-Al2O3/epitaxial-BaTiO3 system showed a typical FE-like hysteresis loop in the polarization - voltage test. This was due to the involvement of the trapped charges at the DE/FE interface, originating from the very high field across the thin Al2O3 layer when the BaTiO3 layer played a role as the NC layer. Therefore, the NC effect in the Al2O3/BaTiO3 system was frustrated by the involvement of reversible interface charge; the highly stored charge by the NC effect of the BaTiO3 during the charging period could not be retrieved during the discharging process because integral part of the polarization charge was retained within the system as a remanent polarization.
The negative capacitance (NC) effects in ferroelectric materials have emerged as the possible solution to low-power transistor devices and high-charge-density capacitors. Although the steep switching ...characteristic (subthreshold swing < sub-60 mV/dec) has been demonstrated in various devices combining the conventional transistors with ferroelectric gates, the actual applications of the NC effects are still some way off owing to the inherent hysteresis problem. This work reinterpreted the hysteretic properties of the NC effects within the time domain and demonstrated that capacitance (charge) boosting could be achieved without the hysteresis from the Al2O3/BaTiO3 bilayer capacitors through short-pulse charging. This work revealed that the hysteresis phenomenon in NC devices originated from the dielectric leakage of the dielectric layer. The suppression of charge injection via the dielectric leakage, which usually takes time, inhibits complete ferroelectric polarization switching during a short pulse time. It was demonstrated that a nonhysteretic NC effect can be achieved only within certain limited time and voltage ranges, but that these are sufficient for critical device applications.
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