Progress on hafnium oxide-based emerging ferroelectric materials and applications

Ferroelectricity

During the early research on HfO₂ thin films, the majority of reported HfO₂ phases were monoclinic (M-phase), tetragonal (T-phase), or cubic (C-phase). In the temperature range from room temperature to 2,000 K, thin films primarily exist in the M-phase (space group: P2₁/c); between 2,000 and 2,900 K, the T-phase (space group: P4₂/nmc) is predominant, and the C-phase (space group: Fm3m)^[15,16] is also found. All three phases are centrosymmetric and thus non-ferroelectric. Furth theoretical and experimental studies later revealed the existence of a metastable orthorhombic phase within HfO₂^[17-19], known as the Pca2₁ phase^[20]. In 2011, a team of German scientists led by Böscke discovered ferroelectric behavior in Si-doped HfO₂ thin films^[4]. They found that when Si:HfO₂ films were capped with an electrode, the crystallization process underwent changes, and a metastable orthorhombic phase with ferroelectricity was formed after cooling. As shown in Figure 1A, as the amount of SiO₂ dopant increases, the shape of the ferroelectric polarization loop gradually changes from ferroelectric to antiferroelectric, as reflected in one or two peaks in the AC C-V sweeps. With a capping layer, the Si:HfO₂ thin film forms an orthorhombic phase after cooling, resulting in the generation of ferroelectricity^[4]. Calculations showed that both the Pca2₁ and Pmn2₁ phases had low free energy and a small energy barrier during polarization switching. It is generally believed that annealing the film with a capping electrode leads to a transformation to an equilibrium phase under the influence of electrode stress. Since then, the notion that the Pca2₁ phase is the ferroelectric phase of HfO₂ has become predominant. Lee et al. have found, through further characterization of the HfO₂, that the Pca2₁ phase is close in structure to the M-phase, but the grain boundaries inhibit the transition between the two phases^[21]. As shown in Figure 1B, this O-phase leads to the existence of polarized and spacer layers within it, with one part polarized and the other unpolarized. The spacer layer acts as a dead layer that shields the elastic interaction between ferroelectric active layers^[21].

Figure 1. (A) The formation of the orthorhombic phase with a capping layer^[4]. (B) An orthorhombic structure consisting of alternating spacer and ferroelectric layers^[21]. Reprinted from Ref.^[4], with permission from AIP Publishing. Reprinted from Ref.^[21], with permission from AAAS.

Subsequent work extensively discussed whether the ferroelectricity in HfO₂ originates from the polar orthorhombic phase Pca2₁. Sang et al. utilized a combination of high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and position averaged convergent beam electron diffraction (PACBED) to provide support for the assertion that the Pca2₁ phase is the source of the ferroelectric structure of the Gd:HfO₂ films^[22]. Based on space groups that are most likely to cause the ferroelectric behavior, Sang et al. observed the crystal structures of five HfO₂ phases along the four major zone axes, as shown in Figure 2A and B^[22]. Two structures, P2₁/c and Pnm2₁, were initially ruled out by comparing the atomic column projections. As shown in Figure 2C-G, the PACBED pattern observed in the experiments lacks mirrors perpendicular to the [001] axis and is almost identical to the simulated pattern of the Pca2₁ phase. The absence of mirrors in the PACBED pattern also excludes the presence of centrosymmetric Pbcm and Pbca phases.

Figure 2. (A) Crystal structure projections of five HfO₂ phases along the four major zone axes; (B) HAADF-STEM images of four different grains (from the perspective zone axes); (C) STEM image of Gd:HfO₂ film; (D) PACBED pattern. Simulated PACBED patterns for the (E) Pca2₁, (F) Pbca, and (G) Pbcm phases^[22]. Reprinted from Ref.^[22], with permission from AIP Publishing.

The research team of Huan^[19] first utilized a first-principles-based structure search algorithm to identify the low free energy phases under different pressures and temperatures. As shown in Figure 3A, it was discovered that the space groups Pca2₁ and Pmn2₁ are the most likely ferroelectric phases in HfO₂, which can transform into other equilibrium phases due to the lower free energy of the two polar orthorhombic phases. As shown in Figure 3B, the T-phase of HfO₂ thin films formed under high temperature and high pressure, and after cooling and annealing, it transformed into two polar phases that can generate switchable polarization. The M-phase under normal temperature and pressure conditions may transform into the T-phase through conditions such as heating and higher pressure, as shown in Figure 3C. Afterward, polarization occurred through stress and other methods, causing the T-phase to transform into the O-phase, which became the main crystal phase in the thin film after annealing^[19]. Whether the Pca2₁ phase can exist stably at normal temperature and pressure, and how to induce this phase have become new questions.

Figure 3. (A) Equilibrium phase diagram of HfO₂. (B) Regimes showing the free energy difference between the Pca2₁ and Pmn2₁ phases. (C) Possible pathways for the formation of the polar phase^[19]. Reprinted figures with permission from Ref.^[19] Copyright by the American Physical Society. (D) Phase diagram illustrating the relationship between grain radius and temperature for HZO thin films. (E, G, I, K, M) Illustrative schematic figures; (F, H, J, L, N) Corresponding free energy curves^[23]. Figures reprinted with permission from Ref.^[23] Copyright by the Wiley Company.

Figure 3D-N collectively shows the conditions in films before and throughout the accelerated thermal process utilized for film crystallization. Figure 3D presents a conceptual representation of the temperature-dependent phase stability of a columnar HZO crystal^[23]. The green region represents that the stable phase is the M-phase, while the crystalline phases inside the parentheses (T-phase and O-phase) are the second stable phases. The red dashed line indicates that at these points, the second stable phase can change between the T-phase and the O-phase^[23]. Therefore, the stable phase at high temperatures becomes the T-phase. The thermodynamically stable phase becomes the M-phase at low temperature. However, due to the high kinetic energy barrier, the transition from the T-phase to the M-phase is suppressed so that the metastable T-phase can be retained. If the temperature and time are enough to overcome the high kinetic barrier, a stable M-phase can be formed in this step^[24]. After a rapid thermal anneal process, the stability of the O-phase increases if the formation of the m-phase is kinetically suppressed, as shown in Figure 3K and L, and the second stable phase transforms from the T-phase to the O-phase at low temperature. Fan et al. investigated the structure and polarization response of HfO₂ with an applied electric field, and then elucidated how the metastable phase can exist stably at room temperature and normal pressure^[25]. As a result, when the lattice parameter and lattice strain are within a certain range, the hafnium oxide lattice phase is dominated by the Pca2₁ phase even at room temperature and normal pressure.

The ferroelectricity origin in hafnium oxide is intrinsically linked to its polarization switching pathways. First-principles calculations reveal that oxygen atom-mediated cross-unit-cell switching pathways dominate during domain nucleation and domain wall motion, creating asymmetric energy barriers that lead to inhomogeneous nucleation-propagation characteristics during polarization reversal^[26,27]. Peng et al. further reveal the spacer layer mechanism in the ferroelectric domain switching dynamics of HZO thin film through phase-field simulations^[28]. The dynamic evolution process of ferroelectric domain structures reveals that 180° polarization reversal is governed by the nucleation of new domains with high nucleation density. Under ultrafast electric fields, polarization switching is governed by high-density nucleation events, with new domains preferentially forming at grain boundaries or defects and propagating rapidly via domain wall migration. Moreover, the isolation layer leads to significantly weakened interactions between adjacent domains. The scale-free independent switching capability of nanoscale polar domains (with 1 nm lateral dimensions and atomically sharp domain walls) originates precisely from this weak inter-domain coupling. This decoupling effect enables individual domains to switch polarization states without triggering collective responses from neighboring regions^[28]. This mechanism fundamentally differs from the uniform polarization switching observed in conventional perovskite materials. Furthermore, researchers have revealed a novel 90° ferroelectric/ferroelastic switching dynamics in fluorite-structured oxides through in-situ visualization of oxygen migration phenomena^[29,30]: the polarization transition involves a transformation from the horizontal polarized O-phase with (O_a) to the non-polar T-phase, followed by a subsequent shift from the T-phase to the vertically polarized O-phase (O_b), with the non-polar T-phase acting as the intermediate stage, as shown in Figure 4A and B. The structural transformation from the O-phase to the T-phase mutually locks polarization and strain, generating a ferroelectric-ferroelastic coupled domain state. The non-polar T-phase provides a lower energy barrier for this ferroelectric/ferroelastic switching. This polarization switching and phase transition process provides deeper insights into the ferroelectric behavior of HfO₂-based materials.

Figure 4. (A) iDPC-STEM images showing a 90-degree polarization switching cycle with intermediate T phase. (B) Atomic schematics of orthorhombic ZrO₂ with ferroelectric order and tetragonal ZrO₂[^[29]. Reprinted figures with permission from Ref.^[29] Copyright by the Wiley Company.

Wei et al. also found that the ferroelectricity of hafnium oxide may originate from other phases^[5]. After analyzing the prepared ferroelectric HfO₂ films in terms of lattice structure and symmetry, they concluded that the ferroelectricity of their prepared films did not originate from the Pca2₁ phase, but from the rhombohedral phase (R-phase) with higher symmetry. Shortly from the first-principles perspective, Qi et al. argued that the observation is due to the application of in-plane shear strain on the tetragonal phase, transforming it into the polar phase Pnm2₁ phase, which is another polar orthorhombic phase^[31]. There are also some subsequent reports about the R-phase in HZO ferroelectric materials. In their work, Wang et al. also reported the discovery of the ferroelectric R-phase in HfZr_1+xO₂ materials^[32]. They studied a method to construct the R-phase in HfO₂ materials by increasing the concentration of Hf and O atoms. They confirmed the excess structure of Hf/Zr through HAADF images. Density functional theory calculations showed that the formation energy of the R-phase in HZO films is lower than that of the O-phase and M-phase when the ratio of Hf/Zr to O is greater than 1.079:2, which favors the formation of the R-phase^[32]. However, some researchers have raised different opinions regarding the conclusions of this work. In response to the significantly reduced coercive field observed in this work, Zhu et al. provided further mechanistic insights^[27]. They systematically investigated how interstitial Hf doping lowers the coercive field in ferroelectric HfO₂. The results revealed that Hf doping diminishes the energy difference between the polar orthorhombic Pca2₁ phase and the intermediate tetragonal P4₂/nmc phase, thereby reducing both the switching energy barrier and coercive field^[27]. So far, the origin of the ferroelectricity of HfO₂ still remains to be further investigated.

Through the research process, STEM technology plays a pivotal role in elucidating the mechanism of hafnium oxide-based ferroelectric materials, particularly in atomic-scale phase structure characterization and visualization of phase transition pathways. Figure 5A displays the HAADF-STEM image of HfZr_1+x O₂ thin films, while Figure 5B further reveals the insertion of excess Hf/Zr atoms into the lattice^[32]. By combining STEM imaging with HAADF image simulations and DFT calculations, researchers confirmed the phase structure and demonstrated that excess Hf/Zr atoms expand the lattice of HfZr_1+xO₂, generating enhanced in-plane and out-of-plane stresses that effectively stabilize ferroelectricity. Clear O-phase features were observed in Y:HfO₂ films, as shown in Figure 5C. Figure 5D presents Integrated Differential Phase Contrast (iDPC)-STEM images revealing distinct non-centrosymmetric polar structures, with Figure 5E highlighting oxygen atom displacements within the marked regions (red boxes), providing microscopic evidence for the in-plane switchable polarization in Y:HfO₂ films^[33]. In studies of HfO₂-based crystals, STEM enabled precise discrimination among orthorhombic ferroelectric (o-FE), orthorhombic antiferroelectric (o-AFE), and tetragonal non-polar phases (t) [Figure 5F], while fully mapping the dynamic migration trajectories of Hf and O atoms during the metastable tetragonal-to-orthorhombic phase transition^[34]. By directly observing the motion and structural changes of different atoms, the complex mechanisms of the phase transition process between the t-phase and o-phase have been effectively explained. The application of STEM for microstructural characterization of HfO₂-based ferroelectric materials, complemented by atomic-scale observations of these systems, has established a crucial experimental framework for advancing mechanistic understanding of ferroelectricity in HfO₂-based material systems.

Figure 5. (A) STEM-HAADF image of HfZr_1+xO₂ thin film, (B) The enlarged inset of (A)^[32]. Reprinted from Ref.^[32], with permission from AAAS. (C) STEM-HAADF image of the Y:HfO₂ film. The inset shows the fast Fourier transformation of (C). (D) iDPC-STEM image of the Y:HfO₂ film, (E) Polarization direction mapping of the red box in (D)^[33]. Reprinted (adapted) with permission from Ref.^[33]. Copyright (2024) American Chemical Society. (F) iDPC-STEM results of hafnium oxide crystals^[34]. Reprinted from Ref.^[34], with permission from Springer Nature.

Doping

Many studies have shown that the ferroelectricity of HfO₂-based thin films can be affected by extrinsic factors, such as doping^[24,35-37], strain^[38-41], defects^[42,43], surface energy and electric field^[11], with the effect of doping being particularly pronounced. The contour plots of the variation of P_r with crystal radius and doping for different doping systems are given in Figure 6A. Böscke et al. have induced ferroelectricity in HfO₂ by using CVD-deposited TiN electrodes on top of Si-doped HfO₂ films^[4]. Schroeder et al. in their study found that different Si doping concentrations in HfO₂ films caused the HfO₂ films to exhibit different properties^[44]; in the work of Kirbach, it was found that the Si:HfO₂ films exhibited the strongest ferroelectric properties when the Si doping concentration was 3.8%, within the studied range of 2.7%-5.6%^[45]. Doping affects the lattice structure of HfO₂ films by changing the bond lengths between metal-oxygen bonds in HfO₂ films and increasing the asymmetry of their lattice structure. Doping elements can stabilize either the T-phase or C-phase at appropriate doping concentrations, promote the generation of the O-phase, and inhibit the conversion of the T-phase to the M-phase.

Figure 6. (A) Contour plots of the variation of P_r value with crystal radius and doping amount^[44]. Reprinted figures with permission from Ref.^[44] Copyright by IOP Publishing, Ltd. (B) P-V curve and C-V curve^[46]. Reprinted from^[46], with permission from AIP Publishing. (C) Piezoelectric measurement and P-V curve of 70 nm Y:HfO₂ capacitor^[47]. Reprinted from^[47], with permission from AIP Publishing. (D) P-E loops for Y:HfO₂ films with a thickness of 930 nm^[48]. (E) Thickness dependence of P_r for doping HfO₂-based films^[48]. Reprinted from^[48], with permission from AIP Publishing. (F) Typical P-V loops of Y:HfO₂ films measured from 20 to 300 K^[49]. Reprinted from Ref.^[49], with permission from Springer Nature.

In recent studies, Y-doped HfO₂ thin films have also demonstrated excellent ferroelectricity^[33]. As shown in Figure 6B, Müller investigated Y-doped HfO₂ films and found that Y:HfO₂ films have low annealing temperatures and are able to exhibit strong ferroelectricity without top electrode clamping^[46]. Starschich et al. prepared Y:HfO₂ ferroelectric films up to a thickness of 70 nm using the chemical solution deposition method^[47]. Figure 6C clearly demonstrates the ferroelectric and piezoelectric properties of the Y:HfO₂ thin films. Mimura et al. have detected ferroelectricity in Y:HfO₂ thin film with a thickness of 930 nm^[48]. In Figure 6D, a clear hysteresis loop is observed in the film with a thickness of 930 nm, revealing that the ferroelectricity is maintained even at nearly micrometer-scale thickness. These results suggest that the ferroelectric phase in Y:HfO₂ thin film is determined by the composition itself, and the surface energy has little influence on it. Figure 6E summarizes the relationship between ferroelectricity and thickness in HfO₂ films with different doping systems. It can be observed that in various doping systems, Y:HfO₂ films can still maintain excellent ferroelectric properties even at the thickest thickness. Xu et al. have taken a step further by achieving ferroelectricity in bulk single crystal Y:HfO₂ using the laser-diode-heated floating technique^[36]. Additionally, Yun et al. have prepared Y-doped HfO₂ thin films that exhibit excellent ferroelectric properties. As illustrated in Figure 6F, to decouple extrinsic contributions (e.g., injected charge carriers and oxygen vacancy-related charge trapping), the authors conducted temperature-dependent polarization measurements on Y:HfO₂ thin films^[49]. The results demonstrate a robust P_r of ≥ 37 μC/cm², with P_r exhibiting enhanced stability at cryogenic temperatures (e.g., negligible variation below 100 K). This thermal invariance confirms that extrinsic interference is effectively suppressed under low-temperature conditions. Furthermore, the moderate temperature dependence of P_r across the studied range (20-300 K) strongly suggests that intrinsic domain switching remains the dominant mechanism even at room temperature.

Recent studies on doped ferroelectric HfO₂-based thin films have shown increasing diversity. Among those doping systems, the atomic radius of Zr is similar to that of Hf, and ZrO₂ and HfO₂ have almost equivalent crystal structures, similar lattice parameters and exhibit similar physical and chemical properties. Müller et al. reported that ferroelectricity can be induced in HZO thin films, and that the ferroelectric properties of HZO are influenced by the Zr content, which is optimal at 50%^[50]. However, when the Zr content exceeds 70%, the ferroelectricity of the films decreases significantly^[51]. Additionally, in MFM-structured devices, the inhomogeneous distribution design of Hf/Zr elements also exerts an influence on the ferroelectric properties and cycling endurance of HZO ferroelectric thin films^[52]. Ansari et al. fabricated V:HfO₂ film and confirmed the negative capacitance effect in V:HfO₂, demonstrating the potential of V-doped HfO₂ thin films for applications in negative capacitance devices^[53]. Zhou et al. prepared HfO₂-based ferroelectric thin films using the method of co-doping. They observed that La³⁺-Ta⁵⁺ co-doped HfO₂-based thin films can exhibit ferroelectricity, even at a thickness as low as 3 nm^[54]. Figure 7 summarizes the evolution of P_r in hafnium oxide thin films with different doping systems from 2011 to 2025. Over the past decade, the ferroelectric properties of HfO₂-based films across various doping configurations have seen continuous improvement, and we anticipate further breakthroughs in the future.

Figure 7. The evolution of P_r in hafnium oxide thin films with different doping systems (including Zr^{[5,24,32,39,44,50-52,55-74]}, Y^[40,46-49], Si^[4], La^[44,54], Gd^[22,75], V^[53], La-HZO^[6], pure HfO₂^[76]) from 2011 to 2025.

Compound structure

In recent years, research on HfO₂-ZrO₂ superlattices has been increasing^[42]. In 2017, Weeks et al. investigated the relationship between the ferroelectric properties of HfO₂-ZrO₂ superlattices and the thickness and deposition temperature^[58]. Thin films prepared at low temperatures exhibited a pseudo-antiferroelectric response, while films prepared at higher temperatures showed ferroelectric behavior. This difference arises because temperature influences the stability of the polar orthorhombic phase. At higher deposition temperatures, microcrystalline nucleation occurred within the film, stabilizing the orthorhombic phase. In 2019, Park et al. studied the structure and electrical properties of HfO₂ and ZrO₂ nanolayers and superlattices with four different layer combinations and thicknesses^[77]. The results showed that for ZrO₂ nanolayers with thickness exceeding 1.1 nm, the fraction of the monoclinic phase was higher compared to solid solutions with the same HfO₂/ZrO₂ ratio or HfO₂-starting samples. This is due to the biaxial tensile strain conditions in the thin film, which promote the phase transition from the T-phase to the M-phase. In the work by Peng et al. in 2022, the HfO₂-ZrO₂ superlattice capacitors exhibited stronger ferroelectricity, better durability, and retention characteristics^[78]. Compared to the HZO capacitor, the HfO₂-ZrO₂ SL MFM capacitor exhibits a significantly improved retention rate. The endurance of SL capacitors exceeds 10¹² cycles per fatigue cycle, which is three orders of magnitude higher than HZO. The HfO₂-ZrO₂ superlattice structure effectively improves the durability of capacitors.

In 2022, Cheema et al. discovered that the capacitive properties of the HfO₂-ZrO₂ superlattice grown on the Si substrate could be enhanced by the negative capacitance effect^[79]. In this work, they proposed an ultra-thin HfO₂-ZrO₂ superlattice gate stack, as shown in Figure 8A. A superlattice thin film with a thickness of 2 nm and a periodic structure, as shown in Figure 8B, was prepared in the study. The potential mixed ferroelectric ordering in these HfO₂-ZrO₂ heterojunctions was confirmed by high-resolution transmission electron microscopy images in Figure 8C. Compared to traditional HfO₂-based high dielectric constant gate stacks, this HfO₂-ZrO₂ superlattice structure was able to generate larger capacitance without compromising the mobility and leakage current, demonstrating excellent prospects for applications.

Figure 8. (A) HfO₂-ZrO₂ fluorite-structure system. (B) Schematic of the HZH fluorite-structure multilayer. (C) High-resolution TEM images of HZH tri-layer (top) and extracted d-lattice spacings^[79]. Reprinted from Ref.^[79], with permission from Springer Nature. The superlattice structures of (D) sample C1, (E) sample C2, (F) sample C4, (G) sample C5^[80]. Reprinted from Ref.^[80], with permission from Wiley.

In 2023, Bai et al. designed a special ferroelectric capacitor based on HfO₂/ZrO₂ superlattice. They reduced the leakage current of the thin film and enhanced the ferroelectricity through one-sided rapid thermal annealing^[80]. The samples with the same compositions as C1 and C2 (shown in Figure 8D and E) were subjected to top heating at 500 °C during the annealing process (named sample C1L and sample C2L, respectively). There is a certain amount of spontaneous polarization observed after 1,000 cycles, indicating the presence of a wake-up effect, which was not observed in the samples annealed at 600 °C. Figure 8F and G displays the structure of samples C4 and C5. After annealing with the addition of the ZrO₂ layer, the leakage current of sample C5 is reduced by six orders of magnitude compared to sample C4. Thermal annealing on the ZrO₂/Pt side effectively reduces the aggregation of oxygen vacancies, thus avoiding dielectric breakdown during high-temperature heat treatment. Jia et al. fabricated three distinct HZO superlattice configurations and revealed that charge injection at electrodes during polarization switching triggers breakdown-induced fatigue failure. By structurally optimizing the HZO superlattice architecture, the device successfully achieved a durability exceeding 10¹² cycles^[74]. Gong et al. discussed the mechanism of durability enhancement in the HfO₂-ZrO₂ superlattice structure through first-principles calculations^[81]. The results show that the superlattice has a higher oxygen vacancy (V_o) potential barrier along the polarization direction compared to HZO thin films, which inhibits the increase of V_O during the cycling process. Materials doped with higher V_O potential barriers can effectively enhance the durability of HfO₂ devices.

Morphotropic phase boundary of HfO₂-ZrO₂

Morphotropic phase boundary (MPB) refers to the boundary between two crystal phases with different chemical compositions. Using MPB can enhance the κ value of advanced unit capacitors in dynamic random-access memory (DRAM) and reduce the Equivalent oxide thickness (EOT) of gate stacks. The HRTEM and EDS mappings in Figure 9A and B characterize a prototypical MPB microstructure^[42]. The properties and structure of the MPB region will differ from the original HfO₂ and ZrO₂ film. In the work of Kim et al., the MPB was achieved in HZO thin films by adjusting the thermal budget during annealing^[82]. It was found that by adjusting the process, MPB could be achieved by overcoming the energy barrier through low-temperature annealing. Das et al. achieved MPB in HZO thin films through high-pressure annealing and rapid cooling processes^[83]. The HZO film they prepared exhibited a maximum dielectric constant of approximately 49 and an EOT of 4.8 Å, which were reported to be the optimal performance at that time^[83]. Kashir et al. achieved a greater formation of MPB region by intertwining ZrO₂ and HfO₂ layers to create HZZ-type devices, as depicted in Figure 9C^[84]. The dielectric constant of the HZZ nanolayers is larger than that of HZO and ZrO₂ films, which is attributed to the formation of the MPB region^[84]. They also prepared HZO films with different proportions of the T-phase by adjusting the oxygen content and heat treatment conditions^[85]. The results showed that the relative dielectric constant decreased with decreasing T-phase content^[85]. This may be due to the different densities of the MPB region. Oh et al. also found that a higher proportion of the T-phase helps achieve optimal MPB conditions^[86]. Gaddam et al. obtained the MPB through ZrO₂/HZO bilayer AFE/FE capacitor films, as depicted in Figure 9D^[87]. The devices they fabricated also exhibited excellent performance (higher κ of ~56 and the lower EOT of 3.8 Å) and could withstand up to 10¹² cycles^[87]. The MPB is of great importance for improving the dielectric performance of HZO film and further applications.

Figure 9. (A) The HRTEM and (B) EDS mappings of the TiN/ZrO₂/HfO₂/TiN capacitor^[42]. Reprinted (adapted) with permission from Ref.^[42]. Copyright (2022) American Chemical Society. (C) HZO/ZrO₂ supercell^[84]. Reprinted from Ref.^[84], with permission from IOP Publishing. (D) Schematic of heterostructure^[87]. Reprinted (adapted) with permission from Ref.^[87]. Copyright (2022) American Chemical Society.

Wake-up and fatigue

In practical applications of HfO₂-based ferroelectric materials, there are phenomena of wake-up and fatigue^[9,88-90]. The HZO ferroelectric thin film exhibits a “wake-up effect”, where P_r of the material increases with the number of electric field cycles. Fatigue, on the other hand, refers to a decrease in P_r as the number of cycles increases beyond a certain threshold. The classic wakefulness and fatigue phenomenon is shown in Figure 10A. In Pešic’s 2016 work, it was proposed that defects are the fundamental cause of the increase in remanent polarization during the wake-up stage and the subsequent degradation of polarization with further cycling^[91]. As shown in Figure 10B-D, the wake-up and fatigue processes primarily involve the redistribution of existing defects within the device. In Figure 10E, phase transitions inside the HZO thin film under different states are observed through HAADF-STEM images, which are caused by defect rearrangement. The degradation of dielectric reduces the ferroelectric switching, and trap density increases with device cycling, indicating that trap generation is the main mechanism leading to the degradation of ferroelectric performance, apart from domain pinning.

Figure 10. (A) Wake-up and fatigue behaviors of HZO devices. In (B) 1, (C) 1,000, and (D) 10⁶ cycles. (E) HAADF-STEM images with HfO₂ doping in the pristine, wake-up, and fatigue states, along with theoretical schematic diagrams of different phases^[91]. Reprinted from Ref.^[91], with permission from Wiley. (F) HAADF cross-sectional image of TiN/HZO/TiN device after 10⁷ cycles of fatigue. (G) Corresponding ABF image of (F). (H) Magnified ABF image obtained from the green square region in (G)^[92]. Reprinted from Ref.^[92], with permission from Springer Nature.

In 2021, McMitchell et al. found reversible tetragonal-orthorhombic, orthorhombic-monoclinic, and other phase transitions during HfO₂ formation^[93]. The low energy barrier of these phase transitions leads to cycle wake-up. They also established a model by describing grain reorientation to explain the changes in the orthorhombic unit cell during the wake-up process. The rotation of the orthorhombic unit cell aligns the polarization direction with the electric field direction, thereby enhancing the remnant polarization. Rushchanskii et al. suggested that in the wake-up cycle, randomly distributed oxygen vacancies are rearranged into the lowest energy configuration, allowing them to transform into the orthorhombic phase^[94]. The monoclinic phase could be seen as domain boundaries between orthorhombic domains with opposite polarization. The increase in polarization may be due to the growth of the orthorhombic phase at the monoclinic phase boundaries. The decrease in polarization could result from the reverse transformation from the orthorhombic phase to the monoclinic phase or from the growth of orthorhombic phases with polarization in the opposite direction, leading to a decrease in the proportion of the monoclinic phase^[94]. In 2022, Cheng et al. conducted detailed studies on the crystal structure of the O-phase grains in HZO ferroelectric films during different processes such as initial state, wake-up, fatigue, and recovery using STEM-HAADF and scanning transmission electron microscopy in annular bright-field mode (STEM-ABF) techniques^[92]. The generation of the interfacial T-phase was observed during the fatigue cycling process affected by oxygen vacancies, but the wake-up, fatigue, and recovery processes were mainly dominated by the reversible transformation between the ferroelectric phase Pca2₁ (O_FE phase) and the antiferroelectric phase Pbca (O_AFE phase). Under low electric fields, these two phases could mutually convert in Figure 10F-H, exhibiting reversible wake-up, fatigue, and recovery processes^[92]. Under high electric fields, irreversible structural changes accompanied the process. The mechanism of ferroelectric performance degradation in HZO films also applies to HfO₂ films in other doping systems. The mechanism and solutions for the wake-up and fatigue mechanisms of HfO₂-based thin films are still under investigation.

Atomic layer deposition

Atomic layer deposition (ALD) is a chemical vapor deposition technique consisting of cyclic gas-solid reactions. The main process is that gas-phase precursors are alternately pulsed into the reaction chamber and undergo a gas-solid phase chemisorption reaction on the surface of the substrate to form a thin film^[95]. Nowadays, literature regarding the production of ferroelectric HfO₂-based films via ALD has become increasingly prevalent. This subsection describes some cutting-edge work on the ALD preparation of hafnium oxide thin films, as well as the improvement of the ALD preparation process for the preparation of HfO₂-based thin films.

Cheema et al. grew ultra-thin HfO₂ on silicon using ALD and confirmed the presence of inversion-symmetry breaking and switchable polarization using second harmonic generation and advanced scanning probe techniques, respectively^[96]. HfO₂ not only has no critical thickness for ferroelectricity, but its polar distortion is also enhanced as the film thickness decreases. At reduced dimensions, higher symmetry phases are energetically favorable. In the classical perovskite, the size effect driven by surface energy favors the higher symmetry ferroelectric phase (cubic phase) rather than the lower symmetry paraelectric phase (tetragonal phase) upon dimensional reduction. In contrast, in the fluorite structure, the non-centrosymmetric phase (orthorhombic Pca2₁, O-phase) has higher symmetry compared to the bulk-stable centrosymmetric phase (P2₁/c, M-phase)^[96]. As shown in Figure 11A, the surface energy promotes the transformation from M-phase to O-phase. Yu et al. grew HZO ferroelectric thin films on Si substrates by the ALD technique^[97], and the 20-nm-thick HZO films exhibited good ferroelectricity. Liang et al. prepared HZO thin films and HfO₂-ZrO₂ superlattices by ALD, which exhibit enhanced ferroelectricity, better durability and retention properties^[98]. This is due to the lower defect density of the HfO₂-ZrO₂ superlattice suppresses the fatigue condition during electric field cycling.

Figure 11. (A) Size effects in fluorite-structure ferroelectrics^[96]. Reprinted from Ref.^[96], with permission from Springer Nature. (B) PEALD process for growing HZO films on a Si wafer^[103]. Reprinted from Ref.^[103], Copyright (2023), with permission from Elsevier (C) TiN/HZO/TiN and (D) RuO₂/HZO/RuO₂ are shown^[65]. Reprinted from Ref.^[65], with permission from the Royal Society of Chemistry.

Appropriate process conditions can effectively improve the performance of HfO₂-based thin films. Lee et al. prepared HZO ferroelectric capacitors with metal-ferroelectric-metal (MFM) structures using an ALD technique incorporating a Sequential No-Atmosphere Process (SNAP)^[99]. The SNAP process can effectively maintain a clean interface between TiN and HZO layers under vacuum conditions, which could effectively suppress the growth of harmful passivation layers and enhance P_r^[99]. Plasma-enhanced atomic layer deposition (PEALD) employs a highly reactive plasma during the reaction, enhancing the deposition rate and the film's quality effectively^[100-102]. The process of PEALD is shown in Figure 11B^[103]. The relationship between the crystallization of HfO₂ films and the leakage current was investigated in the work of Chesnokov et al.^[104]. The PEALD process reduces the crystallization temperature, and energetic particles from the plasma can activate the local crystallization process, which triggers the formation of nanocrystals^[104]. Onaya et al. studied the crystal structures of HZO thin films prepared at different oxidants and different post-metallization annealing (PMA) temperatures^[105]. They found that the spacing of the O/T/C phases gradually increased with rising PMA temperature, confirming that higher annealing temperatures promote the formation of the O-phase. HZO films prepared using PEALD and PMA processes at 400 °C exhibit the highest P_r value and reduced wake-up effect, which is mainly attributed to the increase in the proportion of the O-phase in the HZO film^[105].

Factors in the preparation process, such as the oxygen content^[69], insertion layer^{[57,71,106-109]}, and capping layer^{[38,56,59,68,110-113]}, also affect the properties of HfO₂-based films. Oxygen vacancies in HfO₂-based films are formed due to interfacial reactions between the film and the bottom electrode during deposition and post annealing. Kim et al. used ozone as an oxidizing agent during ALD to oxidize the TiN; the ferroelectricity and durability of the films were significantly improved and P_r was increased by ~1.6 times, with no wake-up effect and no significant fatigue effect, of which the number of cycling is more than 10^8[69]. Inserting the TaO_x layer can well improve the conductivity linearity and durability of HfO₂ devices^[114]. Wang et al. improved the crystallinity and ferroelectricity of 4 nm HZO films by monolayer engineering, and the HZO films can show better crystallinity even at 400 °C^[72]. They replaced one layer of HfO₂ with one layer of ZrO₂ during the ALD process of preparing HZO thin films. The test results showed that this structure can effectively enhance the ferroelectricity of HZO thin films, which provides a feasible solution for the realization of good ferroelectricity below 5 nm^[72].

Goh et al. systematically investigated the effects of metal oxides on the wake-up effect and ferroelectricity of HZO ferroelectric thin films using two electrode materials, TiN and RuO₂^[64,65]. The results indicate that the RuO₂/HZO/RuO₂ capacitor has a smaller wake-up effect and better endurance than the TiN/HZO/TiN capacitor, up to more than 10⁸ cycles. Figure 11C shows that the TiN transition metal electrode reacts with oxygen in the HZO layer, producing oxygen vacancies and defects in the HZO layer. The electron traps on the vacancies generate a built-in electric field and an unswitched pinning domain. Figure 11D explains that, unlike the TiN electrode, the RuO₂ metal oxide electrode can provide oxygen to the HZO layer through its own reduction reaction, thereby suppressing the formation of oxygen vacancies. Therefore, the RuO₂/HZO/RuO₂ device has fewer interface layers, a smaller wake-up effect, and better reliability^[65]. In the RuO₂/HZO/TiN device, the E_c value increases linearly as f increases from 250 Hz to 250 kHz. However, in the TiN/HZO/TiN device, the E_c value is independent of logf. This is attributed to the reaction between the TiN transition metal electrode and oxygen in the HZO layer, whereas the RuO₂ oxide metal electrode supplies oxygen to the HZO layer to inhibit the formation of oxygen vacancies^[64].

Furthermore, top electrodes with different thermal expansion coefficients α and work functions also have an impact on the ferroelectric performances of HfO₂-based films^[59]. It can be observed that the P_r value of ferroelectric capacitors with Au, Pt, TiN, Ta, and W as the top electrode increases continuously. This phenomenon may be due to the beneficial effects of in-plane tensile strain on the phase transition from the T-phase to the O-phase along the c-axis^[55,59]. As α decreases, the in-plane strain changes from compressive strain to tensile strain, and the proportion of the ferroelectric phase increases accordingly^[59,115]. These factors all indicate the importance of fully considering various factors in improving the quality of HfO₂-based thin films during the ALD preparation process.

Pulsed laser deposition

Pulsed laser deposition (PLD) is a highly effective deposition technique that involves subjecting the surface of a target to intense laser pulses, altering the arrangement of atoms on its surface. The high-energy laser pulse causes the target atoms to melt, resulting in the generation of a plasma plume, which subsequently sputters onto the substrate. The preparation of HfO₂-based thin films using PLD is able to obtain high-quality epitaxial films with homogeneous compositions and few impurities, and the quality of the films can be improved quickly and efficiently by adjusting the growth parameters^[116].

There are several crucial factors that require careful attention in preparing HZO thin film with PLD, including oxygen pressure, laser energy density, temperature, bottom electrode, and so on. It has been reported that the use of bottom electrodes with different lattice constants also greatly affects the generation of the O-phase in HZO films^{[11,63,117,118]}. Estand et al. pointed out that different bottom electrodes, even on the same substrate, have different effects on the formation of ferroelectric O-phase and ferroelectric polarization of the HZO films^[118]. The ferroelectric loops plot on the right side of Figure 12A shows that the HZO films on LSMO bottom electrode exhibit the largest ferroelectric polarization, which might be due to the epitaxial stress of LSMO on the HZO film stabilizes the O-phase. However, some researchers have suggested that it is the interfacial interaction between the LSMO and the HZO film^[117]. As shown in Figure 12B, under oxygen-deficient conditions, the Hf/Zr atoms take the place of the Mn at the LSMO/HZO interface, replacing the Mn cations at the MnO₂ interface. This chemical remodeling may have led to the stabilizing effect of LSMO on the ferroelectric O-phase, but the mechanism of this is still under discussion.

Figure 12. (A) XRD images and ferroelectric loops of HZO films grown on different bottom electrodes^[118]. Reprinted from Ref.^[118], with permission from the Royal Society of Chemistry. (B) Substitution of Mn by Hf/Zr in the interface between LSMO and HZO^[117]. Reprinted figures with permission from Ref.^[117] Copyright by the American Physical Society. (C) The work functions of o-HZO, MnO₂-LSMO, LaSrO-LSMO, and LAO-LSMO^[119]. Reprinted from Ref.^[119], with permission from Springer Nature. (D) Intensity of the o-HZO(111) and m-HZO(002) peaks, (E) P_r as a function of the lattice parameter of the substrate^[63]. Reprinted (adapted) with permission from Ref.^[63]. Copyright (2019) American Chemical Society. Dependence of P_r and V_c on (F) temperature, (G) pressure, and (H) thickness^[120]. Reprinted (adapted) with permission from Ref.^[120]. Copyright (2019) American Chemical Society.

Shi et al. demonstrated a crucial interface engineering strategy to stabilize and enhance the ferroelectric orthorhombic phase of HZO thin films by controlling the termination of the underlying LSMO^[119]. In their study, the results showed that MnO₂ termination promoted and stabilized the ferroelectric phase of HZO. As shown in Figure 12C, the work function of MnO₂-terminated LSMO is larger than the work function of o-HZO, leading to the attraction of electrons from o-HZO to MnO₂-terminated LSMO, resulting in a hole-rich condition for HZO. At low hole concentrations, the m-HZO phase is more stable, while at hole concentrations greater than 0.17 h/u.c., the o-HZO phase is more stable. These observations strongly indicate that hole doping and MnO₂ termination in LSMO favor the stability of the o-HZO phase.

Substrate also has an effect on the polarization of HfO₂-based films^[63]. The researchers grew epitaxial bilayer films of HZO and LSMO electrodes on a series of substrates and demonstrated the critical effect of epitaxial stress on the stabilization of the ferroelectric orthorhombic phase. As shown in Figure 12D and E, the lattice strain of LSMO electrodes plays a crucial role in stabilizing the HZO orthorhombic phase, as it is determined by the lattice mismatches with the substrate. When subjected to tensile strain, the majority of HZO films exhibit an orthorhombic phase. As a result, HZO films on TbScO₃ and GdScO₃ substrates exhibit enhanced ferroelectric polarization compared to films on other substrates^[63,121]. Lyu et al. used PLD to grow HZO thin films, and investigated the effects of other factors, temperature, oxygen pressure, and thickness, on the structure and ferroelectricity of the HZO films^[120]. As shown in Figure 12F-H, their work indicated the relationship between coercive field (V_C) and temperature, pressure, and film thickness.

Other fabrication methods

In addition to the ALD and PLD methods mentioned above, magnetron sputtering can also be used to prepare high-quality ferroelectric HZO thin films. Bouaziz et al. used magnetron sputtering to prepare two different structure samples: non-mesa structure (NM) and mesa structure (M)^[62]. These two samples exhibit different electrical characteristics. Under low-stress conditions, the M sample has a higher maximum P_r, while under high-stress conditions, it is the opposite. During the annealing process, different microcrystalline structures are formed in the HZO layer due to the different coverage of the top electrode on the HZO layer. Mittermeier et al. also used RF magnetron sputtering technology to prepare ferroelectric HZO thin films with a thickness of 4 nm and quantitatively studied the neurobehavior of the TiN/HZO/Pt ferroelectric tunnel junction^[122]. Chen et al. prepared hafnium oxide nanocrystals via the hydrothermal method and successfully achieved synaptic behavior^[123,124], further expanding the application prospects of HfO₂in artificial synaptic devices. There are still many unexplored aspects regarding the preparation process of HfO₂-based ferroelectric thin films. For further details, please refer to the corresponding references.

FTJ

The reversal of spontaneous polarization in the ferroelectric tunneling junctions combined with the different charge shielding properties of the two metallic electrodes alters the internal electric field distribution of the FTJ, which in turn alters the tunneling currents to produce two different resistive states^[125,126]. Hafnium oxide has no size effect, and remains ferroelectric even at a thickness of 3-5 nm^[67] with high CMOS compatibility. The work of Bégon-Lours et al. demonstrates the nonvolatile resistive switching behavior of ferroelectric HZO with a thickness of 4.5 nm^[127]. The resistive switching behavior of the films is significantly enhanced after wake-up. The effect of different coefficients of thermal expansion of the bottom electrode on the ferroelectricity of ultrathin HZO films was systematically investigated by Goh et al.^[66]. The 2P_r values of the capacitor decrease gradually in the order of bottom electrodes: W, Mo, Pt, TiN, and Ni, indicating a corresponding reduction in their ferroelectricity. This is attributed to the film undergoing an effective phase transition from the T-phase to the O-phase under the effect of in-plane tensile stresses during annealing at room temperature.

In the work of Prasad et al., ferroelectric HZO thin films with a thickness of up to 2.5 nm were prepared. When applying this thin film to FTJs, the devices showed a significantly high Tunneling Electroresistance ratio^[128]. Specifically, with a 1 nm HZO layer at zero bias voltage, the R_off/R_on ratio reached 135. FTJs with a HZO layer thickness of 2.5 nm demonstrated switch ratios above 10² at different bias voltages, and the R_off/R_on ratio measured at 300 mV can reach up to 10⁵. This work strongly demonstrates the significant potential of HZO thin films for high-density integration of FTJs.

In 2018, Chen et al. prepared a three-dimensional vertical HZO-based FTJ array that meets the requirements of high-density HNN applications^[60]. The FTJ had high integration density and low power consumption (1.8 pJ/pulse). It could simulate synaptic behaviors such as long-term potentiation (LTP), long-term depression (LTD), and Spike Timing-Dependent Plasticity (STDP), and demonstrated good repeatability (> 1,000 cycles). In 2022, Du et al. prepared a single-phase orthorhombic epitaxial HZO-based FTJ with an MFS structure^[129]. They grew a 1uc LSMO and a 4.5 nm HZO film on NSTO. Figure 13A shows the eight resistance states of the FTJ device, while Figure 13B demonstrates the response of the FTJ under different pulse voltages. Figure 13C and D illustrates the cycling endurance and retention performance of the FTJ, respectively. The FTJ device achieves over 1,000 switching cycles and retention times exceeding 10⁴ s in different resistance states.

Figure 13. (A) R-V loops measured at different voltages. (B) Relationship between resistance and pulse voltage duration. (C) Resistance switching cycles. (D) Retention time corresponding to the resistance states in (A)^[129]. Reprinted (adapted) with permission from Ref.^[129]. Copyright (2022) American Chemical Society. (E) Multilevel R-V property, (F) Retention examinations^[70]. Reprinted from Ref.^[70], Copyright (2022), with permission from Elsevier.

Wang et al. further fabricated FTJ devices based on epitaxial HZO and simulated synaptic behavior. The study first investigated the parameters such as crystallinity, strain, interface, and electrical properties of FTJs^[70]. As shown in Figure 13E and F, the HZO-based FTJ can achieve multiple resistance states and retain for over 10⁴ s. Figure 14A-C clearly illustrates its ability to implement various Hebbian rules, exhibiting excellent synaptic plasticity. The researchers have also applied this device to an artificial neural network depicted in Figure 14D. Figure 14E and F presents the relative frequency of the conductance distribution for the optimal weight matrix and mapping matrix, respectively. The recognition rate achieved by this FTJ is 93.7%, slightly lower than the 97.3% achieved by software training, demonstrating great potential for applications. Cao et al. effectively increased the tunneling current and residual polarization of the FTJ by adding ZrO₂ (ZO) and Al₂O₃ (AO) layers to the HZO ferroelectric layer, while reducing leakage current^[73]. Devices with only the AO layer and devices with both AO and ZO layers inserted exhibited larger ferroelectric polarization and better durability and reliability than the original devices. The optimized FTJ also achieved synaptic behavior. The calculated power consumption of the FTJ was found to be as low as 10.44 fJ/spike. The HZO-based FTJ device designed in this work has excellent prospects in the field of artificial synapses.

Figure 14. (A) Asymmetric anti-Hebbian configuration, (B) Symmetric Hebbian configuration, (C) Symmetric anti-Hebbian configuration realized by the device. (D) Schematic diagram of the multilayer neural network used for recognition. (E and F) Weight distribution and accuracy of software training and conductance maps^[70]. Reprinted from Ref.^[70], Copyright (2022), with permission from Elsevier.

Memristor

Hafnium oxide, as a binary oxide, can exhibit good RS behaviors^[130-134] through the formation and dissolution of conductive filaments (CFs)^{[131,135,136]}. The most obvious features of the CF mechanism are the sudden resistance change of SET (from HRS to LRS) and RESET (from LRS to HRS)^[137]. After applying a positive voltage at the top electrode, oxygen ions leave the original lattice and move toward the bottom electrode. After the oxygen ions move, oxygen vacancies are formed in their original positions. Under the influence of the electric field, these vacancies move directionally and begin to accumulate. As they accumulate, a conductive filament channel gradually forms within the oxide film, switching the memory device from a HRS to a LRS, which is known as the SET process. When a corresponding RESET voltage is applied, the oxygen ions recombine with the oxygen vacancies, reducing their concentration within the film, breaking the conductive filaments and transforming the device from LRS to HRS; this is referred to as the RESET process.

Nonlinear behavior and high asymmetry adversely affect the synaptic behavior of memristors. Improving the linearity of memristive devices enhances their performance. The random growth of CFs formed by V_O leads to stochastic CF morphologies, as shown in Figure 15A. Introducing an ion diffusion-limiting layer can regulate CF formation [Figure 15B], but these methods remain suboptimal. Zeng et al. achieved highly linear conductance modulation in HfO₂-based memristors by doping metal nanoparticles during oxide film epitaxial growth, forming well-defined grain boundaries [Figure 15C], which significantly improved device linearity^[138].

Figure 15. Filament growth in (A) common memristor, (B) memristor with a diffusion limiting layer, and (C) memristor with a metal nanoparticle^[138]. Reprinted from Ref.^[138], with permission from Wiley.

Many studies have reported on resistive switching devices with HfO₂ as the functional layer. Covi et al. proposed a TiN/HfO₂/Ti/TiN resistive switching device for use as an artificial synapse in neural-inspired structures^[139]. This device exhibits significant resistive switching behavior, demonstrating potentiation/depression behavior under current stimulation, and has successfully achieved STDP behavior, indicating its ability to realize synaptic plasticity. Jiang et al. developed a Ta/HfO₂/Pt-type resistive switching device^[140]. As shown in Figure 16A, by adjusting the compliance current and testing voltage, they successfully induced 24 different resistive states in the device. Figure 16B demonstrates that these different resistive states can be maintained for more than 10⁴ s, indicating excellent retention performance of the device. Additionally, Figure 16C shows that the device can exhibit enhanced/inhibited behavior by applying positive/negative pulses. Liu et al. fabricated a resistive switching device with a HfO₂/HfO_x ultrathin bilayer structure^[141]. As shown in Figure 16D and E, different resistive states can be induced in the device by applying different voltages or repeating the same voltage. As an artificial synapse, the device was found to exhibit STDP behavior, as shown in Figure 16F. Additionally, the device demonstrated low power consumption, indicating excellent potential for future applications in neuromorphic computing. Zhu et al. prepared Cu/HZO/GeS/Pt and Cu/Ges/Pt memristors, and the addition of the HZO layer can effectively improve the switching ratio and reliability compared to the structure without the HZO layer^[133]. Meanwhile, the memristor with the HZO layer has a switching speed as low as 10 ns, which is due to the different transport rates of copper ions in HZO and GeS, resulting in a more concentrated formation and regeneration of conductive filaments at the interface of the two layers.

Figure 16. (A) The resistance states of the device adjusted by changing the compliance current setting. (B) Retention testing of the device. (C) The simulated switching behavior of the device^[140]. Reprinted from Ref.^[140], with permission from Springer Nature. (D) The measured I-V curve of the resistive device with gradually increasing reset voltages from 1.0 to 3.0 V. (E) The I-V curves of the resistive device measured by applying positive 1.4 V bias voltages six times. (F) The STDP curve of the device^[141]. Reprinted from Ref.^[141], with permission from the Royal Society of Chemistry.

The self-correcting memristor that can be used in large synaptic array structures to reduce leakage currents in neuromorphic systems was then proposed by Ryu with Ti/TiO₂/Si devices and Ti/TiO₂/HfO₂/Si devices^[142]. As shown in Figure 17A and B, the Ti/TiO₂/Si and Ti/TiO₂/HfO₂/Si devices exhibit different I-V characteristics. XPS analysis yielded that the HfO₂ interfacial layer enhanced the interfacial switching characteristics through rectification behavior, and the current suppression by the Schottky barrier between Ti and TiO₂ under negative bias facilitates the realization of high-density synaptic devices. Pattern recognition simulations with a two-layer artificial neural network found that the recognition rate of Ti/TiO₂/HfO₂/Si devices was higher than that of Ti/TiO₂/Si devices, as depicted in Figure 17C. Ismail et al. also prepared RRAMs with TiO₂ and HfO₂ as the functional layers^[143], which is able to achieve more than 10⁷ cycles with retention time of more than 10⁴ s as shown in Figure 17D and E. In addition, as shown in Figure 17F, this RRAM is capable of STDP behaviors and other synaptic behaviors, which has good potential for application in the field of nonvolatile storage and artificial synapses.

Figure 17. The I-V characteristic curves of (A) Ti/TiO₂/Si and (B) Ti/TiO₂/HfO₂/Si devices. (C) Recognition accuracy of Ti/TiO₂/Si devices and Ti/TiO₂/HfO₂/Si devices^[142]. Reprinted from Ref.^[142], Copyright (2020), with permission from Elsevier. (D) W/TiO₂/HfO₂/TaN devices withstand more than 10⁷ switching cycles. (E) W/TiO₂/HfO₂/TaN devices remain unchanged for more than 10⁴ s at 125 °C. (F) LTP, LTD, and STDP behaviors of W/TiO₂/HfO₂/TaN devices^[143]. Reprinted from Ref.^[143], Copyright (2022), with permission from Elsevier. (G) Enhancement and suppression characteristics of Pt/Ta₂O₅/HfO₂/TiN devices. (H) PPD test results. (I) Recognition accuracy of the Fashion MNIST dataset^[108]. Reprinted from Ref.^[108], Copyright (2021), with permission from Elsevier.

By varying the limiting current and SET voltage, the Pt/Ta₂O₅/HfO₂/TiN memristor can achieve multiple resistance states^[108]. This memristor is also capable of emulating synaptic behaviors such as LTD, LTP, and paired-pulse depression (PPD), as depicted in Figure 17G and H. Moreover, the devices demonstrated recognition performance on the Fashion MNIST dataset, as shown in Figure 17I. The inhibitory behavior of the memristor originates from the fact that the rate of accumulation of conductive defects in the switching layer is lower than the relaxation rate of these defects in the device, resulting in a decrease in conductance.

As shown in Figure 18A, a PET/ITO/HZO/Ag flexible memristor was deposited by the ALD process by Wang et al.^[144]. Typical bipolar resistive switching characteristics were observed in this flexible memristor, as depicted in Figure 18B. Long-term plasticity and short-term plasticity were simulated in this artificial synapse by applying continuous pulses to the top electrode, as shown in Figure 18C and D. Gradual modulation of conductance could be attributed to the controllable Ag⁺ conductive filament pathway^[144]. Margolin et al. achieved a W/hydrogenated amorphous silicon(aSi:H) /HZO/TiN ferroelectric memristor on a mica substrate^[145]. The structure and physical image of the flexible device are shown in Figure 18E. The modulation of conductivity is achieved by gradually switching the ferroelectric domains in the structure by affecting the potential barriers. As shown in Figure 18F, even after bending at different bending radii and bending over 100 times at the same bending radius, the device can still maintain its original ferroelectric polarization and switching ratio, indicating that the device can maintain its functional characteristics under both static and repetitive bending, and the enormous potential of HZO-based devices in the field of flexible electronics.

Figure 18. (A) Schematic diagram of ITO/HZO/Ag device structure. Typical (B) I-V characteristic curve, (C) PPF curve, and (D) PPD curve of PET/ITO/HZO/Ag device^[144]. Reprinted from Ref.^[144], with permission from Springer Nature. (E)Schematic diagram (F) properties during bending tests of HZO-based flexible devices^[145]. Reprinted from^[145], with permission from AIP Publishing.

In addition to the CF mechanism, ferroelectric memristors have also received widespread attention. Unlike CF-based memristors that primarily change resistance states by the migration of oxygen vacancies to form CFs, ferroelectric memristors utilize ferroelectric materials as functional layers to switch between HRS and LRS by changing the polarization state of the ferroelectric material^{[13,61,146,147]}. The Pd/HZO/LSMO/LAO ferroelectric memristor fabricated by Xiao et al. undergoes a transition from LRS to HRS when a positive voltage is applied to the top electrode, causing downward polarization in the ferroelectric layer and reducing the depletion layer width and barrier height at the HZO/LSMO interface^[148]. Conversely, applying a negative voltage increases the depletion layer width and barrier height, transitioning the device from LRS to HRS. Yoong et al. grew HZO thin films and investigated their structure, ferroelectricity and brain-like learning behavior^[61]. The device can achieve a retention time of over 6 hours, over 10⁶ cycles, and a switching ratio of over three orders of magnitude. Zhu et al. prepared a memristor device with an Au/HZO/LSMO/STO structure and investigated the effects of cooling rate and oxygen pressure on the performance of HZO^[149]. The results demonstrated that both the annealing rate and oxygen pressure influenced the proportion of the O-phase in HZO. The optimized HZO-based memory device achieved more than 15,000 cycles of high and low resistance switching. Furthermore, the impact of the O-phase on resistive behavior was discussed in this work. This study shows the great significance of the fabrication process to improve the applications of HZO ferroelectric devices in the field of artificial intelligence.

Zhao et al. prepared HZO-based ferroelectric memristors on Si substrates. The results of electrical tests showed that the memristor has typical ferroelectric polarization characteristics, stable switching behavior, and a large R_off/R_on ratio^[150]. The effect of voltage on its conductance regulation was investigated using pulse voltage tests. The conduction mechanism of the HZO film in this work can be explained by the Trap-Assisted Tunneling mechanism. The ferroelectric domain polarization and charge conduction mechanisms inside the device are the key to its performance. The captured electrons are then transferred to the anode through either the Poole-Frenkel emission mechanism or trap-assisted tunneling^[150]. The process of polarization modulation involves modifying the barrier height/width layer at the interface between a ferroelectric material and a semiconductor. When the ferroelectric polarization is directed toward the semiconductor, it generates a depolarizing field in the ferroelectric barrier that opposes the polarization. As a result, the barrier height decreases, leading to a higher tunneling transmittance. In this state, the device is set to LRS. On the contrary, when polarization is reversed and directed toward the metal electrode, the electrons on the semiconductor surface are depleted, increasing the barrier height and reducing the transmission rate of tunneling, causing the device to enter a high resistance state. This work has made significant contributions to explaining the conduction mechanism of the HZO ferroelectric memristor.

FeFETs

HfO₂-based FeFETs with metal/ferroelectric/insulator/semiconductor (MFIS) gate stacks can be used to fabricate high-density and fast-writing nonvolatile memories. However, for practical applications, further improvements in storage window stability and device endurance are still required. Huang et al. fabricated FeFETs using alternative metal gate technology and conducted research on FeFETs with different gate lengths (Lg) and gate widths (Wg), of which the schematic diagram device is shown in Figure 19A and B^[151]. The memory window (MW) is calculated based on the difference between the forward and reverse threshold voltages, and it is one of the important indicators for measuring device performance. It was found that the average memory window of FeFETs with different Lg and Wg reached up to 2.4 V. By utilizing an electronic synapse array, the device can achieve a recognition accuracy rate as high as 95.2%, as illustrated in Figure 19C. Tsai et al. proposed a method to improve the dielectric breakdown strength, on/off current ratio, and MW of HZO-based FeFET^[152]. The device structure is shown in Figure 19D. This method mainly involves controlling the surface morphology and stress of the HZO layer by adjusting the roughness of the bottom electrode TiN and the thermal expansion mismatch with the TiN/W stack covering layer, to optimize the ferroelectric orthorhombic phase of HZO. After applying this FeFET to a neural network, the device achieves high accuracies of 91.4% and 92.2%, as shown in Figure 19E and F, respectively.

Figure 19. (A) Top-view scanning electron microscopy image, (B) HRTEM image, (C) recognition accuracy of different experimental devices and the ideal numeric values of FeFET^[151]. Reprinted figures with permission from Ref.^[151] Copyright by IOP Publishing, Ltd. (D) Schematic diagram of the device structure. The potentiation and depression behavior in FeFETs with (E) IPP and (F) NPP operating modes^[152]. Reprinted (adapted) with permission from Ref.^[152]. Copyright (2022) American Chemical Society. (G) Schematic illustration of the FeFET. (H) Retention tests for different resistance states. (I) The potentiation and depression behavior^[153]. Reprinted (adapted) with permission from Ref.^[153]. Copyright (2020) American Chemical Society.

Halter et al. proposed a device concept based on FeFET^[153], the structure of which is shown in Figure 19G. The device utilizes HZO gate dielectric as hardware support, which exhibits good linearity and CMOS compatibility. Furthermore, as shown in Figure 19H, each induced resistance state in the device demonstrates a retention time of over 1,500 s, showcasing excellent retention performance. It is found that the channel resistance of the device is directly coupled to the polarization of the HZO layer, and it can achieve highly symmetrical potentiation and depression, as illustrated in Figure 19I. The device's good linearity in conductivity is advantageous for its application in artificial neural networks.

In the work of Xiao et al., the storage window, retention, and durability of HZO-based FeFETs with MFIS gate stacks were studied using a fast voltage pulse measurement method^[106]. The results showed that HZO-based FeFETs with an additional ZrO₂ crystal seed layer exhibited longer retention and larger initial storage window. This is mainly attributed to the improved crystalline quality of the HZO layer and suppression of interface defects after inserting the ZrO₂ crystal seed layer.

As shown in Table 1, compared to traditional ferroelectric materials, hafnium oxide-based devices exhibit no size effect, facilitating device miniaturization, while demonstrating strong compatibility with silicon without adverse reactions and excellent CMOS compatibility^[154-159]. Currently, HfO₂-based devices have already achieved diverse synaptic behaviors and hold potential for application as artificial synaptic components in artificial neural networks, with promising prospects even in the field of flexible electronics.