Hf0.5Zr0.5O2 Film Research Articles

Improved leakage and fatigue properties of W/Hf0.5Zr0.5O2/W capacitor through the insertion of Pt metallic layer

The successful integration of Hf0.5Zr0.5O2 (HZO) ferroelectric (FE) thin film into memory devices necessitates addressing the challenges associated with achieving high remanent polarization (Pr) and low leakage current, in order to ensure excellent reliability. The limitations of HZO film are overcome in this paper through the design of a Pt/W bilayer electrode, wherein a thin Pt metallic layer (ML) is embedded at the top interface of the W/HZO/W capacitor. This innovative approach combines the unique advantages offered by both W and Pt electrodes. The results reveal that the 2Pr values of the W/HZO/5 Pt/W and W/HZO/10 Pt/W capacitors, with Pt ML thicknesses of 5 nm and 10 nm, respectively, are slightly lower at 42.1 μC/cm2 and 35.9 μC/cm2 compared to the reference W/HZO/W capacitor's value of 47.9 μC/cm2. This notwithstanding, these 2Pr values still maintain comparability with previously reported findings. Moreover, compared to the reference W/HZO/W capacitor, the W/HZO/10 Pt/W capacitor exhibits a reduction of one order of magnitude in leakage current, a 30 % increase in breakdown field from 3.0 MV/cm to 3.9 MV/cm, and an improvement of one order of magnitude in endurance performance, reaching 1.3 × 108 cycles. This improvement can be attributed to the incorporation of a thin Pt ML, which enhances the schottky barrier and prevents potential oxidation of adjacent W electrode near the HZO film, effectively alleviating the leakage current. The present work demonstrates the successful mitigation of leakage current and improved fatigue properties while preserving FE characteristics through strategic insertion of Pt ML. The research findings can offer valuable scientific guidance for optimizing the performance of HZO-based devices.

Characterization of HZO Films Fabricated by Co-Plasma Atomic Layer Deposition for Ferroelectric Memory Applications.

Plasma-enhanced atomic layer deposition (ALD) is a common method for fabricating Hf0.5Zr0.5O2 (HZO) ferroelectric thin films that can be performed using direct-plasma (DP) and remote-plasma (RP) methods. This study proposed co-plasma ALD (CPALD), where DPALD and RPALD are applied simultaneously. HZO films fabricated using this method showed wake-up-free polarization properties, no anti-ferroelectricity, and high fatigue endurance when DPALD and RPALD started simultaneously. To minimize defects in the film that could negatively affect the low polarization properties and fatigue endurance, the direct plasma power was reduced to 75 W. Thus, excellent fatigue endurance for at least 109 cycles was obtained under a high total remanent polarization of 47.3 μC/cm2 and an applied voltage of 2.5 V. X-ray photoelectron spectroscopy and transmission electron microscopy were used to investigate the mechanisms responsible for these properties. The HZO films fabricated by CPALD contained few lattice defects (such as nonstoichiometric hafnium, nonlattice oxygen, and residual carbon) and no paraelectric phase (m-phase). This was attributed to the low-carbon residuals in the film, as high-energy activated radicals were supplied by the adsorbed precursors during film formation. This facilitated a smooth transition to the o-phase during heat treatment, which possessed ferroelectric properties.

High performance and nearly wake-up free Hf0.5Zr0.5O2 ferroelectric capacitor realized by middle layer strategy with BEOL compatibility

Hf0.5Zr0.5O2(HZO) has drawn great attention owing to its excellent ferroelectricity, sub-10 nm scalability, and CMOS compatibility. With regard to increasingly restrict thermal budget and power consumption, conventional HZO films need further optimization to meet these demands. Here, we propose a middle layer (ML) strategy aiming to enhance ferroelectricity and inhibit wake-up effect of ferroelectric (FE) capacitors compatible with back-end of line under the low operating electric field. ZrO2, HfO2, and Al2O3were integrated into HZO film as different MLs. Among them, the device with ZrO2ML achieves the excellent double remnant polarization (2Pr) of 41.7μC cm-2under the operating electric field of 2 MV cm-1. Moreover, ultralow wake-up ratios of around 0.08 and 0.05 were observed under 2 MV cm-1and 3 MV cm-1, respectively. Additionally, the FE capacitor with ZrO2ML demonstrated an enhanced reliability characterizations, including a stable 2Prof 40.7μC cm-2after 4.3 × 109cycles. This work provides the perspective to optimize both the ferroelectricity and reliability, while maintains the ultralow wake-up ratio in HfO2-based FE through ML engineering.

Stabilization of Tetragonal Phase in Hafnium Zirconium Oxide by Cation Doping for High-K Dielectric Insulators.

As electronic circuit integration intensifies, there is a rising demand for dielectric insulators that provide both superior insulation and high dielectric constants. This study focuses on developing high-k dielectric insulators by controlling the phase of the Hf0.5Zr0.5O2 (HZO) film with additional doping, utilizing yttrium (Y), tantalum (Ta), gallium (Ga), silicon (Si), and aluminum (Al) as dopants. Doping changes the ratio of tetragonal to monoclinic phases in doped HZO films, and Y-doped HZO (Y:HZO) films specifically exhibit a high tetragonal phase ratio and a dielectric constant of 40.9, indicating superior insulating properties compared to undoped HZO films. To clarify the fundamental mechanism driving the enhancement in dielectric properties, we have carried out various analyses combined with density functional theory (DFT) calculations. Through the optimization of the post-deposition annealing (PDA) process and the heterojunction structure with Al2O3, an Al2O3/Y:HZO heterojunction with a high dielectric constant and even lower leakage current compared to a single layer was developed. The thin-film transistor (TFT) with the Au/Ti/amorphous InGaZnO4 (a-IGZO)/Al2O3/YHZO/TiN heterojunction structure exhibits low subthreshold swing (SS) values within a narrow gate-source voltage (Vsg) range. This study advances knowledge on how the controlled-phase doped HZO films affect the dielectric constant and leakage current and will contribute to semiconductor technology advancements by overcoming the limitations of conventional high-k dielectric insulators.

Epitaxial Orientation-Controlled High Crystallinity and Ferroelectric Properties in Hf0.5Zr0.5O2 Films.

Hafnium-based binary oxides are essential for fabricating nanoscale high-density ferroelectric memory devices. However, effective strategies to control and improve their thin-film single crystallinity and metastable ferroelectricity remain elusive, hindering potential applications. Here, using NdGaO3 (NGO) substrates with four crystalline orientations, we report a systematic study of the structural characterizations and ferroelectric properties of epitaxial Hf0.5Zr0.5O2 (HZO) films, demonstrating orientation-controlled high crystallinity and enhanced ferroelectric properties. HZO films grown on NGO(001) and NGO(110) substrates exhibit relatively low crystallinity and a significant presence of the monoclinic phase. In contrast, HZO films grown on NGO(100) and NGO(010) possess high single crystallinity and a dominant ferroelectric phase. These differences are attributed to the surface symmetry of the NGO substrate, which favors the formation of 4- or 2-fold domain configurations. Moreover, the optimized HZO films exhibit a large polarization (2Pr) of ∼50 μC/cm2, enhanced fatigue behavior up to 1011 cycles, improved retention of 2Pr ∼ 40 μC/cm2 after 10 years, and characteristic polarization switching speeds in the submicrosecond range. Our results highlight the importance of modulating the single crystallinity and ferroelectric phase fraction of HfO2-based films to enhance ferroelectric properties, further revealing the potential of epitaxial symmetry engineering.

Bias voltage modulated electric transport properties in Fe65Co35/Hf0.5Zr0.5O2 films

We demonstrate the bias voltage modulated electric transport properties in Fe65Co35 (FeCo) films deposited on Hf0.5Zr0.5O2 (HZO) films. Intrinsic multiferroicity have been characterized for the FeCo/HZO heterostructures by analyzing the ferroelectric polarizations and imaging the magnetic domains. We show that, upon the application of bias voltages on the FeCo/HZO heterostructures, the current-voltage (I-V) curves of FeCo films can be regulated, giving rise to different resistance states, enabled partly by strain-mediated magnetoelectric coupling effect. The strain coupling effect between HZO and FeCo films is elucidated by simulating the volumetric strain of the FeCo/HZO films under the action of applied voltages. Through additional analysis, the bias voltage modulated electric transport properties could also be linked with charge-mediated magnetoelectric coupling effect. We anticipate that our work will inspire further studies on low-power consumption and high-density electronic devices.

Improving the ferroelectric properties of Lu doped Hf0.5Zr0.5O2 thin films by capping a CeO x layer

Lu doped Hf0.5Zr0.5O2 (HZO) ferroelectric films were prepared on Pt/TiN/SiO2/Si substrate by chemical solution deposition method, and an interfacial engineering strategy for improving the ferroelectric property was explored by capping the Lu doped HZO films with a cerium oxide layer. Compared with the Lu doped HZO film without the CeO x coating layer, the Lu doped HZO film with the CeO x coating layer has a larger remanent polarization (2P r = 34.72 µC cm−2) and presents weaker wake-up behavior, which result from the higher orthogonal phase ratio and the lower oxygen vacancy of the CeO x coated Lu doped HZO film. In addition, the CeO x coating can remarkably improve the fatigue resistance and retention performance of the Lu doped HZO films. It is hoped that the results can provide an effective approach for the realization of high-performance and highly reliable hafnium oxide based ferroelectric thin films.

Magnetically-induced enhancement of photocatalytic dye decomposition in ferroelectric Hf0.5Zr0.5O2 films

The film photocatalyst is easily recycled and has attracted the widely research interesting. Here the ferroelectric Hf0.5Zr0·5O2(HZO) thin film synthesized through the atomic layer deposition (ALD) method is used as a photocatalyst for the rhodamine B (RhB) dye decomposition under visible light irradiation. The photocatalytic RhB dye decomposition performances with or without an external magnetic field excitation provided by a commercial NdFeB magnet are experimentally characterized. With the increasing of the external direct-current (DC) magnetic field from 0 to 3000 Gs, the photocatalytic RhB dye decomposition ratio by HZO film obviously increases from 87.20 % to 98.17 %, which is attributed to the enhanced separation of the photo-induced positive and negative carrier due to the magnetic Lorentz effect. The magnetically-induced enhancement of photocatalysis in the ferroelectric HZO film, provides a method with these advantages of simple external excitation method, remarkable promotion effect and friendly-environment to enhance the dye decomposition ratio, and is potential in the practical application of photocatalytic dye wastewater treatment through harvesting the clean solar energy in future.

Symmetry Engineering of Epitaxial Hf0.5Zr0.5O2 Ultrathin Films.

Robust ferroelectricity in HfO2-based ultrathin films has the potential to revolutionize nonvolatile memory applications in nanoscale electronic devices because of their compatibility with the existing Si technology. However, to fully exploit the potential of ferroelectric HfO2-based thin films, it is crucial to develop strategies for the controlled stabilization of various HfO2-based polymorphs in nanoscale heterostructures. This study demonstrates how substrate-orientation-induced anisotropic strain can engineer the crystal symmetry, structural domain morphology, and growth orientation of ultrathin Hf0.5Zr0.5O2 (HZO) films. Epitaxial ultrathin HZO films were grown on the heterostructures of (001)- and (110)-oriented La2/3Sr1/3MnO3/SrTiO3 (LSMO/STO) substrate. Various structural analyses revealed that the (110)-oriented substrate promotes a higher degree of structural order (crystallinity) with improved stability of the (111)-oriented orthorhombic phase (Pca21) of HZO. Conversely, the (001)-oriented substrate not only induces a distorted orthorhombic structure but also facilitates the partial stabilization of nonpolar phases. Electrical measurements revealed robust ferroelectric properties in epitaxial thin films without any wake-up effect, where the well-ordered crystal symmetry stabilized by STO(110) facilitated better ferroelectric characteristics. This study suggests that tuning the epitaxial growth of ferroelectric HZO through substrate orientation can improve the stability of the metastable ferroelectric orthorhombic phase and thereby offer a better understanding of device applications.

Precise control of fatigue, wake-up, charge injection, and break-down in Hf0.5Zr0.5O2-based ferroelectric memories

Hf0.5Zr0.5O2 (HZO) thin films are promising for applications in ferroelectric memories. However, these materials often face challenges, such as polarization fluctuations (e.g., fatigue and wake-up) and electric break-down incidents during the “1/0” write/read cycles, hindering their industrial application. Herein, fatigue, wake-up, and electric break-down seriously depend on both the electric field for polarization switching (Es) and the charge accumulation in the HZO film. More and more charges pin ferroelectric domains, and the sub-switching polarization introduces serious ferroelectric fatigue during the 1010 write/read cycles at Es ∼ 1.2Ec, where Ec is a ferroelectric coercive field. On the contrary, new charges increase rather slowly, and complete polarization switching is realized during the 1010 cycles at Es ∼ 1.5Ec, so the HZO film presents excellent polarization stability. A high Es ∼ 2Ec introduces the strong wake-up effect first; however, the serious charge accumulation largely increases leakage current and quickly introduces an electric break-down of the HZO film. Furthermore, it was found that the leakage current and dielectric permittivity can effectively monitor the charge accumulation and provide an early warning for fatigue and electric break-down.

Low oxidation conditions in pulsed laser deposition enhance polarization without degradation of endurance and retention in Hf0.5Zr0.5O2 films

Interplay between oxygen vacancies and the stabilization of the ferroelectric orthorhombic phase in doped HfO2, as well as the resulting impact on endurance and retention, is far from being well understood. In Hf0.5Zr0.5O2 (HZO) thin films, it is commonly found that high polarization occurs usually at the the expense of robustness upon cycling due to the polarization–endurance dilemma. It has been reported that HZO thin films grown by pulsed laser deposition under the mixed Ar and O2 atmosphere exhibit a high polarization. Here, we show that this strategy enables functional properties tuning, allowing to obtain HZO films with high polarization at low oxidation conditions without degradation of endurance and retention.

Analysis of ferroelectric properties of ALD-Hf0.5Zr0.5O2 thin films according to oxygen sources

Ferroelectric Hf0.5Zr0.5O2 (HZO) thin films are mostly deposited with a thickness of less than 10 nm by an atomic layer deposition (ALD) process. Since the oxygen source used in the ALD process affects the residues in the deposited HZO films, the choice of oxygen source can be one of the important factors to improve ferroelectricity. From this point of view, the ferroelectric properties of 10-nm-thick ALD-HZO films according to the oxygen source (O3, H2O, and D2O) were comprehensively analyzed in this study. Heavy water (deuterium water, D2O) was used as a tracer to pinpoint the origin of hydrogen that could be derived from unreacted metal precursors or unreacted hydroxyl groups. As a result, it was revealed that the decrease in ferroelectric polarization and increase in leakage current observed in the H2O- and D2O-based HZO capacitors compared to the O3-based HZO capacitor were due to the oxygen source. These results highlight the importance of using O3 as a hydrogen-free oxygen source in the ALD process to achieve better ferroelectricity.

Texture in atomic layer deposited Hf0.5Zr0.5O2 ferroelectric thin films

HfO2-based ferroelectric materials have excellent CMOS compatibility and polarization even in ultrathin films, which is regarded as promising candidate material for memory devices and related electronic devices. However, as the ferroelectric layer thickness scaling down, the consistency of device performance is inevitably affected by the complex polyphase and polycrystalline nature of HfO2-based ferroelectric thin films. Therefore, obtaining and controlling the texture of HfO2-based ferroelectric thin films is extremely important for regulating polarization orientation and improving ferroelectric polarization. In this work, 10 nm Hf0.5Zr0.5O2 (HZO) films were prepared by atomic layer deposition (ALD) on Nb-doped SrTiO3 (NSTO) single crystal substrates. The texture and orientation of HZO films were analyzed through the state-of-the-art spherical aberration corrected transmission electron microscope (Cs-TEM) technique. The domain matching epitaxy between HZO films and NSTO substrate helps eliminate lattice mismatch and residual misfit strain, and ultimately leads to the formation of <112> texture in HZO ferroelectric films. The obtained results represent lattice and interface play an extremely important role in regulating the ferroelectricity of HfO2-based films, which provides a way to design ultra-thin HfO2-based materials with excellent ferroelectric polarization by controlling the crystal structure and orientation.

Improvement in the polarization properties in thin ferroelectric Hf0.5Zr0.5O2 films by two-step flash lamp annealing

In this study, we systematically studied the polarization properties of thin ferroelectric Hf0.5Zr0.5O2 (HZO) films annealed by flash lamp annealing (FLA). We have recently proposed a unique annealing method, two-step FLA, which features high temperature annealing with a minimal increase in thermal budget. Using this technique, we observed improvements in the polarization properties in 5 nm HZO films. These were an increase in the remanent polarization (2Pr) to 24.2 μC cm−2 and better durability compared with other more conventional annealing techniques. In addition, we confirmed that there was no clear degradation in 2Pr under thermal stress. Two-step FLA is one of the effective ways to obtain ferroelectricity with high values of 2Pr and good durability in HZO films thinner than 5 nm.

Epitaxy‐Driven Ferroelectric/Non‐Ferroelectric Polymorph Selection in an All‐Fluorite System

AbstractFilms of ferroelectric hafnia have hitherto been deposited on electrodes with a non‐fluorite crystal structure. As a result, they are polycrystalline, contain fractions of non‐ferroelectric polymorphs or have poor crystal quality. Here, a strategy that circumvents all these limitations is shown. Seven nanometers‐thick epitaxial Hf0.5Zr0.5O2 (HZO) films are deposited directly on yttria‐stabilized zirconia (YSZ) single‐crystals. The fluorite structure of the whole system enables coherent epitaxy, while the substrate orientation induces polymorph‐selective growth, being the HZO films orthorhombic on YSZ(111) and monoclinic on YSZ(001). Besides, the YSZ substrate can play the role of a buried floating electrode under the appropriate measuring conditions (temperature and frequency) thanks to its thermally‐activated oxygen conductivity. Indeed, out‐of‐plane ferroelectric switching is confirmed in the orthorhombic HZO samples at 185 °C and 0.01 Hz frequency. This original approach avoids the need to deposit conducting bottom layers, allowing high‐quality orthorhombic hafnia to be obtained directly on the substrate and its ferroelectric nature to be studied. Moreover, it constitutes a case of ion‐driven ferroelectric switching, and thus gives support to the recently proposed relationship between ionic conductivity, and ferroelectricity in fluorite systems.

Quantification of Crystalline Phases in Hf0.5Zr0.5O2 Thin Films through Complementary Infrared Spectroscopy and Ab Initio Supercell Simulations.

In the quest for thinner and more efficient ferroelectric devices, Hf0.5Zr0.5O2 (HZO) has emerged as a potential ultrathin and lead-free ferroelectric material. Indeed, when deposited on a TiN electrode, 1-25 nm thick HZO exhibits excellent ferroelectricity capability, allowing the prospective miniaturization of capacitors and transistor devices. To investigate the origin of ferroelectricity in HZO thin films, we conducted a far-infrared (FIR) spectroscopic study on 5 HZO films with thicknesses ranging from 10 to 52 nm, both within and out of the ferroelectric thickness range where ferroelectric properties are observed. Based on X-ray diffraction, these HZO films are estimated to contain various proportions of monoclinic (m-), tetragonal (t-), and polar orthorhombic (polar o-) phases, while only the 11, 17, and 21 nm thick are expected to include a higher amount of polar o-phase. We coupled the HZO infrared measurements with DFT simulations for these m-, t-, and polar o-crystallographic structures. The approach used was based on the supercell method, which combines all possible Hf/Zr mixed atomic sites in the solid solution. The excellent agreement between measured and simulated spectra allows assigning most bands and provides infrared signatures for the various HZO structures, including the polar orthorhombic form. Beyond pure assignment of bands, the DFT IR spectra averaging using a mix of different compositions (e.g., 70% polar o-phase +30% m-phase) of HZO DFT crystal phases allows quantification of the percentage of different structures inside the different HZO film thicknesses. Regarding the experimental data analysis, we used the spectroscopic data to perform a Kramers-Kronig constrained variational fit to extract the optical functions of the films using a Drude-Lorentz-based model. We found that the ferroelectric films could be described using a set of about 7 oscillators, which results in static dielectric constants in good agreement with theoretical values and previously reported ones for HfO2-doped ferroelectric films.

Spatially selective crystallization of ferroelectric Hf0.5Zr0.5O2 films induced by sub-nanosecond laser annealing

In this work we study the sub-nanosecond laser-induced crystallization of 10 nm-thick atomic layer deposited amorphous Hf0.5Zr0.5O2 (HZO) films in an air atmosphere. We used an infrared laser with 1064 nm wavelength and 800 ps pulses to anneal TiN/HZO/TiN capacitors by scanning an 80 μm-diameter spot along their top surface in a controlled way. The laser annealing process was optimised in terms of fluence to achieve the complete crystallization of HZO into a non-monoclinic polymorph, as demonstrated by X-ray diffraction and transmission electron microscopy. Piezoresponse force microscopy and polarization-field loops confirm that the optimal as-annealed HZO films are piezoelectric and ferroelectric from the first cycle on. Spatial selectivity was accomplished by scanning the laser on selected areas of the samples. Micro-diffraction experiments show that the transition between the crystallized and the amorphous region is abrupt within a distance of several hundred µm.

Ultrathin Al2O3 interfacial layer for Hf0.5Zr0.5O2-based ferroelectric field-effect transistors

In this study, the effects of an Al2O3 interfacial layer (IL) on the characteristics of ferroelectric field-effect transistors (FeFETs) with Hf0.5Zr0.5O2 (HZO) gate dielectrics on Si substrates were investigated. FeFETs with HZO gate dielectrics have gained considerable attention owing to their compatibility with modern fabrication processes and scalability. However, during HZO deposition on Si substrates, an ultrathin metal silicate IL with a low dielectric constant is formed in an uncontrolled manner, leading to a significant voltage drop and the generation of interface traps during device operation. To address this issue, an ultrathin Al2O3 IL with a thickness less than 2 nm was introduced between the HZO film and Si substrate via in situ atomic layer deposition. The impact of this IL on a memory window (MW) and endurance characteristics was evaluated by comparing the devices with and without an intentional Al2O3 IL. The obtained results revealed that the Al2O3 IL effectively suppressed the interface trap generation, expanded the MW, and enhanced the transistor endurance characteristics. This described approach can be potentially used for improving the reliability of FeFETs fabricated on Si substrates.

Giant Electromechanical Effect in Piezoelectric Nanomembranes Based on Hf0.5Zr0.5O2.

Since ultrathin ferroelectric HfO2 films can be conformally grown by atomic layer deposition even on complex three-dimensional structures, new horizons in the development of next-generation piezoelectric devices are opened. However, hafnium oxide has a significant drawback for piezoelectric applications: its piezoelectric coefficients are much smaller than those of classical materials currently used in piezoelectric devices. Therefore, new approaches to the development of high-performance piezoelectric devices based on exploiting the unique properties of HfO2 are of paramount importance. In this work, a giant electromechanical effect in miniature piezoelectric membrane devices based on a 10 nm-thick ferroelectric Hf0.5Zr0.5O2 (HZO) film is experimentally demonstrated. Compared to the pure piezoelectric effect in the HZO film, the gain of the electromechanical response in membrane devices reaches 25 times. Numerical simulations confirm that this effect stems from the asymmetric shape of the membranes and can be further improved by designing the device geometry. Furthermore, according to first-principles calculations, an additional opportunity to improve the piezoelectric coefficient, and hence, the device efficiency is provided by the engineering of the mechanical stress in the HZO film. The proposed approach enables the development of new promising piezoelectric devices including miniature reflectors, nanoactuators, and nanoswitches.

Interface-type tunable oxygen ion dynamics for physical reservoir computing

Reservoir computing can more efficiently be used to solve time-dependent tasks than conventional feedforward network owing to various advantages, such as easy training and low hardware overhead. Physical reservoirs that contain intrinsic nonlinear dynamic processes could serve as next-generation dynamic computing systems. High-efficiency reservoir systems require nonlinear and dynamic responses to distinguish time-series input data. Herein, an interface-type dynamic transistor gated by an Hf0.5Zr0.5O2 (HZO) film was introduced to perform reservoir computing. The channel conductance of Mott material La0.67Sr0.33MnO3 (LSMO) can effectively be modulated by taking advantage of the unique coupled property of the polarization process and oxygen migration in hafnium-based ferroelectrics. The large positive value of the oxygen vacancy formation energy and negative value of the oxygen affinity energy resulted in the spontaneous migration of accumulated oxygen ions in the HZO films to the channel, leading to the dynamic relaxation process. The modulation of the channel conductance was found to be closely related to the current state, identified as the origin of the nonlinear response. In the time series recognition and prediction tasks, the proposed reservoir system showed an extremely low decision-making error. This work provides a promising pathway for exploiting dynamic ion systems for high-performance neural network devices.