Polarization Switching Speed Research Articles

Large ferroelectric polarization and high dielectric constant in HfO2-based thin films via Hf0.5Zr0.5O2/ZrO2 nanobilayer engineering

HfO2-based ferroelectric films have been extensively explored and utilized in the field of non-volatile memory and electrical programmability. However, the trade-off between ferroelectric polarization and dielectric constant in HfO2 has limited the overall performance improvement of devices in practical applications. Herein, a novel approach is proposed for the Hf0.5Zr0.5O2/ZrO2 (HZO/ZrO2) nanobilayer engineering, which can effectively regulate the phase structure evolution of HfO2 films to construct a suitable morphotropic phase boundary (MPB). The findings highlight that the top ZrO2 layer can regularly promote the formation of either the ferroelectric o-phase or the antiferroelectric t-phase. An ideal MPB is successfully established in HZO/ZrO2 (6/9 nm) nanobilayer film by carefully optimizing the HZO/ZrO2 thickness ratio, which presents a high dielectric constant of 52.7 and a large 2Pr value of up to 72.3 μC/cm2 without any wake-up operation. Moreover, the HZO/ZrO2 nanobilayer thin films demonstrate faster polarization switching speed (1.09 μs) and better fatigue performance (109 cycles) compared to the conventional HZO solid solution films. The relationship between ferroelectric and dielectric properties can be harmoniously balanced through the designation. The results indicate that the HZO/ZrO2 nanobilayer engineering strategy is quite potential to pave the way for the development of next-generation memory technologies with superior performance and reliability.

Nanoscale Investigation of the Effect of Annealing Temperature on the Polarization Switching Dynamics of Hf0.5Zr0.5O2 Thin Films

AbstractRecently, HfO2‐based ferroelectric thin films have attracted widespread interest in developing next‐generation nonvolatile memories. To form a metastable ferroelectric orthorhombic phase in HfO2, a post‐annealing process is typically necessary. However, the microscopic mechanism underlying the effect of annealing temperature on ferroelectric domain nucleation and growth is still obscure, despite its importance in optimizing the operation speed of HfO2‐based devices. In this study, the ferroelectric properties and polarization switching of Hf0.5Zr0.5O2 thin films annealed at different temperatures (550–700 °C) are systematically investigated. Evidently, the crystal structure, remnant polarization, and dielectric constant monotonically change with annealing temperature. However, microscopic piezoresponse force microscopy images as well as macroscopic switching current measurements reveal non‐monotonic changes in the polarization switching speed with annealing temperature. This intriguing behavior is ascribed to the difference in the ferroelectric‐domain nucleation process induced by the amount of oxygen vacancies in the Hf0.5Zr0.5O2 thin films annealed at different temperatures. This work demonstrates that controlling the defect concentration of ferroelectric HfO2 by tuning the post‐annealing process is critical for optimizing device performance, particularly polarization switching speed.

Grain-size adjustment in Hf0.5Zr0.5O2 ferroelectric film to improve the switching time in Hf0.5Zr0.5O2-based ferroelectric capacitor

By adjusting the rising time in annealing ferroelectric HfO2-based films, the grain size of the film can be controlled. In this study, we found that increasing the rising time from 10 to 30 s at an annealing temperature of 700 °C in N2 atmosphere resulted in improved ferroelectric switching speed. This is because the larger grain size reduces the internal resistance components, such as the grain bulk resistance and grain boundary resistance, of the HZO film. This in turn lowers the overall equivalent resistance. By minimizing the RC time constants, increasing the grain size plays a key role in improving the polarization switching speed of ferroelectric films.

Coupling effects of interface charge trapping and polarization switching in HfO2-based ferroelectric field effect transistors

HfO2-based ferroelectric field-effect transistors (FeFETs) are regarded as one of the most promising non-volatile memory technologies in the future. However, the charge trapping phenomenon during the program/erase operation is still a challenge. In this work, we comprehensively investigate the behaviors of semiconductor/insulator interface charge trapping in HfO2-based FeFETs. Through analyzing the effects of the spatial distribution of interface traps and the polarization switching speed, the coupling effects of semiconductor/insulator interface charge trapping and polarization switching are recognized. We also find that the band tail state traps have much less influence on the electrical characteristics of the FeFETs than the deep level state traps. Through engineering the devices with band tail state traps with concentrations as small as possible, the influences of charge trapping could be effectively suppressed. Moreover, the gate voltage (VG) scanning rate has a significant influence on the interface charge trapping process due to the time dependent change of ferroelectric polarization. The largest memory window could be obtained by carefully choosing the VG scanning rate of the FeFETs based on the polarization switching speed. This work represents a key step for realizing highly reliable HfO2-based FeFETs.

Effect of Domain Structure and Dielectric Interlayer on Switching Speed of Ferroelectric Hf0.5Zr0.5O2 Film

The nanosecond speed of information writing and reading is recognized as one of the main advantages of next-generation non-volatile ferroelectric memory based on hafnium oxide thin films. However, the kinetics of polarization switching in this material have a complex nature, and despite the high speed of internal switching, the real speed can deteriorate significantly due to various external reasons. In this work, we reveal that the domain structure and the dielectric layer formed at the electrode interface contribute significantly to the polarization switching speed of 10 nm thick Hf0.5Zr0.5O2 (HZO) film. The mechanism of speed degradation is related to the generation of charged defects in the film which accompany the formation of the interfacial dielectric layer during oxidization of the electrode. Such defects are pinning centers that prevent domain propagation upon polarization switching. To clarify this issue, we fabricate two types of similar W/HZO/TiN capacitor structures, differing only in the thickness of the electrode interlayer, and compare their ferroelectric (including local ferroelectric), dielectric, structural (including microstructural), chemical, and morphological properties, which are comprehensively investigated using several advanced techniques, in particular, hard X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, and electron beam induced current technique.

Re-Annealing-Induced Recovery in 7nm Hf0.5Zr0.5O2 Ferroelectric Film: Phase Transition and Non-Switchable Region Repair

To achieve HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -based ferroelectric (FE) devices with robust reliabilities, the impacts of re-annealing on 7nm FE-Hf <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> Zr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (HZO) capacitors are comprehensively studied in this work. Impressively, the re-initialization phenomenon can be clearly observed by re-annealing cycled HZO capacitors. It is found that FE properties (remanent polarization (Pr), coercive electric field (E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> ), polarization switching speed, wakeup/fatigue effect, and symmetry) can be obviously improved after re-annealing. With in-depth discussions, it is considered that temperature-dependent phase transition and non-switchable region repairing could be the dominant mechanisms. Our results indicate that re-annealing could effectively improve FE-HZO performance and shed light on reliability optimizations.

Highly Enhanced Polarization Switching Speed in HfO2‐based Ferroelectric Thin Films via a Composition Gradient Strategy

AbstractThe next‐generation semiconductor memories are essentially required for the advancements in modern electronic devices. Ferroelectric memories by HfO2‐based ferroelectric thin films (FE‐HfO2) have opened promising directions in recent years. Nevertheless, improving the polarization switching speed of FE‐HfO2 remains a critical task. In this study, it is demonstrated that the composition‐graded Hf1‐xZrxO2 (HZO) ferroelectric thin film has more than two times faster polarization switching speed than the conventional composition‐uniform one. Meanwhile, it has excellent ferroelectricity and improved endurance characteristics. It is also discovered that when the HZO thin film has a gradient composition, the polarization‐switching dynamics shifts from the nucleation‐limited‐switching mechanism to the domain‐wall growth mechanism. Moreover, the transition of switching dynamics is responsible for the faster speed and better endurance of the composition‐graded HZO thin film. These findings not only reveal the physical mechanisms of this material system but also provide a new strategy for memory devices having faster speed and higher endurance.

Sub-Nanosecond Switching of Si:HfO2 Ferroelectric Field-Effect Transistor.

The discovery of ferroelectric doped HfO2 enabled the emergence of scalable and CMOS-compatible ferroelectric field-effect transistor (FeFET) technology which has the potential to meet the growing need for fast, low-power, low-cost, and high-density nonvolatile memory, and neuromorphic devices. Although HfO2 FeFETs have been widely studied in the past few years, their fundamental switching speed is yet to be explored. Importantly, the shortest polarization time demonstrated to date in HfO2-based FeFET was ∼10 ns. Here, we report that a single subnanosecond pulse can fully switch HfO2-based FeFET. We also study the polarization switching kinetics across 11 orders of magnitude in time (300 ps to 8 s) and find a remarkably steep time-voltage relation, which is captured by the classical nucleation theory across this wide range of pulse widths. These results demonstrate the high-speed capabilities of FeFETs and help better understand their fundamental polarization switching speed limits and switching kinetics.

Mesoscopic-scale grain formation in HfO2-based ferroelectric thin films and its impact on electrical characteristics

Ferroelectric memory devices are expected for low-power and high-speed memory applications. HfO2-based ferroelectric is attracting attention for its CMOS-compatibility and high scalability. Mesoscopic-scale grains, of which size is almost comparable to device size, are formed in HfO2-based ferroelectric poly-crystalline thin films, which largely influences electrical characteristics in memory devices. It is important to study the impact of mesoscopic-scale grain formation on the electrical characteristics. In this work, first, we have studied the thickness dependence of the polarization switching kinetics in HfO2-based ferroelectric. While static low-frequency polarization is comparable for different thickness, dynamic polarization switching speed is slower in thin Hf0.5Zr0.5O2 (HZO) capacitors. Based on the analysis using the NLS model and physical characterization, thinner HZO contains smaller grains with orientation non-uniformity and more grain boundaries than thicker HZO, which can impede macroscopic polarization switching. We have also theoretically and experimentally studied the polar-axis alignment of a HfO2-based ferroelectric thin film. While in-plane polar orientation is stable in as-grown HZO, out-of-plane polarization can be dominant by applying electric field, which indicates the transition from in-plane polar to out-of-plane polar orientation in the ferroelectric phase grains. This is confirmed by calculating kinetic pathway using ab-initio calculation.

Improvement of endurance and switching speed in Hf1−x Zr x O2 thin films using a nanolaminate structure

To improve the endurance and polarization switching speed of Hf1−x Zr x O2 (HZO) ferroelectric films, we designed a 10 nm Hf0.5Zr0.5O2 + ZrO2 (HZZ) nanolaminate structure. Three films with different ZrO2 interlayers thicknesses were compared to find the optimal condition to implement the effect of the topological domain wall which was proposed recently. The HZZ film were deposited by repeatedly stacking ten HZO (∼0.92 nm) and six ZrO2 (∼0.53 nm) layers; they exhibited a dramatic reduction of coercive field without an effective loss of remnant polarization. The endurance at operation voltage increased by more than 100 times compared with that of the solid solution HZO film, and the switching speed was increased by more than two times. The formation of the tetragonal phase-like spacer between the ferroelectric polar regions appears to be the main factor associated with the reduction of the switching barrier and leads to the acceleration of the switching propagation over multiple domains.

Effect of thickness scaling on the switching dynamics of ferroelectric HfO2–ZrO2 capacitors

HfO2-based ferroelectric (FE) films have attracted immense attention in the research of ferroelectric random access memory (FeRAM) and ferroelectric field-effect transistors (FeFETs) due to their excellent ferroelectric performance and high compatibility with CMOS integration. The study of switching dynamics is essential in understanding and optimizing HfO2-based FE films for nonvolatile memory applications. To explore the effect of thickness scaling on the polarization switching speed of the HfO2-ZrO2 (HZO) FE film, we demonstrate TaN/HZO/TaN capacitors fabricated with 6, 10, and 20 nm-thick HZO films. The inhomogeneous field mechanism (IFM) model was used to characterize the polarization switching of polycrystalline HZO films. The results showed that the characteristic time constants for the 6, 10, and 20-nm-thick HZO samples were 283, 43.7, and 20.5 ps, respectively, which is inconsistent with the variation in coercive electric field (EC) with sample thickness. These results demonstrate the distribution of the local fields in capacitors with various HZO thicknesses and contribute to the design and optimization of HZO films for advanced nanonode applications.

Damage-Induced Ferroelectricity in HfOx-Based Thin Film

In the last decade, a surge in research on hafnium oxide (HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> )-based ferroelectricity has become a significant part of the semiconductor research trend. In this work, we present a new factor, that is, process damage to the HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> -based ferroelectric implementation. By reversely exploiting harmful damage in the metal-oxide-semiconductor process, the ferroelectricity in HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> is improved by eliciting early amorphization suitable for orthorhombic phase formation. In addition, through various pulsed polarization measurements, it is found that the damaged HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> film has a fast polarization switching speed, with an initial pinning site that is suitable for domain wall growth motion; advantageous for high-performance memory applications.

Preferential growth characteristics and ferroelectric properties of epitaxial SrBi2Nb2O9 thin films along the a-axis direction due to the misfit strains

Epitaxial SrBi2Nb2O9 (SBN) thin films were grown on single crystal (100) Nb-doped SrTiO3 (Nb:STO) substrates and (100) Rh substrates with different lattice constants, respectively. The Nb:STO substrates have a lattice constant similar to that of SBN and the epitaxial SBN thin films on the Nb:STO substrates were grown along the c-orientation, and the epitaxial SBN thin films on the Rh substrates were grown along a-orientation with a small lattice constant by compressive stress. The epitaxially a-oriented SBN thin films showed excellent ferroelectric properties compared to the epitaxially c-oriented SBN thin films because the SBN as a layered perovskite material has polar axis perpendicular to the c-orientation. It was confirmed that the epitaxially a-oriented SBN thin film having the polar axis parallel to the switching electric field has faster polarization switching speed than the epitaxially c-oriented SBN thin film having the polar axis perpendicular to the switching electric field.

Effects of Process-Induced Defects on Polarization Switching in Ferroelectric Tunneling Junction Memory

The scaling effects of ferroelectric tunnel junction (FTJ) memory are investigated by evaluating remanent polarization ( ${P} _{\mathrm {r}}$ ), coercive field ( ${E} _{\mathrm {c}}$ ), and polarization switching speed. Through the analysis on dc tunneling currents and frequency responses of ${E} _{\mathrm {c}}$ and ${P} _{\mathrm {r}}$ for FTJs with various sizes, it is found that polarization switching mechanism (domain nucleation limit) is not changed by the width reduction. However, by the length scaling, domain wall spreading-limited switching becomes dominant by process-induced damaged edge regions, leading to the larger ${P}_{\mathrm {r}}$ and the faster switching speed.

Perspective on the switching behavior of HfO2-based ferroelectrics

Since ferroelectricity was first observed in 2011, HfO2-based ferroelectrics have garnered significant attention, owing to their compatibility with complementary metal–oxide–semiconductors. Moreover, their thickness scalability facilitates the miniaturization of integrated circuit systems. The ultrafast polarization switching speed in the range of sub-nanoseconds helps in the fabrication of fast-operation devices. The origins of ferroelectricity in HfO2-based ferroelectrics differ from those of conventional perovskite ferroelectrics, with more complex behaviors associated with polarization switching. In this Perspective, recent investigations on the complex behaviors pertaining to polarization switching, including wake-up, split-up, fatigue, negative capacitance, accumulative switching, and some of their relations are discussed. Furthermore, the polarization switching dynamics have also been studied. Finally, the potential applications and investigations of HfO2-based ferroelectrics are discussed.

Fabrication of Nb nanopin electrodes and their influence on multiferroic properties of BiFeO3 thin films

We report enhanced multiferroic properties of BiFeO3 (BFO) thin films by depositing them on Nb nanopin bottom electrodes via spin-coating. The well-aligned Nb nanopin array was prepared from wet etching of an anodic aluminum oxide (AAO) nanotemplate. X-ray diffraction (XRD) measurement revealed that the BFO thin films were polycrystalline. The BFO thin films fabricated on Nb nanopin electrodes exhibited improved ferroelectricity with a remnant polarization (Pr) of 37.5 μC/cm2 and a coercive electric field (Ec) of 98 kV/cm, which was better than those (Pr ~ 32.9 μC/cm2 and Ec ~ 217 kV/cm) on the Nb film electrodes. We investigated leakage current, polarization switching speed as well as ferromagnetism, which also exhibited the improvement of BFO thin films on Nb nanopin electrodes. Significantly, we suggest that an enhanced electric field and surface-volume ratio of the Nb nanopin electrodes contribute to enhancement of the multiferroic properties of the BFO thin films.

Ultrafast measurements of polarization switching dynamics on ferroelectric and anti-ferroelectric hafnium zirconium oxide.

The ultrafast measurements of polarization switching dynamics on ferroelectric (FE) and antiferroelectric (AFE) hafnium zirconium oxide (HZO) are studied. The transient current during the polarization switching process is probed directly on the nanosecond scale. The switching time is determined to be as fast as 10 ns to reach fully switched polarization with characteristic switching times of 5.4 ns for FE HZO and 4.5 ns for AFE HZO by the nucleation limited switching model. The limitation by the parasitic effect on capacitor charging is found to be critical in the correct and accurate measurements of intrinsic polarization switching speed of HZO.

Ferroelectric domain structure and polarization switching speed of highly (111)-oriented polycrystalline YMnO3 thin films on glass substrates

Polycrystalline YMnO3 (YMO) thin films were deposited on (111) Pt/Ta/glass substrates via pulsed laser deposition (PLD). The low PLD deposition rate made it possible to reduce the leakage currents of the YMO thin film due to the resulting high level of crystallinity. The YMO thin film exhibited a strong ferroelectric response, including a remnant polarization of 4.2μC/cm2. The piezoresponse force microscopy study revealed that the YMO thin films had a mosaic domain structure and a ferroelectric domain wall energy lower than that of PbTiO3 thin films. It is suggested that the fast switching speed of YMO capacitors, which is below 85ns, originates from this low domain wall energy.

Dependence of threshold thickness of crystallization and film morphology on film processing conditions in poly(vinylidene fluoride–trifluoroethylene) copolymer thin films

In spin-cast films of poly(vinylidene fluoride–trifluoroethylene) on metalized silicon substrate, there exists a threshold thickness of crystallization dth, below which the crystallinity drops precipitously. Due to the direct link between the crystallinity and functional properties in the polymer, there is a corresponding large change in the film ferroelectric properties, including the dielectric constant, the polarization level, and polarization switching speed, as the thickness is reduced to below dth. Detail microstructure studies show that this threshold thickness is controlled by the stable crystal lamellar size along the film thickness direction. By varying the film processing condition to reduce the crystal lamellar size in the thickness direction, dth can be reduced markedly. As a result, better ferroelectric responses were obtained in ultrathin films.

Effect of Interface and Crystallinity on the Ferroelectric Properties of Poly(Vinylidene Fluoride-Trifluoroethylene) Copolymer Thin Films

ABSTRACTFerroelectric polymer thin films are attractive for a wide range of applications such as MEMS, IR sensors, and memory devices. We present the results of a recent investigation on the thickness dependence of the ferroelectric properties of poly(vinylidene fluoridetrifluoroethylene) copolymer spin cast films on electroded Si substrate. We show that as the film thickness is reduced, there exist two thickness regions. For films at thickness above 100 nm, the thickness dependence of the ferroelectric properties can be attributed to the interface effect. However, for thinner films, there is a large change in the ferroelectric properties such as the polarization level, the coercive field, and polarization switching speed, which is related to the large drop of the crystallinity in the ultrathin film region (below 100 nm). The results from Xray, dielectric measurement, and AFM all indicate that there is a threshold thickness at about 100 nm below which the crystallinity in the film reduces abruptly.