Nonvolatile Ferroelectric Memory Research Articles

LiNbO3-based ferroelectric tunnel junctions with changeable electroresistance for data storage

Ferroelectric tunnel junctions (FTJs) have attracted considerable attention for potential applications in the next-generation data storage technologies. In this work, we have grown high-quality LiNbO3 single crystal films on STO (111) substrates by rotational epitaxy. The certain ferroelectricity was achieved in nanoscale epitaxial LiNbO3 films. Then, the Au/LiNbO3/Nb: SrTiO3 FTJ was fabricated, and the nonvolatile resistive switching controlled by the nonvolatile polarization switching was observed. The Au/LiNbO3/Nb: SrTiO3 FTJs regulate the quantum tunneling effect through ferroelectric polarization reversal to obtain multi-level resistive states, thereby achieving data storage functionality. At room temperature, the ON/OFF current ratio can exceed 103. Furthermore, the FTJs also exhibit excellent retention for more than 103 s and good switching endurance for 2000 cycles. The results suggest the application potential of this LiNbO3-based FTJ for next generation nonvolatile ferroelectric memories.

Electrical conduction of ferroelectric domains and domain walls in polycrystalline BiFeO3 and Bi5Ti3FeO15 thin films

The unique properties of domain wall conductivity have garnered significant interest for their potential application in non-volatile ferroelectric domain wall memory. In this study, we investigated the electrical conduction within ferroelectric domains and domain walls of polycrystalline BiFeO3 (BFO) and Bi5Ti3FeO15 (BTFO) thin films, which were deposited on Pt/Ta/glass substrates via pulsed laser deposition. BFO thin film consistently demonstrated a (111) orientation, while BTFO thin film exhibited mixed crystallinity, featuring both c-axis and a-axis orientations. This mixed crystallinity in BTFO thin film contributed to a higher remanent polarization of 38.2 μC/cm2 compared to 20.3 μC/cm2 in BFO thin film, which is attributed to the a-oriented crystallinity within the Bi-layered perovskite structure of BTFO thin film. Additionally, BTFO thin film displayed a greater prevalence of 90° domain walls, which enhanced electrical conduction due to charge accumulation, particularly when compared to 180° domain walls. A significant change in resistance was observed when the domain wall was present versus absent, with a more pronounced effect in the BTFO capacitor compared to the BFO capacitor. This is attributed to the higher domain wall conductivity in BTFO thin film, confirming their superiority for use in ferroelectric capacitor devices that leverage domain wall conductivity.

A scalable ferroelectric non-volatile memory operating at 600 °C

A scalable ferroelectric non-volatile memory operating at 600 °C

Ferrielectricity controlled widely-tunable magnetoelectric coupling in van der Waals multiferroics

The discovery of various primary ferroic phases in atomically-thin van der Waals crystals have created a new two-dimensional wonderland for exploring and manipulating exotic quantum phases. It may also bring technical breakthroughs in device applications, as evident by prototypical functionalities of giant tunneling magnetoresistance, gate-tunable ferromagnetism and non-volatile ferroelectric memory etc. However, two-dimensional multiferroics with effective magnetoelectric coupling, which ultimately decides the future of multiferroic-based information technology, has not been realized yet. Here, we show that an unconventional magnetoelectric coupling mechanism interlocked with heterogeneous ferrielectric transitions emerges at the two-dimensional limit in van der Waals multiferroic CuCrP2S6 with inherent antiferromagnetism and antiferroelectricity. Distinct from the homogeneous antiferroelectric bulk, thin-layer CuCrP2S6 under external electric field makes layer-dependent heterogeneous ferrielectric transitions, minimizing the depolarization effect introduced by the rearrangements of Cu+ ions within the ferromagnetic van der Waals cages of CrS6 and P2S6 octahedrons. The resulting ferrielectric phases are characterized by substantially reduced interlayer magnetic coupling energy of nearly 50% with a moderate electric field of 0.3 V nm−1, producing widely-tunable magnetoelectric coupling which can be further engineered by asymmetrical electrode work functions.

Nonlocal erasing and writing of ferroelectric domains using a femtosecond laser in lithium niobate.

We experimentally demonstrate the highly-efficient nonlocal erasing and writing of ferroelectric domains using a femtosecond laser in lithium niobate. Based on the induction of a focused infrared femtosecond laser without any relative displacement or additional treatment, the original multiple ferroelectric domains can be either erased (erasing operation) or elongated (writing operation) simultaneously in the crystal, depending on the laser focusing depth and the laser pulse energy. In the erasing operation, the original multiple ferroelectric domains can be cleared completely by just one laser induction, while in the writing operation, the average length of the ferroelectric domains can be elongated up to 235 µm by three laser inductions. A model has been proposed in which a thermoelectric field and a space charge field are used cooperatively to successfully explain the mechanism of nonlocal erasing and writing. This method greatly improves the efficiency and flexibility of tailoring ferroelectric domain structures, paving the way to large-scale all-optical industrial production for nonlinear photonic crystals and nonvolatile ferroelectric domain wall memories.

Structural and resistive properties of Pt/BiFeO3/La0.5Sr0.5CoO3 heterostructure for storage application

BiFeO3 has promissing application in non-volatile ferroelectric memory, but the dominant mechanism has not been unified. Here, Pt/BiFeO3 (100 nm)/La0.5Sr0.5CoO3 (Pt/BFO/LSCO) heterostructures have been fabricated on (001) SrTiO3 (STO) single crystal substrates by off-axis magnetron sputtering. Both XRD and TEM analyses confirm that the (001) oriented BFO/LSCO heterostructures have been epitaxially grown on STO. In addition, XRD, TEM along with XPS analyses further indicate that the BFO film (001) presents epitaxial perovskite structure. On the basis of electric field regulation of Pt/BFO and BFO/LSCO interface barrier, the switching interface state is controlled by charge filling and trapping in interface potential well, which is responsible for I–V resistance and G-V hysteretic curve. Based on the fitting results of the conduction mechanism of I–V characteristic curve, it proves that the space charges limiting current play a dominant role, and the filling and trapping of interface traps are considered as the main resistance mechanisms. Moreover, ultraviolet irradiation can modulate the I–V resistance curve but does not change its conduction mechanism. This work provides experimental data for the practical application of integrated BFO resistive memories.

Epitaxial Strain Enhanced Ferroelectric Polarization toward a Giant Tunneling Electroresistance.

A substantial ferroelectric polarization is the key for designing high-performance ferroelectric nonvolatile memories. As a promising candidate system, the BaTiO3/La0.67Sr0.33MnO3 (BTO/LSMO) ferroelectric/ferromagnetic heterostructure has attracted a lot of attention thanks to the merits of high Curie temperature, large spin polarization, and low ferroelectric coercivity. Nevertheless, the BTO/LSMO heterostructure suffers from a moderate FE polarization, primarily due to the quick film-thickness-driven strain relaxation. In response to this challenge, we propose an approach for enhancing the FE properties of BTO films by using a Sr3Al2O6 (SAO) buffering layer to mitigate the interfacial strain relaxation. The continuously tunable strain allows us to illustrate the linear dependence of polarization on epitaxial strain with a large strain-sensitive coefficient of ∼27 μC/cm2 per percent strain. This results in a giant polarization of ∼80 μC/cm2 on the BTO/LSMO interface. Leveraging this large polarization, we achieved a giant tunneling electroresistance (TER) of ∼105 in SAO-buffered Pt/BTO/LSMO ferroelectric tunnel junctions (FTJs). Our research uncovers the fundamental interplay between strain, polarization magnitude, and device performance, such as on/off ratio, thereby advancing the potential of FTJs for next-generation information storage applications.

Ultra-High-Density Ferroelectric Array Formed by Sliding Ferroelectric Moiré Superlattices.

2D van der Waals (vdW) materials offer infinite possibilities for constructing unique ferroelectrics through simple layer stacking and rotation. In this work, we stack nonferroelectric GeS2 and ferroelectric CuInP2S6 to form heterostructures by combining sliding ferroelectric polarization with displacement ferroelectric polarization to achieve multiple polarization states. First-principles calculations reveal that the polarization reversal of the CuInP2S6 component in the GeS2/CuInP2S6/GeS2 heterostructure can simultaneously drive the switching of sliding ferroelectric polarization, displaying a robust coupling of the two polarizations and leading to the overall polarization switching. Based on this, ferroelectric arrays with a density of 6.55 × 1012 cm-2 (equivalent to a storage density of 0.7 TB cm-2) were constructed in a moiré superlattice, and the polarization strength of array elements was 11.77 pC/m, higher than that of all reported 2D vdW out-of-plane ferroelectrics. High density, large polarization, and electrically switchable array elements in ferroelectric arrays provide unprecedented opportunities to design 2D high-density nonvolatile ferroelectric memories.

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.

Microscopic insight into domain configuration in orthorhombic K0.52Na0.48NbO3 single crystals driven by electric-field

Domain switching not only exhibits a great effect on piezoelectric performance, but also serves as foundation of some ferroelectric applications, such as ferroelectric nonvolatile memories and nonlinear optical devices. Therefore, a thorough understanding in the process and mechanism of domain switching is of vital significance. In this paper, we observed the process of domain switching under electric field by controlling poling conditions. The domain structures, polarization orientation, and piezoelectric performance of K0.52Na0.48NbO3 single crystals poled at different temperatures and electric fields were investigated. We found that the polarization-rotation in poling process of the crystals occurs in steps and along a specific conversion path: [1¯01] - [011] - [1¯10]. Furthermore, poling under specific conditions can construct specific domains, which can be used in optical-function devices and may significantly promote the design of new generation ferroelectric memory devices.

Wake-Up Free Ultrathin Ferroelectric Hf0.5Zr0.5O2 Films.

The development of the new generation of non-volatile high-density ferroelectric memory requires the utilization of ultrathin ferroelectric films. The most promising candidates are polycrystalline-doped HfO2 films because of their perfect compatibility with silicon technology and excellent ferroelectric properties. However, the remanent polarization of HfO2 films is known to degrade when their thickness is reduced to a few nanometers. One of the reasons for this phenomenon is the wake-up effect, which is more pronounced in the thinner the film. For the ultrathin HfO2 films, it can be so long-lasting that degradation occurs even before the wake-up procedure is completed. In this work, an approach to suppress the wake-up in ultrathin Hf0.5Zr0.5O2 films is elucidated. By engineering internal built-in fields in an as-prepared structure, a stable ferroelectricity without a wake-up effect is induced in 4.5 nm thick Hf0.5Zr0.5O2 film. By analysis of the functional characteristics of ferroelectric structures with a different pattern of internal built-in fields and their comparison with the results of in situ piezoresponse force microscopy and synchrotron X-ray micro-diffraction, the important role of built-in fields in ferroelectricity of ultrathin Hf0.5Zr0.5O2 films as well as the origin of stable ferroelectric properties is revealed.

Effect of ZnSnO3 on dielectric and ferroelectric properties of Sr2Bi4Ti5O18 ceramics

AbstractSr2Bi4Ti5O18 is one of the promising candidates for ferroelectric nonvolatile random access memory device applications. Albeit, this ceramic material is not suitable for certain ferroelectric device applications because of the merits and demerits associated with this material. Solid‐solution or composite ceramics in bulk configuration is the better choice to overcome the drawbacks associated with the individual compounds. Concurrently, solid‐solution of functional electro‐ceramics comprising Aurivillius family‐based lead‐free bismuth‐layered structured ceramics (1 − x) Sr2Bi4Ti5O18 (SBT) and x mol% of ilmenite structured ZnSnO3 (ZS) (where x = 0, 5, 10, and 20 mol%) were synthesized via conventional solid‐state reaction route to investigate the effect of ZS on the physical properties of SBT. X‐ray diffraction diffractograms revealed that a single phase was formed maximum up to 5 mol% of ZS in SBT and scanning electron microscopy studies suggested platelike microstructure of the samples. Intriguingly, positive‐up and negative‐down results indicated that the true switchable polarization for the 10 mol% ZS in SBT (10.87 μC/cm2), 33% higher than that of pure SBT ceramics in addition to the lower leakage current density and enhanced dielectric response in the SBT–ZS samples. The observed enhancement in electrical characteristics of SBT–ZS composite ceramics compared to pure SBT highlights the decisive role played by the ZS in lowering the thermodynamic barrier resulting in ease of polarization switching and mitigating the leakage current of SBT ceramics.

Engineering Ferroelectricity and Large Piezoelectricity in h-BN.

Hexagonal boron nitride (h-BN) is a well-known layered van der Waals (vdW) material that exhibits no spontaneous electric polarization due to its centrosymmetric structure. Extensive density functional theory (DFT) calculations are used to demonstrate that doping through the substitution of B by isovalent Al and Ga breaks the inversion symmetry and induces local dipole moments along the c-axis, which promotes a ferroelectric (FE) alignment over antiferroelectric. For doping concentrations below 25%, a "protruded layered" structure in which the dopant atoms protrude out of the planar h-BN layers is energetically more stable than the flat layered structure of pristine h-BN or a wurtzite structure similar to w-AlN. The computed polarization, between 7.227 and 21.117 μC/cm2, depending on dopant concentration and the switching barrier (16.684 and 45.838 meV/atom) for the FE polarization reversal are comparable to that of other well-known FEs. Interestingly, doping of h-BN also induces a large negative piezoelectric response in otherwise nonpiezoelectric h-BN. For example, we compute d33 of -24.214 pC/N for Ga0.125B0.875N, which is about 5 times larger than that of pure w-AlN (5 pC/N), although the computed e33 (-1.164 C/m2) is about 1.6 times lower than that of pure w-AlN (1.462 C/m2). Because of the layered structure, the rather small elastic constant C33 provides the origin of the large d33. Moreover, doping makes h-BN an electric auxetic piezoelectric. We also show that ferroelectricity in doped h-BN may persist down to its trilayer, which indicates high potential for applications in FE nonvolatile memories.

Ferroelectricity in Epitaxial Perovskite Oxide Bi2WO6 Films with One-Unit-Cell Thickness.

Retaining ferroelectricity in ultrathin films or nanostructures is crucial for miniaturizing ferroelectric devices, but it is a challenging task due to intrinsic depolarization and size effects. In this study, we have shown that it is possible to stably maintain in-plane polarization in an extremely thin, one-unit-cell thick epitaxial Bi2WO6 film. The use of a perfectly lattice-matched NdGaO3 (110) substrate for the Bi2WO6 film minimizes strain and enhances stability. We attribute the residual polarization in this ultrathin film to the crystal stability of the Bi-O octahedral framework against structural distortions. Our findings suggest that ferroelectricity can surpass the critical thickness limit through proper strain engineering, and the Bi2WO6/NdGaO3 (110) system presents a potential platform for designing low-energy consumption, nonvolatile ferroelectric memories.

Perspectives on MXene-PZT based ferroelectric memristor in computation in memory applications

Lead zirconate titanate (PZT) is the promising candidate in advanced ferroelectric memory application due to its excellent piezoelectricity, ferroelectricity, pyroelectricity, non-linear dielectric behavior, multiferroic properties, high ferroelectric Curie temperature, and extremely strong stability. It has gained attention in the field beyond von-Neumann computing, which inspires the development of computation in memory applications. Various structures of the ferroelectric memristive device, including ferroelectric field effect transistor, tunnel junctions, nonvolatile memory, and capacitor, based on PZT have been proposed for the realization of computation in memory application. On the other hand, unique designs realize the performance enhancement of PZT ferroelectric memristive devices, i.e., the insertion of 2D material MXene. This perspective further points out some of the challenges that MXene-PZT based ferroelectric memristive devices encounter in reality and finally give our viewpoint on possible developments toward computation in memory in a neuromorphic platform.

Ferroelectric SnPz/In2Se3 as a Stable and Durable Non-Volatile 2D Ferroelectric Memory Material

In ferroelectric memory, the repeated application of external electric fields can cause ferroelectric fatigue, limiting its stability and service life, especially as the storage unit size decreases. To address this issue, we conducted first-principles research on a SnPz/In2Se3 structure and examined its structure under different polarization directions. Our analysis revealed significant differences in the adsorption position of Sn atoms depending on the polarization direction, suggesting that SnPz/In2Se3 could be a highly stable ferroelectric storage material. Moreover, the polarization-induced changes in the electronic structure near the Fermi level, which allowed for the use of tunneling current and obtaining stored information without causing the ferroelectric fatigue effect during information readout. These findings highlight the potential of SnPz/In2Se3 to significantly extend the lifespan of ferroelectric materials, reduce energy consumption, and minimize the environmental impact of discarded electronic devices.

Periodic Ferroelectric Stripe Domains in α-In2Se3 Nanoflakes Grown via Reverse-Flow Chemical Vapor Deposition.

The two-dimensional (2D) layered semiconductor α-In2Se3 has aroused great interest in atomic-scale ferroelectric transistors, artificial synapses, and nonvolatile memory devices due to its distinguished 2D ferroelectric properties. We have synthesized α-In2Se3 nanosheets with rare in-plane ferroelectric stripe domains at room temperature on mica substrates using a reverse flow chemical vapor deposition (RFCVD) method and optimized growth parameters. This stripe domain contrast is found to be strongly correlated to the stacking of layers, and the interrelated out-of-plane (OOP) and in-plane (IP) polarization can be manipulated by mapping the artificial domain structure. The acquisition of amplitude and phase hysteresis loops confirms the OOP polarization ferroelectric property. The emergence of striped domains enriches the variety of the ferroelectric structure types and novel properties of 2D In2Se3. This work paves a new way for the controllable growth of van der Waals ferroelectrics and facilitates the development of novel ferroelectric memory device applications.

Leakage mechanism in ferroelectric Hf0.5Zr0.5O2 epitaxial thin films

Fluorite-structured Hf0.5Zr0.5O2 (HZO) thin films have attracted considerable attention in recent years because of their good CMOS-compatibility and robust ferroelectricity down to a thickness of few unit-cell. The main challenges remaining to be overcome before HZO films will be the ideal candidates for next-generation, non-volatile ferroelectric memory devices are lowering its leakage current and improving its fatigue endurance. However, the leakage mechanism of the ferroelectric HZO thin films still remains elusive, which hinders the applicability of HZO in microelectronic devices. Herein, we studied the electric properties of Pt/HZO/La0.7Sr0.3MnO3 (LSMO) heterostructures grown on (001) SrTiO3 (STO) substrates. The leakage mechanism is found to be dominated by Schottky emission and the Schottky barrier heights are 0.48 eV and 0.58 eV for Pt/HZO and LSMO/HZO interfaces, respectively. In turn, by post-annealing in oxygen atmosphere, we are able to increase the barrier height by 0.1 eV and thus effectively reduce the leakage current and improve the endurance by an order of magnitude. Our studies help guide future work to integrate HZO thin films into microelectronic devices.

A Study on Imprint Behavior of Ferroelectric Hafnium Oxide Caused by High‐Temperature Annealing

Hafnium oxide is found to be a favorable material for ferroelectric nonvolatile memory devices. Its compatibility with complementary metal–oxide–semiconductor processes, the relatively low crystallization temperature when zirconium‐doped, and the thickness scaling are among the advantageous properties of hafnium oxide. Different requirements must be fulfilled for different applications of hafnium oxide. Herein, high‐temperature annealing and operation conditions are analyzed in order to investigate nonvolatile memories for automotive applications. A strong imprint behavior (shift in coercive voltages) is observed after annealing hafnium–zirconium–oxide thin films at temperatures varied between 100 and 200 °C. The imprint behavior is a significant challenge in many applications. Therefore, to reduce/recover the undesirable imprint behavior caused by high‐temperature treatment, two different ways are successfully examined and delineated here: endurance cycling and applying high electric fields.

Impacts of pulse conditions on endurance behavior of ferroelectric thin-film transistor non-volatile memory

In this work, the ferroelectric thin-film transistor (Fe-TFT) with polycrystalline-silicon (poly-Si) channel and HfZrO x gate dielectric is fabricated to study the characteristics of non-volatile memory (NVM). Significant threshold voltage (V TH) modulation can be achieved with low pulse voltages less than ±3.5 V and pulse widths within 1 μs. In order to achieve the NVM characteristics of low voltage and high speed operation, the impact of the program/erase (PRG/ERS) pulse voltage (V PRG/V ERS) and pulse width on endurance is a critical consideration. In the study of the pulse width effect on endurance, it can be observed that the V TH in PRG-state exhibits the wake-up effect at both short and long pulse widths. In addition, with the increase of pulse width, the V TH in the PRG-state exhibits significant fatigue effect and subthreshold swing (SS) degradation effect. For V TH in the ERS-state, the increase of the pulse width also exhibits the fatigue effect and the SS degradation effect, which is dominated by the SS degradation effect at long pulse widths. In the study of the pulse voltage effect on endurance, the increase of V PRG shows the imprint effect that the V TH in either PRG- or ERS-state reveals a decreasing trend. When the V ERS increases, the SS of the PRG- and ERS-states is degraded, and the fatigue effect of the PRG-state is enhanced. Moreover, the retention characteristics of poly-Si Fe-TFTs exhibit stable characteristics at both room temperature and 50 °C.