Nonvolatile Ferroelectric Memory Research Articles

Improving the Scalability of Ferroelectric FET Nonvolatile Memories With High-k Spacers

This paper investigates scaled ferroelectric field-effect transistor (FeFET) nonvolatile memories (NVMs) with high-k spacer device design considering ferroelectric-dielectric random phase variations with TCAD atomistic simulations. Our study indicates that, in addition to raising the orthorhombic phase and reducing the grain size of the ferroelectric, using high-k spacers can serve as another way to enhance the scalability of FeFETs because it improves both the mean memory window (MW) and the worst-case MW. More importantly, these improvements increase with the down-scaling of gate length. In addition, we have investigated the impact of high-k spacers on the critical electric field across interfacial layer (EIL) for the reliability of FeFET NVMs. Our study suggests that, for scaled FeFETs with high-k spacers, the highest EIL during write operation is no longer located near the S/D edge but at the mid channel. Using high-k spacers can reduce the mid-channel EIL, and the reduction increases with decreasing gate length due to the increasing impact of high-k spacers. Our study may provide insights for future high-density FeFET design.

High-performance ferroelectric nonvolatile memory based on Gd-and Ni-codoped BiFeO3 films

BiFeO3 (BFO), Bi0.92Gd0.08FeO3 (BGFO) and Bi0.92Gd0.08Fe0.95Ni0.05O3 (BGFNO) films are epitaxially grown on 0.7 wt% Nb-SrTiO3 (NSTO) substrates. The strong ferroelectric property in BGFNO film is confirmed by piezoresponse force microscopy (PFM) and polarization versus voltage (P–V) measurement. It is also found that the Au/BGFNO/NSTO devices possess a ferroelectric resistance switching (RS) effect. Gd- and Ni-codoped BiFeO3 is found to strongly enhance the resistance on/off ratio. A resistance on/off ratio as large as 3 × 106 is achieved with an applied pulse voltage of −8 V and +4 V. In addition, the devices exhibit excellent retention and anti-fatigue characteristics. The memristor behavior of Au/BGFNO/NSTO is attributed to the switching of polarization states, which modulate the width and height of the barrier at the BGFNO/NSTO interface. The excellent resistive switching properties in Au/BGFNO/NSTO devices indicate the promising application in nonvolatile memory.

Excellent ferroelectric Hf0.5Zr0.5O2 thin films with ultra-thin Al2O3 serving as capping layer

Hf0.5Zr0.5O2 (HZO) has become one of the most popular HfO2 based ferroelectric thin films due to its huge potential to integrate low-cost high-density nonvolatile ferroelectric memory. Most researchers sandwiched the HZO between metals, such as TiN, and then adopted post-deposition high temperature anneal to improve the ferroelectricity and reliability of the film. In this work, the effect of a thin dielectric Al2O3 layer with different thicknesses to replace a metallic capping layer on the ferroelectric properties of a HZO (10 nm) thin film is evaluated, and we also compared the effects of TiN and W bottom electrodes on the properties of a capacitor. The results showed that the TiN/Al2O3 (1 nm)/HZO/W capacitor performed the best with a maximum 2Pr as high as 31.4 μC/cm2 at an electric field of ±3 MV/cm and very low leakage currents. In addition, the fatigue studies demonstrated the capacitor's excellent endurance properties with continuous cycling up to 1010 cycles. The use of an ultra-thin Al2O3 layer with excellent capping effects would significantly simplify the integration process of HfO2-based ferroelectric memory.

Stabilization of thick, rhombohedral Hf0.5Zr0.5O2 epilayer on c-plane ZnO

Metastable rhombohedral hafnia-based ferroelectric films are emerging as a promising candidate in ferroelectric nonvolatile memory technologies, but the limited critical thickness impedes their applications. Herein, a 35-nm-thick rhombohedral Hf0.5Zr0.5O2 epilayer was stabilized on ZnO(0001) under an oxygen-deficient condition. Domain matching epitaxy, which facilitates the accommodation of misfit strain, allows the epitaxial growth of the (111)-oriented rhombohedral Hf0.5Zr0.5O2 film. We propose that a strong symmetry constraint is imposed on the epilayer at the initial epitaxial growth stage, i.e., the plane adjacent to ZnO(0001) should have a threefold symmetry. Although the bulk monoclinic phase is much more stable than the rhombohedral phase, our first principles calculations reveal that these two phases are energetically comparable with each other when this symmetry constraint is considered. Moreover, our results show that the incorporation of doubly charged oxygen vacancies is also powerful in shifting the energy balance between competing phases, making the metastable rhombohedral phase more stable.

Controlled manipulation of conductive ferroelectric domain walls and nanoscale domains in BiFeO3 thin films

Recently, there is a surge of research interest in configurable ferroelectric conductive domain walls which have been considered as possible fundamental building blocks for future electronic devices. In this work, by using piezoresponse force microscopy and conductive atomic force microscopy, we demonstrated the controlled manipulation of various conductive domain walls in epitaxial BiFeO3 thin films, e.g. neutral domain walls (NDW) and charged domain walls (CDWs). More interestingly, a specific type of nanoscale domains was also identified, which are surrounded by highly conductive circular CWDs. Similar nanoscale domains can also be controlled created and erasured by applying local field via conductive probe, which allow nondestructive current readout of different domain states with a large on/off resistance ratio up to 102. The results indicate the potential to design and develop high-density non-volatile ferroelectric memories by utilizing these programable conductive nanoscale domain walls.

Current change due to artificial patterning of the number of ferroelectric domain walls and nonvolatile memory characteristics

Domain walls (DWs) are formed at the boundaries between domains formed in a ferroelectric, and experimental results have been reported on the phenomenon of electrical conductivity in the DW. DW conduction nonvolatile memory applications are possible by forming and removing DWs with the high DW conductivity (DWC). Here, we investigated two-electrode devices and three-electrode DWC nonvolatile devices with current–voltage curves that change according to the number of DWs. When the number of DWs formed in the epitaxial PbTiO3 thin film was changed to 0, 2, and 4, the resistance of DWC was observed to decrease in the two-electrode device. For a three-electrode DWC nonvolatile memory having three electrodes with a structure similar to that of a flash memory structure, the slope of the source-drain current–voltage curve was adjusted by the gate electrode, and showed nonvolatile characteristics that can replace the flash memory.

High Speed and Large Memory Window Ferroelectric HfZrO₂ FinFET for High-Density Nonvolatile Memory

We fabricated the HfZrO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (HZO) ferroelectric fin field-effect transistors (Fe-FinFET) with fin width of 60 nm and gate length of 100 nm for ferroelectric nonvolatile memory operations. The fabricated Fe-FinFET exhibited a large memory window (MW) of 1.5 V and high (100 ns) program/erase speeds at ±5 V. After 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> program/erase cycles, the MW was maintained at 1.09 V and the retention time was measured up to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> s with no degradation. The fabricated HZO Fe-FinFET is compatible with the current FinFET process and has a high MW, a fast program/erase speed, and excellent reliability. Therefore, the fabricated Fe-FinFET is a promising candidate for high-density ferroelectric field-effect transistor memory applications.

Metal (Pt)/ferroelectric (SrBi2Ta2O9)/insulator (La2O3)/semiconductor (Si), MFIS structures for nonvolatile memory applications

Extended data retention is a cardinal impediment for ferroelectric memories and serves a pivotal role for nonvolatile memory applications. Here, nonvolatile Metal–Ferroelectric–Insulator–Semiconductor (MFIS) structures are fabricated by using thin films of Strontium Bismuth Tantalum Oxide (SrBi2Ta2O9) as ferroelectric and high-κ lanthanum oxide (La2O3) as a buffer insulator on p-Si substrates via RF magnetron sputtering. The grazing incidence x-ray diffraction analysis confirms the dominant (111) and (115) ferroelectric perovskite phases of SrBi2Ta2O9 thin films. Albeit, atomic force microscopy surface micrographs revealed highly smooth La2O3 and SBT (SrBi2Ta2O9) thin films with a surface roughness of ∼0.22 ± 0.04 nm and ∼1.05 ± 0.03 nm, respectively. Capacitance–voltage (C–V), capacitance–time (C–T), and current–voltage (I–V) characteristics of Pt/SrBi2Ta2O9/La2O3/Si, MFIS structures, exhibited a high memory window of ∼1.1 V at ±5 V sweep voltage, data retention measured until ∼104 s even on the extrapolation up to 10 years, and a low leakage current density of ∼12.8 μA/cm2 at −1 V and 300 K. Far from it, the probed conduction mechanism is studied for Pt/SrBi2Ta2O9/La2O3/Si MFIS device structures. The optimum nonvolatile memory characteristics are attributed to the high-quality SBT ferroelectric and the buffer layer La2O3/Si interface of the investigated MFIS structure and also assert from the control Pt/SBT/Pt and Pt/La2O3/Si results. Thus, the proposed Pt/SrBi2Ta2O9/La2O3/Si structure is a potential candidate for a gate stack of one-transistor (1T) type Ferroelectric Field-Effect Transistors nonvolatile memory applications.

Nanoscale bubble domains with polar topologies in bulk ferroelectrics

Multitudinous topological configurations spawn oases of many physical properties and phenomena in condensed-matter physics. Nano-sized ferroelectric bubble domains with various polar topologies (e.g., vortices, skyrmions) achieved in ferroelectric films present great potential for valuable physical properties. However, experimentally manipulating bubble domains has remained elusive especially in the bulk form. Here, in any bulk material, we achieve self-confined bubble domains with multiple polar topologies in bulk Bi0.5Na0.5TiO3 ferroelectrics, especially skyrmions, as validated by direct Z-contrast imaging. This phenomenon is driven by the interplay of bulk, elastic and electrostatic energies of coexisting modulated phases with strong and weak spontaneous polarizations. We demonstrate reversable and tip-voltage magnitude/time-dependent donut-like domain morphology evolution towards continuously and reversibly modulated high-density nonvolatile ferroelectric memories.

Determination of Hafnium Zirconium Oxide Interfacial Band Alignments Using Internal Photoemission Spectroscopy and X-ray Photoelectron Spectroscopy.

Doped ferroelectric HfO2 is highly promising for integration into complementary metal-oxide semiconductor (CMOS) technology for devices such as ferroelectric nonvolatile memory and low-power field-effect transistors (FETs). We report the direct measurement of the energy barriers between various metal electrodes (Pt, Au, Ta, TaN, Ti/Pt, Ni, Al) and hafnium zirconium oxide (Hf0.58Zr0.42O2, HZO) using internal photoemission (IPE) spectroscopy. Results are compared with valence band offsets determined using the three-sample X-ray photoelectron spectroscopy (XPS) as well as the two-sample hard X-ray photoelectron spectroscopy (HAXPES) techniques. Both XPS and IPE indicate roughly the same dependence of the HZO barrier on metal work function with a slope of 0.8 ± 0.5. XPS and HAXPES-derived barrier heights are on average about 1.1 eV smaller than barrier heights determined by IPE, suggesting the presence of negative charge in the HZO.

Band Excitation Piezoresponse Force Microscopy Adapted for Weak Ferroelectrics: On-the-Fly Tuning of the Central Band Frequency.

New interest in microscopic studies of ferroelectric materials with low piezoelectric coefficient, $d_{33}^\ast$, has emerged after the discovery of ferroelectric properties in HfO2 thin films, which are the main candidate for the next generation of nonvolatile ferroelectric memory. The study of the microscopic structure of ferroelectric HfO2 capacitors is crucial to get insights into the device behavior and performance. However, a small $d_{33}^\ast$ of ferroelectric HfO2 films leads to a low piezoresponse, especially in band excitation piezoresponse force microscopy (BE-PFM). In this work, we have implemented the BE-PFM technique with an increased scanning rate, thus improving this versatile tool for weak ferroelectrics. The acceleration of measurement was achieved by focusing excitation into a narrow frequency band and tuning the central frequency on-the-fly using an online real-time model estimation by fitting a complex BE response. The tracking of the contact resonance frequency was implemented using a pure mechanical cantilever response acquired in BE atomic force acoustic microscopy. To obtain optimal excitation parameters, we perform statistical analysis by minimizing estimator variance. The measurement precision of several PFM techniques was compared both by the simulation and experimentally using a Hf0.5Zr0.5O2-based ferroelectric capacitor.

High-performance polymer semiconductor-based ferroelectric transistor nonvolatile memory with a self-organized ferroelectric/dielectric gate insulator

Ferroelectric organic field-effect transistor nonvolatile memories (Fe-OFET-NVMs) offer attractive features for future memory applications, such as flexible and wearable electronics. Polymer semiconductor-based top-gate Fe-OFET-NVMs possess natural advantages in the device structure and processing manufacturing, compared to small-molecule semiconductor-based bottom-gate Fe-OFET-NVMs. However, their performances, such as mobility and operating voltages, should be further improved to be comparable to those of the latter. In this Letter, we develop a route to achieve high-performance top-gate Fe-OFET-NVMs, by employing a polymer semiconductor channel and self-organized ferroelectric/dielectric gate insulators, which were processed by a solution spin-coating technique. The optimal Fe-OFET-NVM exhibits a high mobility of 1.96 cm2/V s on average, a reliable endurance over 400 cycles, a stable retention capability over 6 × 104 s, and a life more than one year. Furthermore, the operating voltage of the Fe-OFET-NVM is reduced to ±20 V by scaling down the thickness of the ferroelectric/dielectric gate insulator. The whole performances of our memories are comparable to or better than those of the previous Fe-OFET-NVMs.

Domain modulation in LiNbO3 films using litho piezoresponse force microscopy

Domain engineering plays a pivotal role in the development of ferroelectric non-volatile memory devices. In this work, we mainly focus on the domain kinetic in ion-sliced single crystal LiNbO3 thin films under tip-induced electric fields using piezoresponse force microscope (PFM). Polarization reversal takes place when the electric fields are above threshold value (coercive voltage V c) of films. Besides, the dependence of domain dynamic on pulse duration and amplitude were investigated in detail, and specific local domain reversal (5 μm) was completed by the optimized poling condition. All the results reveal that tip-induced polarization reversal could be an effective way to domain engineering, which gives much more promising prospects regarding to the high density non-volatile ferroelectric memory devices.

Defects in ferroelectric HfO2.

The discovery of ferroelectricity in polycrystalline thin films of doped HfO2 has reignited the expectations of developing competitive ferroelectric non-volatile memory devices. To date, it is widely accepted that the performance of HfO2-based ferroelectric devices during their life cycle is critically dependent on the presence of point defects as well as structural phase polymorphism, which mainly originates from defects either. The purpose of this review article is to overview the impact of defects in ferroelectric HfO2 on its functional properties and the resulting performance of memory devices. Starting from the brief summary of defects in classical perovskite ferroelectrics, we then introduce the known types of point defects in dielectric HfO2 thin films. Further, we discuss main analytical techniques used to characterize the concentration and distribution of defects in doped ferroelectric HfO2 thin films as well as at their interfaces with electrodes. The main part of the review is devoted to the recent experimental studies reporting the impact of defects in ferroelectric HfO2 structures on the performance of different memory devices. We end up with the summary and perspectives of HfO2-based ferroelectric competitive non-volatile memory devices.

Erasable Ferroelectric Domain Wall Diodes* *Supported by the National Key Basic Research Program of China (Grant No. 2019YFA0308500), the Basic Research Project of Shanghai Science and Technology Innovation Action (Grant No. 17JC1400300), and the National Natural Science Foundation of China (Grant Nos. 61674044 and 61904034).

The unipolar diode-like domain wall currents in LiNbO3 single-crystal nanodevices are not only attractive in terms of their applications in nonvolatile ferroelectric domain wall memory, but also useful in half-wave and full-wave rectifier systems, as well as detector, power protection, and steady voltage circuits. Unlike traditional diodes, where the rectification functionality arises from the contact between n-type and p-type conductors, which are unchanged after off-line production, ferroelectric domain wall diodes can be reversibly created, erased, positioned, and shaped, using electric fields. We demonstrate such functionality using ferroelectric mesa-like cells, formed at the surface of an insulating X-cut LiNbO3 single crystal. Under the application of an in-plane electric field above a coercive field along the polar Z axis, the domain within the cell is reversed to be antiparallel to the unswitched bottom domain via the formation of a conducting domain wall. The wall current was rectified using two interfacial volatile domains in contact with two side Pt electrodes. Unlike the nonvolatile inner domain wall, the interfacial domain walls disappear to turn off the wall current path after the removal of the applied electric field, or under a negative applied voltage, due to the built-in interfacial imprint fields. These novel devices have the potential to facilitate the random definition of diode-like elements in modern large-scale integrated circuits.

Interfacial Regulation of Dielectric Properties in Ferroelectric Hf0.5Zr0.5O2 Thin Films

The discovery of ferroelectricity in hafnium zirconium oxide (HZO) thin films has attracted wide attention from academia to industry due to the application in ferroelectric non-volatile random access memories (FeRAM) with prominent performance in scalability and CMOS process compatibility. Dielectric behavior of ferroelectric HZO thin films is a key factor affecting the dynamic effect, piezoelectric and electrostrictive effect. Interface between HZO and capping electrodes plays an important role in regulating the dielectric properties. In this paper, the impedance frequency response and dielectric spectrum of ferroelectric HZO thin films were analyzed. Parameters of the interface were extracted to analyze the regulating effect on the dielectric properties based on an impedance model with constant phase element (CPE). Besides, dielectric spectrums at elevated temperatures were identified to verify this analysis.

Large domain-wall current in BiFeO3 epitaxial thin films

Large domain wall conductivity in insulating ferroelectric thin films provides a new idea for non-destructive readout of domain information of a nonvolatile ferroelectric domain wall memory. However, there are several challenges hindering its development, including the excessive leakage current and insufficient on-state read current. A lot of experimental and theoretical data have referred to the accumulation of oxygen vacancies at the domain wall regions of ferroelectric thin films as the extrinsic contribution of the wall conduction. In this work, we found that the wall current varied with the angle between the applied electric field and the initial polarization, and that the wall current could be increased to 1.9 μA for a bismuth ferrite thin-film device with the reduced oxygen vacancies in contrast to the previous value of 2 nA for the device with rich oxygen vacancies. The oxygen vacancy concentration can be controlled through the thermal annealing of the sample at two atmospheres of 5% hydrogen mixed with argon gas and pure oxygen gas. For the former, the enhanced oxygen vacancies within the annealed sample at the reduced atmosphere decreased the domain wall current by over a few orders of magnitude, in contrast to the significant enhancement of the wall current for the sample in the latter annealed at the oxidizing atmosphere. The change of oxygen vacancy concentration was implied from the Fe2+⇔Fe3+ transition by the X-ray photoelectron spectroscopy analysis. This change confirmed the rigid domain wall conduction mechanism as well as the extrinsic adverse defect compensation of the wall current, paving the way to the exploration of high-power domain-wall nanodevices with sufficient output currents.

Ferroelectric control of electron half-metallicity in A -type antiferromagnets and its application to nonvolatile memory devices

Two-dimensional (2D) $A$-type antiferromagnetic van der Waals (vdW) materials with either intralayer ferromagnetic and/or intralayer antiferromagnetic coupling have sparked great interest due to their potential applications in spintronics. By first-principles design we predict that a heterostructure formed by sandwiching the 2D $A$-type antiferromagnet, $2\mathrm{H}\text{\ensuremath{-}}\mathrm{V}{\mathrm{Se}}_{2}$, between the 2D out-of-plane ferroelectric, ${\mathrm{Sc}}_{2}{\mathrm{CO}}_{2}$, presents a semimetal band structure with a half-metallic conduction band. The mechanism giving rise to this peculiar structure cooperatively originates from the strong built-in electric field of the double-layer ferroelectric ${\mathrm{Sc}}_{2}{\mathrm{CO}}_{2}$, and from the charge transfer selectively occurring only at one interface. Based on the so-designed ${\mathrm{Sc}}_{2}{\mathrm{CO}}_{2}\text{/}\mathrm{V}{\mathrm{Se}}_{2}$ multiferroic heterostructure, two concepts for nonvolatile memory devices are proposed, in which two states (``1'' and ``0'') are realized by switching the polarization direction of the ferroelectric layers. These results demonstrate that the combination of 2D ferroelectrics and $A$-type antiferromagnetic vdW materials provides not only a fascinating way to achieve half-metallicity in 2D materials, but also a route to the design of new types of nonvolatile ferroelectric memory devices.

Simulations of single event effects in 6T2C-based ferroelectric non-volatile static random access memory

The single event effects (SEEs) on ferroelectric Hf0.5Zr0.5O2 capacitor-based non-volatile static random access memory (nvSRAM) were investigated by simulation. A nvSRAM cell integrated with two ferroelectric Hf0.5Zr0.5O2 capacitors is proposed in this study. A macro-model of the ferroelectric Hf0.5Zr0.5O2 capacitor, extracted from the real fabricated devices, is utilized for simulation analysis. Fundamental store and recall operations of the proposed nvSRAM design have been demonstrated. An independent double exponential current source was utilized and injected into specific circuit nodes to simulate the heavy ion induced single event transient current. The simulation results show that the transient pulse current is possible to upset the logic state of the memory cell from 1 to 0, but whether it can recover in a short time period after the upset errors depends on the exact value of linear energy transfer for the injected particles. In addition, increasing the remnant polarization (P r) and decreasing the coercive voltage (V c) and film thickness of ferroelectric capacitors can mitigate the influence of SEEs, which provides guidance for process hardening techniques aiming at space applications.

Charge-Ferroelectric Transition in Ultrathin Na0.5 Bi4.5 Ti4 O15 Flakes Probed via a Dual-Gated Full van der Waals Transistor.

Ferroelectric field-effect transistors (FeFETs) have recently attracted enormous attention owing to their applications in nonvolatile memories and low-power logic electronics. However, the current mainstream thin-film-based ferroelectrics lack good compatibility with the emergent 2D van der Waals (vdW) heterostructures. In this work, the synthesis of thin ferroelectric Na0.5 Bi4.5 Ti4 O15 (NBIT) flakes by a molten-salt method is reported. With a dry-transferred NBIT flake serving as the top-gate dielectric, dual-gate molybdenum disulfide (MoS2 ) FeFETs are fabricated in a full vdW stacking structure. Barrier-free graphene contacts allow the investigation of intrinsic carrier transport of MoS2 governed by lattice scattering. Thanks to the high dielectric constant of ≈94 in NBIT, a metal to insulator transition with a high electron concentration of 3.0 × 1013 cm-2 is achieved in MoS2 under top-gate modulation. The electron field-effect mobility as high as 182 cm2 V-1 s-1 at 88 K is obtained. The as-fabricated MoS2 FeFET exhibits clockwise hysteresis transfer curves that originate from charge trapping/release with either top-gate or back-gate modulation. Interestingly, hysteresis behavior can be controlled from clockwise to counterclockwise using dual-gate. A multifunctional device utilizing this unique property of NBIT, which is switchable electrostatically between short-term memory and nonvolatile ferroelectric memory, is envisaged.