Redistribution Of Oxygen Vacancies Research Articles

Resetting the Drift of Oxygen Vacancies in Ultrathin HZO Ferroelectric Memories by Electrical Pulse Engineering

Ferroelectric HfO2‐based films incorporated in nonvolatile memory devices offer a low‐energy, high‐speed alternative to conventional memory systems. Oxygen vacancies have been rigorously cited in literature to be pivotal in stabilizing the polar noncentrosymmetric phase responsible for ferroelectricity in HfO2‐based films. Thus, the ability to regulate and control oxygen vacancy migration in operando in such materials would potentially offer step changing new functionalities, tunable electrical properties, and enhanced device lifespan. Herein, a novel in‐ operando approach to control both wake‐up and fatigue device dynamics is reported. Via clever design of short ad hoc square electrical pulses, both wake‐up can be sped up and both fatigue and leakage inside the film can be reduced, key factors for enhancing the performance of memory devices. Using plasmon‐enhanced photoluminescence and dark‐field spectroscopy (sensitive to <1% vacancy variation), evidence that the electrical pulses give rise to oxygen vacancy redistribution is provided and it is shown that pulse engineering effectively delays wake‐up and reduces fatigue characteristics of the HfO2‐based films. Comprehensive analysis also includes impedance spectroscopy measurements, which exclude any influence of polarization reversal or domain wall movement in interpretation of results.

In situ grazing incidence synchrotron x-ray diffraction studies on the wakeup effect in ferroelectric Hf0.5Zr0.5O2 thin films

The discovery of ferroelectricity in HfO2 ultrathin films has gathered considerable interest from the microelectronic industry owing to their compatibility with complementary metal oxide semiconductor technology. However, a significant challenge in utilizing HfO2 thin films for commercial devices is the wakeup effect, which is an increase in polarization with the number of electric field cycles in HfO2-based capacitors. Despite efforts to develop wakeup-free HfO2 thin films, the root cause of this effect remains elusive. Some studies attribute it to the tetragonal (T) to orthorhombic (O) phase transformation, while others suggest it is due to the redistribution of oxygen vacancies within the HfO2 layer during electric field cycling. This study investigated the phase transformation dynamics and oxygen vacancy distributions in TiN/Hf0.5Zr0.5O2/TiN capacitors subjected to electric field cycling using in situ grazing incidence synchrotron x-ray diffraction and ex situ x-ray photoelectron spectroscopy. The as-grown HZO films were crystallized in a mixed phase consisting of monoclinic (M), tetragonal, and orthorhombic fractions. The T-phase volume fraction decreased continuously up to 10k electric field cycles. In contrast, the O-phase fraction increased within the first 100 cycles and then stabilized with further cycling. The redistribution of oxygen vacancies occurred continuously throughout the cycling process. The results revealed that continuous oxygen vacancy redistribution during electric field cycling resulted in phase transformation between the T-, O-, and M-phases. The study provides insight into the phase transformation dynamics and oxygen vacancy redistribution in TiN/Hf0.5Zr0.5O2/TiN capacitors during electric field cycling, shedding light on the underlying mechanisms of the wakeup effect.

Oxygen vacancy redistribution and ferroelectric polarization relaxation on epitaxial perovskite films during an electrocatalytic process

Ferroelectric catalysis BaTiO3 films show negative ferroelectric polarization drives the oxygen vacancies redistribution to the surface accelerating the adsorption of reactants and charge transfer, resulting in an enhanced OER performance.

(Invited) Optically and Chemically Modulated Oxygen Transport across Interfaces

Materials that conduct oxygen are central to many technologies that promise more highly efficient chemical to electrical and electrical to chemical energy conversion, as exemplified by solid oxide fuel and electrolysis cells (SOFC/SOEC), permeation membranes for partial oxidation of hydrocarbons and solar thermal driven chemical conversions. We begin by describing recent insights achieved in controlling the oxygen reduction reaction (ORR) on mixed ionic electronic conductors (MIEC). We have been able to demonstrate the ability to reversibly depress or enhance the oxygen exchange reaction at MIEC surfaces by orders of magnitude by infiltrating the surfaces with binary oxides with controlled acidity. This we demonstrate can be correlated with the ability of the surface additives to deplete or accumulate electrons needed for the ORR. With these insights, we have been able to understand more fundamentally the source of chromia-induced poisoning of SOFC cathodes and the means to recover performance following poisoning. We next turn our attention to the well known blocking of oxygen ions at grain boundaries in solid oxide electrolytes where grain boundary resistances can drop the overall oxygen ion conductance in polycrystalline solid electrolytes by orders of magnitude. The barriers to oxygen transport across grain boundaries are largely attributed to space charge barriers that form at the grain boundary due to the positive core charge of the grain boundaries. We have demonstrated the ability to modulate the barrier potentials by illumination with above band gap light, illustrating the ability to enhance ionic conduction in solid electrolytes at reduced temperatures, while at the same time demonstrating a novel method for interrogating the distribution of barrier heights at grain boundaries and the dynamics associated with the resultant redistribution of oxygen vacancies in space charge regions under conditions of photo-induced electron-hole generation and recombination. This phenomenum is yet another example of the rapidly developing field of opto-ionics.

Significant Degradation Reduction in Metal Oxide Thin-Film Transistors via the Interaction of Ionized Oxygen Vacancy Redistribution, Self-Heating Effect, and Hot Carrier Effect

In this work, device degradation of metal oxide (MO) thin-film transistors (TFTs) under positive bias stress, linear stress, and saturation stress is systematically investigated. A significant degradation reduction in InGaZnO TFTs is observed for the first time under saturation stress. Incorporated with electric field simulations and temperature simulations, a degradation model, considering the interaction of ionized oxygen vacancy redistribution, self-heating effect, and hot carrier effect, is tentatively proposed to understand the degradation behaviors under various stress combinations. Moreover, a similar degradation reduction phenomenon is also reproduced in InSnZnO TFTs, demonstrating the applicability of the proposed degradation model in MO TFTs.

Tunable voltage polarity-dependent resistive switching characteristics by interface energy barrier modulation in ceria-based bilayer memristors for neuromorphic computing

Synaptic characteristics with tunable dependence on the voltage polarity are demonstrated in ceria (CeO2) and Gd-doped ceria (GDC) bilayer memristors with respect to their stacking orders. Both Pt/GDC/CeO2/Pt and Pt/CeO2/GDC/Pt memristors with different oxide stacking orders exhibit analog, linear, and symmetric synaptic weights for potentiation and depression, paired-pulse facilitation, and short- and long-term plasticity (STP and LTP, respectively). Potentiation and depression behaviors are highly linear and symmetric thanks to the stacking of more oxygen-deficient GDC layer with CeO2, which facilitates the redistribution of oxygen vacancies for analog resistance change. These memristors have opposite dependence of synaptic weight updates on the polarity of potentiation and depression voltages for the stacking order of CeO2 and GDC, which are consistently interpreted by voltage-driven energy barrier modulation at the interface between CeO2 and GDC or with Pt electrodes via oxygen vacancy redistribution. Recognition simulation with modified handwritten digits patterns using a two-layer perceptron neural network exhibits accuracy of approximately 88% with achieved dynamic range, linearity, symmetry, and precision states. These synaptic characteristics are also demonstrated in a 32 × 32 crossbar array of GDC-top memristors. This verifies the potential of bilayer memristors for application in artificial synapse networks in neuromorphic computing systems.

Effects of temperature and DC cycling stress on resistive switching mechanisms in hafnia-based ferroelectric tunnel junction

We propose an accurate and effective method, low-frequency noise (LFN) spectroscopy, to examine the resistive switching mechanism in ferroelectric tunnel junctions (FTJs) based on pure hafnium oxide (HfOx). Contrary to previous studies that primarily focused on the ferroelectric (FE) resistive switching (RS) in HfOx-based FTJs, the results of this study demonstrate that non-FE RS affected by the redistribution of oxygen vacancies also plays a significant role in determining the performance of FTJs. LFN spectroscopy is conducted in different conditions by changing the operating temperature and inducing DC cycling stress. The results reveal that the RS mechanism changes from FE to non-FE RS with increased program bias in all conditions. This change is facilitated by the rise in temperature and the number of DC cycling stress.

High‐Throughput Screening Thickness‐Dependent Resistive Switching in SrTiO3 Thin Films for Robust Electronic Synapse

AbstractThe functionalities and applications of oxide thin films are highly dependent on their thickness. Most thickness‐dependent studies on oxide thin films require the preparation of independent samples, which is labor‐intensive and time‐consuming and inevitably introduces experimental errors. To address this challenge, a general strategy based on high‐throughput pulsed laser deposition technology is proposed to precisely control the thin‐film thickness in local regions under similar growth conditions. The as‐proposed synthesis strategy is demonstrated using typical complex oxide materials of SrTiO3 (STO). Consequently, high‐throughput STO thin films with nine gradient thicknesses ranging from 10.1 to 30.5 nm are fabricated. Notably, a transition from the unipolar to the bipolar resistive switching mode is observed with increasing STO thickness. Moreover, a physical mechanism based on the heterostructure‐mediated redistribution of oxygen vacancies is employed to interpret the transition between the two memristive patterns. The screening of STO thin films with different resistive switching behaviors revealed that the STO thin film with a thickness of 20.3 nm exhibit excellent conductance modulation properties under the application of electrical pulses as well as significant reliability for the emulation of various synaptic functions, rendering it a promising material for artificial neuromorphic computing applications.

Bipolar Resistive Switching Behaviour of Polycrystalline BiFeO3 Thin Films Synthesized via Sol-gel Assisted Spin Coating Technique

Polycrystalline BiFeO3 thin films have been grown on glass substrates using a simple but efficient method commonly known as the spin coating technique. When used in a Cu / BiFeO3 / Cu configuration, the annealed BiFeO3 film (at 350 °C) exhibits bipolar resistive switching behaviour. The device shows stable resistive switching behaviour, where a stable hysteresis in the current–voltage curve was well developed by applying +/- 10 V at room temperature. The ratio of resistance in the high resistance state to the low resistance state of the device is ~ 104 with a good retention time of more than 106 min. The Poole–Frenkel emission at the Cu / BiFeO3 interface is proposed, and a redistribution of oxygen vacancies along the grain boundaries is found to play a key role in the resistance switching in the polycrystalline pure BiFeO3 films.

Unveiling Resistance Switching Mechanisms in Undoped HfOx Ferroelectric Tunnel Junction Using Low-Frequency Noise Spectroscopy

We demonstrate that the resistance switching (RS) of an undoped hafnium oxide (HfOx)-based ferroelectric tunnel junction (FTJ) is affected not only by ferroelectric domain switching of HfOx but also by the redistribution of oxygen vacancies inside HfOx, known as the working principle of resistive random-access memory. It is revealed that the RS mechanism varies depending on the program bias applied to FTJ. Through low-frequency noise spectroscopy, a precise method for distinguishing two distinct RS processes intrinsic to FTJ is presented.

Investigation of Recovery Phenomena in Hf0.5Zr0.5O2-Based 1T1C FeRAM

We have previously studied fatigue and its recovery phenomenon on 64 kbits hafnium-based one-transistor and one-capacitor (1T1C) ferroelectric random-access memory (FeRAM) with PVD-TiN (30 nm)/ALD- Hf0.5Zr0.5O2 (8 nm)/CVD-TiN (50 nm) capacitors. In this study, we characterized a single large capacitor fabricated using the same process as the 1T1C FeRAM to clearly understand the recovery mechanism and comprehensively qualify the recovery effect. The results reveal that the recovery effect is caused by domain depinning and new domains switching owing to a redistribution of oxygen vacancy. Furthermore, it is evident from recovery voltage and recovery pulse width dependence of the recovery effect that the recovery voltage can be reduced by applying a longer recovery pulse width. This enables a more flexible circuit design of 1T1C FeRAM when the recovery method is applied to enhance the cycling endurance.

Room-temperature multiferroicity and magnetoelectric couplings in (Co0.75Al0.25)2(Fe0.75Mg0.25)O4 spinel films

The electric field manipulation of a magnetic order is a key feature of advanced information technologies and the development of room-temperature multiferroic magnetoelectrics remains an important task. In this work, we report novel single-phase magnetoelectric (ME) films consisting of a (Co0.75Al0.25)2(Fe0.75Mg0.25)O4 spinel ferrite. The (Co0.75Al0.25)2(Fe0.75Mg0.25)O4 thin films display a switchable polarization and magnetization with a remnant polarization of about 2.1 µC cm–2 and a remnant magnetization of about 82.0 emu cm–3 at 300 K. Electric field control of magnetism is detected, the magnetization modulation reaches 8.5 % at room temperature with an electric field of 500 kV cm−1, revealing the existence of magnetoelectric coupling. We believe that the substitutions of Al3+ and Mg2+ cations at the spinel sublattices might induce local distortions and local electric field and modulate the superexchange interactions between magnetic ions, which give rise to the ferroelectricity and ME effect in the (Co0.75Al0.25)2(Fe0.75Mg0.25)O4 thin films. The electric field-induced charge modulation may also contribute to the observed magnetoelectric effect, which likely results from the migration and redistribution of oxygen vacancies in the prepared nanofilms. The findings of this work can stimulate research studies on the development of room temperature multiferroics and magnetoelectrics based on the spinel ferrite compounds.

Unravelling the role of oxygen vacancies on the current transport mechanisms in all-perovskite nickelate/titanate heterojunctions for nonvolatile memory applications

The micrometer-sized nickelate–titanate heterojunctions with LaNiO3 (LNO) electrode have been fabricated to investigate the dominant current transport mechanisms under positive and negative bias. The LNO/SmNiO3 (SNO)/Nb:SrTiO3 (NSTO) heterojunction exhibits a highly rectifying feature with a very low leakage in a broad temperature region (from 200 to 425 K), which is attributed to the formation of a Schottky-like barrier at the SNO/NSTO interface. In addition, it is found that the trap defects (i.e., oxygen vacancies) play an essential role in determining the current density (J)–voltage (V) characteristics irrespective of the voltage polarity. The leakage current at low electric fields (<0.25 MV/cm) is dominated by temperature-enhanced trap assisted tunneling process, which is caused by the interface oxygen vacancy induced states. Further analysis suggests that, at high fields (>1.2 MV/cm), the leakage is ascribed to the bulk-limited field enhanced thermal ionization of trapped carriers in the SNO film (i.e., Poole–Frenkel emission). Specially, the oxygen vacancy redistribution near the SNO/NSTO heterointerface driven by a high temperature (425 K) or high electrical field (>3.8 MV/cm) stress is emphasized to account for the transition from the Schottky contact limited to bulk-limited conduction mechanism (i.e., space charge limited conduction). This work will benefit the further analysis of the resistive switching phenomena in nickelate-based devices, showing a potential for nonvolatile memory applications.

All‐Electric Nonassociative Learning in Nickel Oxide

Habituation and sensitization represent nonassociative learning mechanisms in both non‐neural and neural organisms. They are essential for a range of functions from survival to adaptation in dynamic environments. Design of hardware for neuroinspired computing strives to emulate such features driven by electric bias and can also be incorporated into neural network algorithms. Herein, cellular‐like learning in oxygen‐deficient NiOx devices is demonstrated. Both habituation learning and sensitization response can be achieved in a single device by simply controlling the magnitude of the electric field. Spontaneous memory relaxations and dynamic redistribution of oxygen vacancies under electric bias enable such learning behavior of NiOx under sequential training. These characteristics in simple device arrays are implemented to learn alphabets as well as demonstrate simulated algorithmic use cases in digit recognition. Transition metal oxides with carefully prepared defect concentrations can be highly sensitive to electronic structure perturbations under moderate electrical stimulus and serve as building blocks for next‐generation neuroinspired computing hardware.

(Digital Presentation) CeOx Capping for Ferroelectric Y:HfO2 Films

In recent years, HfO2 has been paid a lot of attention as a ferroelectric material due to its excellent scalability and complementary-metal-oxide-semiconductor (CMOS) process compatibility [1]. Some of the issues of ferroelectric HfO2 are wake-up and fatigue effect, increase and decrease of remanent polarization (P r) along with switching cycles, and low endurance. Generally, these phenomena are understood as a result of the increase and redistribution of oxygen vacancies (VO) in HfO2 films [2]. By the way, it is reported that CeOx can release and absorb oxygen from adjacent oxides [3]. In this study, we propose the stacking structure of CeOx and ferroelectric HfO2 to improve endurance and investigate electric properties and voltage application sequence to suppress the fatigue effect.5 mol% Y-doped HfO2 (Y:HfO2) and CeOx capping layers of metal-ferroelectric-metal (MFM) capacitors were deposited by atomic-layer-deposition (ALD) with W bottom and top electrodes by sputtering. Post metallization annealing (PMA) was performed at 450 and 500oC for the various duration. Figure 1 shows hysteresis loops of the capacitors with and without CeOx capping annealed at 450oC for 20 minutes. Large hysteresis on CeOx capping is observed even same annealing temperature and duration. Enhancement of transformation to ferroelectric phase can be explained as tensile stress to Y:HfO2 due to the difference of coefficient of thermal expansion (CTE) between CeOx and Y:HfO2. Variation of switching polarization (P SW), the difference between positive and negative P r, as a function of the number of switching cycles is also investigated and shown in figure 2. Both capacitors are annealed at 500oC for 100 minutes to crystallize. In the case of without CeOx capping, continuous wake-up effect and breakdown over 3×107 times switching are observed. On the other hand, the capacitor with CeOx capping shows a slight wake-up effect and fatigue after 106 times switching, additionally, endurance is improved to 3×109 times switching. The reason why fatigue and prolonging endurance occur is suspected that oxygen ions in the CeOx layer pin polarization domains and compensate VO in Y:HfO2. To suppress fatigue, we propose the recovery process by positive DC voltage application to top electrodes. In figure 3, hysteresis loops of the capacitor with CeOx capping before and after recovery voltage application. One can observe the larger hysteresis after recovery.This work was supported by JST-COI (JPMJCE1309). One of the authors, Kazuto Mizutani, was supported by The FUTABA Foundation.[1] T. S. Boscke, et al., Appl. Phys. Lett., 99, 102903 (2011). [2] M. Pesic, et al., Adv. Funct. Mater., 26, pp. 4601–4612 (2016). [3] M. Mamatrishat, et al., Vacuum, 86, pp. 1513-1513 (2012). Figure 1

Electric field induced degradation of high-voltage PTCR ceramics

This study investigates the degradation of the PTCR material BaTiO3 in high DC voltage, as it is important for the application as heater in electric cars. Impedance spectroscopy was used to gain information on the degradation. The different individual contributions (Ri, Qi) are correlated to the respective features of the microstructure. Besides degradation in electric fields, field-free oxidation and reduction were conducted to examine the influence of oxygen vacancy concentration on the resistivity. The results show a higher resistivity after degradation in the electric field and after oxidation. In contrast, reducing the sample decreases the resistivity. The impedance analysis shows that mostly the grain boundary resistance is influenced during degradation. Likely, a local oxygen vacancy redistribution occurs at grain boundaries. This redistribution is caused by the locally very high electric fields, which in turn influence space charge and Schottky barriers, leading to a change in resistivity of the PTCR material.

Analysis of flexochemical effect and its application in scanning probe microscopy

Electrochemical processes in solids are affected by the properties of various interfaces, where the flexoelectric effect manifests itself considerably due to the inevitable strong gradient fields. Thus, it is crucial to study the coupling between the electrochemical process and the flexoelectric effect. Based on the continuum theory, we conducted the finite element implementation for the flexochemical effect, being the coupling between flexoelectricity, Vegard effect and chemical reactions. Then, the developed method is employed to investigate the flexochemical effect arising in scanning probe microscopy (SPM), including evaluating the contributions from the flexoelectric effect and Vegard effect to the electromechanical response on material SrTiO3 (STO) in piezoresponse force microscopy (PFM) as well as to mechanical redistribution of oxygen vacancy in STO. It is found that at room temperature the nanoscale electromechanical response of the undoped STO in PFM imaging is mainly induced by the converse flexoelectricity while the contribution of direct Vegard effect is negligible. Furthermore, the contact force exerted by SPM tip in manipulating the redistribution of oxygen vacancies is multifunctional, including diminishing vacancies underneath the contact area and enriching the regions around the tip-surface contact edge and inside the sample below the tip, resulting from the synergy of the converse Vegard effect and the direct flexoelectricity. These analyses explain some experimental observations well. This paper provides a continuum framework for the analysis of electrochemomechanical systems with the flexoelectric effect.

Wake-up effect in Hf0.4Zr0.6O2 ferroelectric thin-film capacitors under a cycling electric field

We examined the wake-up effect in a TiN/Hf0.4Zr0.6O2/TiN structure. The increased polarization was affected by the cumulative duration of a switched electric field and the single application time of the field during each switching cycle. The space-charge-limited current was stable, indicating that the trap density did not change during the wake-up. The effective charge density in the space-charge region was extracted from capacitance–voltage curves, which demonstrated an increase in free charges at the interface. Based on changing characteristics in these properties, the wake-up effect can be attributed to the redistribution of oxygen vacancies under the electric field.

Chemical Structure of Conductive Filaments in Tantalum Oxide Memristive Devices and Its Implications for the Formation Mechanism

AbstractResistive switching in metal oxides is believed to be caused by a temperature and electric field driven redistribution of oxygen vacancies within a nanometer sized conductive filament. Accordingly, gaining detailed information about the chemical composition of conductive filaments is of key importance for a comprehensive understanding of the switching process. In this work, spectromicroscopy is used to probe the electronic structure of conductive filaments in Ta2O5‐based memristive devices. It is found that resistive switching leads to the formation of a conductive filament with an oxygen vacancy concentration of ≈20%. Spectroscopic insights provide detailed information about the chemical state of the tantalum cations and show that the filament is not composed of a metallic Ta0 phase. As an extreme case, devices after an irreversible dielectric breakdown are investigated. These devices feature larger conductive channels with higher oxygen vacancy concentrations. Using the experimental data as input for finite element simulations, the role of thermodiffusion for the formation process of conductive filaments is revealed. It is demonstrated that thermodiffusion is not the dominating effect for the filament formation here but might play a role in accelerating the forming process, as well as in the stabilization of the filament.

Electric-field modulated photovoltaic effect of ferroelectric double-perovskite Bi2FeMnO6 films

Ferroelectric perovskite oxide materials for photovoltaics (PV) have received considerable attention for their switchable PV responses and above-bandgap photovoltages as a type of new-generation PV device. Relatively large bandgap and low photocurrent remain major problems for their PV applications. Herein, we report the PV response of ferroelectric double-perovskite Bi2FeMnO6 (BFMO) thin films. It was found that the photocurrent density (Jsc) of a Sn:In2O3 (ITO)/BFMO/SrRuO3 (SRO) thin film device is two orders of magnitude higher than that of BiFeO3, and its power conversion efficiency is about 430 times larger than that of BiFeO3 under AM 1.5G illumination. The built-in electric field near the BFMO/SRO interface, which represents the Schottky contact, leads to the separation of photon-generated carriers. More importantly, the electric poling treatment on the BFMO device can manipulate significantly the magnitude of Jsc, which is independent of the polarization direction. This electric-field modulated PV effect in the poled BFMO device originates from the modulation of the Schottky barrier height at the BFMO/SRO interface. The redistribution of oxygen vacancies after electric poling treatment is mainly responsible for the modulation of the Schottky barrier height.