Ferroelectric Resistive Switching Research Articles

Epitaxial Ferroelectric Hf0.5Zr0.5O2 Thin Films and Their Implementations in Memristors for Brain‐Inspired Computing

AbstractDoped‐HfO2 thin films with ferroelectricity have attracted great attention due to their potential application in semiconductor industry as negative capacitance and resistance switching memory. Despite Hf0.5Zr0.5O2 (HZO) thin films having the most robust ferroelectric properties among all doped‐HfO2 thin films, the realization of single orthorhombic phase HZO thin films is not achieved, while the direct evidence between the structural–properties relationship of orthorhombic phase HZO and ferroelectricity is not confirmed. In this work, the growth of single orthorhombic phase HZO thin films with decent ferroelectricity and resistive switching behavior is reported. With the aid of advanced structural characterization techniques, the HZO thin film is confirmed to be in the single orthorhombic phase. Next, using scanning probe microscopy techniques and macroscopic ferroelectric measurements, the single phase HZO thin films exhibit ferroelectric properties with a remanent polarization of about 20 µC cm−2. Interestingly, the HZO thin film shows ferroelectric resistive switching with an ROFF/RON ratio of about 16 100% with excellent device performance. Furthermore, brain‐like learning behavior is also observed in the HZO thin film. These results may serve to stimulate the study of ferroelectric properties of HZO thin films and their application in the electronic industry.

Ferroelectric resistive switching behavior in two-dimensional materials/BiFeO3 hetero-junctions.

Integrating two-dimensional (2D) materials with ferroelectric thin films may result in unique characteristics and novel applications due to the coupling between their intrinsic characters. Here, we observed the ferroelectric resistive switching behavior in both graphene/BFO and MoS2/BFO heterojunctions, which stems from the modulation of contact barriers and depletion width at the hetero-interface induced by the ferroelectric polarization. Besides, the ferroelectric resistive switching behavior in both graphene/BFO and MoS2/BFO depends on the thicknesses of the corresponding 2D materials, because the thickness-dependent work function or conductivity of 2D materials could change the contact barrier heights and widths at the interface of 2D materials and ferroelectrics. Our results will widen the memristive applications of 2D/ferroelectrics hetero-junctions and provide a pathway for the novel memory devices based on hetero-structures with 2D/3D materials in the future.

High resistance ratio of bipolar resistive switching in a multiferroic/high-K Bi(Fe0.95Cr0.05)O3/ZrO2/Pt heterostructure

An novel heterostructure composed of multiferroic Bi(Fe0.95Cr0.05)O3 (BFCO) and high-K ZrO2 (ZO) layers is investigated. Ferroelectric and electrical properties of the BFZO/ZO heterostructure have been investigated. A pronounced bipolar ferroelectric resistive switching characteristic was achieved in the heterostructure at room temperature. Interestingly, the BFCO/ZO structures exhibit a reproducible resistive switching with a high On/Off resistance ratio ∼2×103 and long retention time. The relationship between polarization and band structure at the interface of BFCO/ZO bilayer under the positive and negative sweepings has been discussed. As a result, the BFCO/ZO multiferroic/high-K heterostructure with high On/Off resistance ratio and long retention characterizes, exhibits a potential in future nonvolatile memory application.

Multi-Nonvolatile State Resistive Switching Arising from Ferroelectricity and Oxygen Vacancy Migration.

Resistive switching phenomena form the basis of competing memory technologies. Among them, resistive switching, originating from oxygen vacancy migration (OVM), and ferroelectric switching offer two promising approaches. OVM in oxide films/heterostructures can exhibit high/low resistive state via conducting filament forming/deforming, while the resistive switching of ferroelectric tunnel junctions (FTJs) arises from barrier height or width variation while ferroelectric polarization reverses between asymmetric electrodes. Here the authors demonstrate a coexistence of OVM and ferroelectric induced resistive switching in a BaTiO3 FTJ by comparing BaTiO3 with SrTiO3 based tunnel junctions. This coexistence results in two distinguishable loops with multi-nonvolatile resistive states. The primary loop originates from the ferroelectric switching. The second loop emerges at a voltage close to the SrTiO3 switching voltage, showing OVM being its origin. BaTiO3 based devices with controlled oxygen vacancies enable us to combine the benefits of both OVM and ferroelectric tunneling to produce multistate nonvolatile memory devices.

BaTiO3 thin films on platinized silicon: Growth, characterization and resistive memory behavior

We report on the fabrication and characterization of Ti/BaTiO3/Pt memristive devices. BaTiO3 films were grown on platinized silicon by pulsed laser deposition with different laser pulse energies. We prove the existence of a correlation between the fabrication conditions and the microstructure and stoichiometry of the films. It is suggested that the small grain size found on our BaTiO3 films destabilizes the structural tetragonal distortion and inhibits the appearance of long-range ferroelectric ordering. We show that even in absence of ferroelectric resistive switching (RS), two different RS mechanisms (metallic filament formation and oxidation/reduction of the Ti top electrode) compete, and can be selected by controlling the films stoichiometry and microstructure.

In-situ TEM observation of Multilevel Storage Behavior in low power FeRAM device

The quest for non-volatile memories has attracted tremendous attention, especially in mature ferroelectric random access memory (FeRAM) with properties of high read/write speed and low power consumption. Strain engineering of multiferroic BiFeO3 (BFO) has recently become the subject of broad research interest because of its intriguing properties. In this study, we demonstrate the switchable diode characteristics in highly strained BFO thin films. Using a unique in situ electrical transmission electron microscopy (TEM), we verify the correlation between ferroelectric resistive switching with multilevel states and polarization reversal. Structural investigation confirms that the phase transition from mixed phase to pure T-like phase, accompanying with the polarization reversal by external bias, is the origin of the multilevel states. The switchable diode with multilevel resistive switching can be explained in terms of the variation of the barrier height, governed by ferroelectric polarization and polarity of the external bias. This research model, i.e., engineering of the room inside, can offer an approach toward high-density memories.

Direct Observation of Domain Motion Synchronized with Resistive Switching in Multiferroic Thin Films.

The room-temperature resistive switching characteristics of ferroelectric, ferroelastic, and multiferroic materials are promising for application in nonvolatile memory devices. These resistive switching characteristics can be accompanied by a change in the ferroic order parameters via applied external electric and magnetic excitations. However, the dynamic evolution of the order parameters between two electrodes, which is synchronized with resistive switching, has rarely been investigated. In this study, for the first time, we directly monitor the ferroelectric/ferroelastic domain switching dynamics between two electrodes in multiferroic BiFeO3 (BFO) planar devices, which cause resistive switching, using piezoresponse force microscopy. It is demonstrated that the geometrical relationship between the ferroelectric domain and electrode in BFO planar capacitors with only 71° domain walls significantly affects both the ferroelectric domain dynamics and the resistive switching. The direct observation of domain dynamics relevant to resistive switching in planar devices may pave the way to a controllable combination of ferroelectric characteristics and resistive switching in multiferroic materials.

Resistive Switching and Modulation of Pb(Zr0.4Ti0.6)O3/Nb:SrTiO3 Heterostructures

In this work, epitaxial Pb(Zr0.4Ti0.6)O3 (PZT) thin films with different thicknesses were deposited on Nb-doped SrTiO3 (NSTO) single-crystal substrates by chemical solution deposition (CSD), and their ferroelectric resistive switching behaviors were investigated. The results showed that the maximum ON/OFF ratio up to 850 could be obtained in the PZT/NSTO heterostructure with the 150 nm thick PZT film. On the basis of the Schottky-Simmons model and the modified semiconductor theory, we also evaluated the interfacial built-in field and the depletion layer at the PZT/NSTO interface, which can be modulated strongly by the ferroelectric polarization, but are independent of the thickness of the PZT thin films. It is clear that the ferroelectric resistive switching is related to the ferroelectric polarization and modulated by the thickness of ferroelectric films. Therefore, there is an optimal thickness of the PZT film for the maximum ON/OFF ratio due to the ferroelectricity and conductivity mutually restricting. It can be expected that by adjusting the ferroelectricity and conductivity of the ferroelectric thin film and its thickness, the maximum switching ratio can be further improved.

Ferroelectric Resistive Switching in High-Density Nanocapacitor Arrays Based on BiFeO3 Ultrathin Films and Ordered Pt Nanoelectrodes.

Ferroelectric resistive switching (RS), manifested as a switchable ferroelectric diode effect, was observed in well-ordered and high-density nanocapacitor arrays based on continuous BiFeO3 (BFO) ultrathin films and isolated Pt nanonelectrodes. The thickness of BFO films and the lateral dimension of Pt electrodes were aggressively scaled down to <10 nm and ∼60 nm, respectively, representing an ultrahigh ferroelectric memory density of ∼100 Gbit/inch(2). Moreover, the RS behavior in those nanocapacitors showed a large ON/OFF ratio (above 10(3)) and a long retention time of over 6,000 s. Our results not only demonstrate for the first time that the switchable ferroelectric diode effect could be realized in BFO films down to <10 nm in thickness, but also suggest the great potentials of those nanocapacitors for applications in high-density data storage.

Ferroelectricity and ferroelectric resistive switching in sputtered Hf0.5Zr0.5O2 thin films

Ferroelectric properties and ferroelectric resistive switching (FE-RS) of sputtered Hf0.5Zr0.5O2 (HZO) thin films were investigated. The HZO films with the orthorhombic phase were obtained without capping or post-deposition annealing. Ferroelectricity was demonstrated by polarization-voltage (P-V) hysteresis loops measured in a positive-up negative-down manner and piezoresponse force microscopy. However, defects such as oxygen vacancies caused the films to become leaky. The observed ferroelectricity and semiconducting characteristics led to the FE-RS effect. The FE-RS effect may be explained by a polarization modulated trap-assisted tunneling model. Our study not only provides a facile route to develop ferroelectric HfO2-based thin films but also explores their potential applications in FE-RS memories.

Ferroelectric and flexible barrier resistive switching of epitaxial BiFeO3 films studied by temperature-dependent current and capacitance spectroscopy

The resistive switching of epitaxial ferroelectric BiFeO3 thin films was investigated by the combined current and capacitance spectroscopy, in addition with the piezoresponse force microscopy and conducting atomic force microscopy. A strong correlation between the resistance states and the capacitance states was found in the temperature range from 77 to 300 K, revealing the role of interface traps. It is demonstrated that the trap-assisted current transport contributes to the large ON current of the ferroelectric and flexible barrier resistive switching. The combination of ferroelectric polarization reversal, electromigration of oxygen vacancies, and interface traps leads to the nonvolatile memory performance with the ON/OFF ratio of 102 for >105 s and the endurance of over 8000 cycles at room temperature.

Ferroelectric polarization and resistive switching characteristics of ion beam assisted sputter deposited BaTiO3 thin films

In this work, 150nm thick polycrystalline BaTiO3 (BTO) films were deposited on Pt/TiO2/SiO2/Si substrate by ion beam assisted sputter deposition technique. The bias voltage dependent resistive switching (RS) and ferroelectric polarization characteristics of Au/BTO/Pt devices are investigated. The devices display the stable bipolar RS characteristics without an initial electroforming process. Fittings to current–voltage (I–V) curves suggest that low and high resistance states are governed, respectively, by filamentary model and trap controlled space charge limited conduction mechanism, where the oxygen vacancies act as traps. Presence of oxygen vacancies is evidenced from the photoluminescence spectrum. The devices also display P–V loops with remnant polarization (Pr) of 5.7μC/cm2 and a coercive electric field (Ec) of 173.0kV/cm. The coupling between the ferroelectric polarization and RS effect in BTO films is demonstrated.

A ferroelectric memristor based on the migration of oxygen vacancies

Ferroelectric resistive switching memory is a non-destructive and easy to achieve multilevel storage, which is a breakthrough for further improving the density in the random access memory.

Temperature dependences of ferroelectricity and resistive switching behavior of epitaxial BiFeO3 thin films**Project supported by the National Natural Science Foundation of China (Grant Nos. 51272078 and 51332007), the State Key Program for Basic Research of China (Grant No 2015CB921202), the Guangdong Provincial Universities and Colleges Pearl River Scholar Funded Scheme, China (2014), the International Science & Technology Cooperation Platform Program

We investigate the resistive switching and ferroelectric polarization properties of high-quality epitaxial BiFeO3 thin films in various temperature ranges. The room temperature current–voltage (I–V) curve exhibits a well-established polarization-modulated memristor behavior. At low temperatures (< 253 K), the I–V curve shows an open circuit voltage (OCV), which possibly originates from the dielectric relaxation effects, accompanied with a current hump due to the polarization reversal displacement current. While at relative higher temperatures (> 253 K), the I–V behaviors are governed by both space-charge-limited conduction (SCLC) and Ohmic behavior. The polarization reversal is able to trigger the conduction switching from Ohmic to SCLC behavior, leading to the observed ferroelectric resistive switching. At a temperature of > 298 K, there occurs a new resistive switching hysteresis at high bias voltages, which may be related to defect-mediated effects.

The effect of in situ annealing oxygen pressure on the ferroelectric resistive switching characteristic

Bismuth ferrite (BiFeO3, BFO) epitaxial thin films with the thickness of 100nm have been grown on SrRuO3 (SRO) coated SrTiO3 (STO) (111) substrates, forming the structure of Pt/BFO (111)/SRO (111)/STO (111) stacking layers fabricated by pulsed laser deposition (PLD). From electrical measurements of current–voltage curves and polarization–voltage hysteresis loops, it is found that oxygen vacancies depending on different in situ annealing oxygen pressures can influence the electrical characteristics of BFO thin film significantly. High in situ annealing oxygen pressure can reduce the leakage current, brings a steeper current–voltage curve, which arises from the polarization reverse. Meanwhile, the sample under high annealing oxygen pressure has a better rectification, with the On/Off diode currents ratio of around 40:1, which is obviously larger than those under low annealing oxygen pressures. By different conduction mechanism fittings, it is observed that Schottky emission mainly dominates the conduction mechanism at On state, while space-charge limited current emission becomes dominating the leakage at Off state.

Mechanism of ferroelectric resistive switching in Bi0.9La0.1FeO3 thin films

Abstract Resistive switching devices are considered as one of the most promising candidates for the next generation memories and nonvolatile logic applications. In this paper, we report an anomalous resistive switching effect in BiFeO 3 based hetero-structures. Different from conventional resistive switching devices made of metal oxides, no forming process is needed to obtain a stable resistive switching effect in the ferroelectric resistive switching devices. Both positive and negative current peaks are observed under forward bias and reverse bias, respectively, suggesting that flexible Schottky-like barriers and conductive channels form at the top and bottom interfaces, which play important roles in the resistive switching of Ag/Bi 0.9 La 0.1 FeO 3 /La 0.3 Sr 0.7 MnO 3 sandwiched structures. These flexible Schottky-like barriers may come from the migration of charged oxygen vacancies/ions under the electric field of sweeping bias and the redistribution of carriers with ferroelectric switching, while the conductive channel resulted from charged oxygen vacancies/ions. These results demonstrate promising prospects for application of the ferroelectric resistive switching effect at interfaces to nonvolatile memory.

Resistive Switching Behavior in Pt/La&lt;sub&gt;0.7&lt;/sub&gt;Sr&lt;sub&gt;0.3&lt;/sub&gt;MnO&lt;sub&gt;3&lt;/sub&gt;/Nb&lt;sub&gt;0.05&lt;/sub&gt;Bi&lt;sub&gt;0.95&lt;/sub&gt;FeO&lt;sub&gt;3&lt;/sub&gt;/Nb:SrTiO&lt;sub&gt;3&lt;/sub&gt; Ferroelectric Heterostructure

we report reproducible resistive switching performance and relevant physical mechanism of Pt/La0.7Sr0.3MnO3/Nb0.05Bi0.95FeO3/Nb:SrTiO3 ferroelectric heterostructure which was fabricated by pulsed laser deposition. This device exhibits a nonvolatile resistive switching with a resistance ratio of up to 60 under 2V/-3V pulse voltages at room temperature. Low voltage readout, reliable resistance switching reproducibility and good time retention, indicating promise for non-destructive readout nonvolatile memories. In this metal/p-semiconductor/ferroelectric/n-semiconductor heterostructure, the mechanism of resistive switching behavior would be attributed to the ferroelectric polarization enhanced field-induced charge redistribution at the semiconductor/ferroelectric interface, resulting in the modulation of the interface barrier height. Keywords: Resistive switching, Ferroelectric resistive switching, Ferroelectric field effect.

Ferroelectric tunnel junctions with graphene electrodes.

Polarization-driven resistive switching in ferroelectric tunnel junctions (FTJs)--structures composed of two electrodes separated by an ultrathin ferroelectric barrier--offers new physics and materials functionalities, as well as exciting opportunities for the next generation of non-volatile memories and logic devices. Performance of FTJs is highly sensitive to the electrical boundary conditions, which can be controlled by electrode material and/or interface engineering. Here, we demonstrate the use of graphene as electrodes in FTJs that allows control of interface properties for significant enhancement of device performance. Ferroelectric polarization stability and resistive switching are strongly affected by a molecular layer at the graphene/BaTiO3 interface. For the FTJ with the interfacial ammonia layer we find an enhanced tunnelling electroresistance (TER) effect of 6 × 10(5)%. The obtained results demonstrate a new approach based on using graphene electrodes for interface-facilitated polarization stability and enhancement of the TER effect, which can be exploited in the FTJ-based devices.

A light-modified ferroelectric resistive switching behavior in Ag/BaMoO4/FTO device at ambient temperature

BaMoO4 powder was prepared by a facile hydrothermal synthesis. And the BaMoO4/FTO device was fabricated by a spin-coated method, in which the thickness of BaMoO4 layer is about 20µm. The bipolar resistive switching effect has been observed in Ag/BaMoO4/FTO device. Moreover, the resistive switching effect of the device is greatly improved by white light irradiation. The resistive switching behavior is explained by the polarization reversal that changes the charge distribution and modulates the Schottky barriers.

Ferroelectric memristive effect in BaTiO3 epitaxial thin films

Epitaxial BaTiO3 (BTO) films have been grown on Nb-doped SrTiO3 (NSTO) (0 0 1) substrate. The ferroelectric resistive switching effect and memristive behaviours have been observed in Pt/BTO/NSTO heterostructures. It exhibits two resistance states under different polarization, and both current–voltage curves are nonlinear, slightly asymmetric and free of discontinuities, in contrast with what often occurs with other resistive switching devices. In addition, the resistance can be continuously tuned up to 1.3 × 105% by varying pulse voltage amplitudes for the ferroelectric memristor application. Moreover, the device exhibits excellent retention and reproducibility. The above phenomena can be qualitatively explained by considering the modulation of the width and height of the potential barrier at the BTO/NSTO interface by ferroelectric polarization.