Resistive Switching Characteristics Research Articles

Effect of annealing conditions on resistive switching in hafnium oxide-based MIM devices for low-power RRAM

Abstract This work presents resistive switching (RS) behaviour in HfO2-based low-power resistive random-access memory (RRAM) devices. A metal-insulator-metal (MIM) structure (Au/HfO2/Pt) was fabricated by sandwiching a thin insulating layer of HfO2 between Pt and Au electrodes. HfO2 films deposited by RF sputtering at room temperature were rapid thermally annealed in N2 ambient at 400 °C and 500 °C. Grazing angle x-ray diffraction (GIXRD), Field emission gun-scanning electron microscopy (FEG-SEM), atomic force microscopy (AFM), and x-ray photoelectron spectroscopy (XPS) were employed to analyse the phase, crystal structure, morphology, surface roughness and chemical composition of the HfO2 films. The bipolar RS could be observed in both as-deposited and annealed HfO2 film-based devices from I–V characteristics measured using a source meter. We have investigated the effect of annealing temperature and annealing ambient on the phase formation of HfO2 as well as the RS characteristics and compared with as-deposited film-based device. Annealed HfO2 film-based devices exhibited improved electrical characteristics, including stable and repeatable RS at significantly lower switching voltages (<1 V) which indicates low power consumption in these devices. The relatively lower processing temperature of the HfO2 films and that too in the films deposited by physical vapor deposition (PVD) technique-RF magnetron sputtering makes this study significantly useful for resistive switching based non-volatile memories.

Anomalous resistive switching effect in La0.8Ca0.2MnO3/Nb:SrTiO3 structure

A barrier-type resistive switching (RS) unit, composed of a metal and Nb:SrTiO3 (NSTO), holds significant potential for data storage applications due to its high storage density, low operating voltage, and excellent stability. While extensive research has been conducted on conductive oxides (COs), there has been relatively less focus on the RS properties of heterogeneous structures combing CO electrodes and NSTO. Epitaxial growth of CO on NSTO is expected to yield devices with enhanced stability and repeatability. This study explores the RS characteristics of La0.8Ca0.2MnO3 (LCMO)/NSTO heterostructures through epitaxy of both conventional and anoxic LCMO films on (00 l)-oriented NSTO single crystal substrates. The results reveal that the conventional LCMO/NSTO structure exhibits a conventional counterclockwise bipolar RS (BRS) effect, while the anoxic LCMO/NSTO heterostructure demonstrates a unique clockwise (CW) BRS effect (exhibiting different RS characteristics under different applied voltages). The study concludes that the CW-BRS effect mechanism is attributed to a high concentration of oxygen vacancies (Vo) in LCMO. Under different external electric fields, Vo in LCMO and NSTO migrate to the LCMO/NSTO interface, respectively, leading to multiple changes in the interface barrier. These findings offer valuable experimental insights for utilizing CO in the field of RS applications.

Role of sulphur in resistive switching behavior of natural rubber-based memory

The rising environmental awareness has spurred the extensive use of green materials in electronic applications, with bio-organic materials emerging as attractive alternatives to inorganic and organic materials due to their natural biocompatibility, biodegradability, and eco-friendliness. This study showcases the natural rubber based resistive switching memory devices and how varying sulphur concentrations (0 - 0.8 wt.%) in natural rubber thin films impact the resistive switching characteristics. The natural rubber was formulated and processed into a thin film deposited on an ITO substrate as the bottom electrode and with an Ag film as the top electrode. The addition of sulphur modifies the degree of crosslinking in the natural rubber thin film, from which the concentration of -C=C- group and density of defect site (S+) are affected, and hence the resistive switching behavior of the memory device. The devices exhibit bipolar resistance with symmetric switching characteristics which are attributed to the formation of conductive paths facilitate by electron transport along -C=C- and S+ defect sites between the two electrodes. Notably, a sample with 0.2 wt.% sulphur exhibits a high ON/OFF ratio (104), a large memory window (5.5 V), prolonged data retention (10 years), and reliable endurance (120 cycles). These findings highlight the potential of natural rubber as a promising material for eco-friendly resistive-switching random access memory applications.&#xD.

Scalable fabrication of vertically arranged Bi2Se3 crossbar arrays for memristive device applications

Despite the unique advantages of the memristive switching devices based on two-dimensional (2D) transition metal dichalcogenides, scalable growth technologies of such 2D materials and wafer-level fabrication remain challenging. In this work, we present the gold-assisted large-area physical vapor deposition (PVD) growth of Bi2Se3 features for the scalable fabrication of 2D-material-based crossbar arrays of memristor devices. This work indicates that gold layers, prepatterned by photolithography processes, can catalyze PVD growth of few-layer Bi2Se3 with 100-folds larger crystal grain size in comparison with that grown on bare Si/SiO2 substrates. We also present a fluid-guided growth strategy to improve growth selectivity of Bi2Se3 on Au layers. Through the experimental and computational analyses, we identify two key processing parameters, i.e., the distance between Bi2Se3 powder and the target substrate and the distance between the leading edges of the substrate and the substrate holder with a hollow interior, which plays a critical role in realizing large-scale growth. By optimizing these growth parameters, we have successfully demonstrated cm-scale highly-selective Bi2Se3 growth on crossbar-arrayed structures with an in-lab yield of 86%. The whole process is etch- and plasma-free, substantially minimizing the damage to the crystal structure and also preventing the formation of rough 2D-material surfaces. Furthermore, we also preliminarily demonstrated memristive devices, which exhibit reproducible resistance switching characteristics (over 50 cycles) and a retention time of up to 106 s. This work provides a useful guideline for the scalable fabrication of vertically arranged crossbar arrays of 2D-material-based memristive devices, which is critical to the implementation of such devices for practical neuromorphic applications.

Resistive Switching Characteristics of NiO Thin Films Influenced by Changes in the Diameter of Nanometer-Scale Top Electrodes.

We investigated the forming, set, and reset voltages affected by the area of the top electrodes of a resistive random access memory (RRAM) capacitor fabricated by epitaxial NiO thin films. NiO RRAM capacitors with Au top electrode with a diameter of 100 μm showed typical unipolar switching characteristics. Au top electrodes with diameters of 10, 20, and 30 nm were formed on the surface of epitaxial NiO thin films by e-beam lithography. The forming, set, and reset voltages tended to decrease as the diameter of NiO RRAM capacitors with nanosized Au top electrodes decreased. As the area of the Au top electrodes increases, the volume of space in which the conductive filaments formed within the NiO RRAM capacitors can be formed becomes larger. This causes the conductive filament to be formed at relatively low energy, giving low forming voltages, while increasing the diversity of paths, causing a wide forming voltage distribution.

Photo‐synaptic Memristor Devices from Solution‐processed Ga2O3 Thin Films

AbstractHardware integration with biological synaptic function is the key to realizing brain‐like computing. Resistive Random Access Memory (RRAM), with a similar structure to biological synapses, are important candidate for the simulation of biological synaptic function. In this work, Ga2O3 film as a functional layer of RRAM is prepared by the solution method, and an RRAM‐based photo‐synaptic device with an Ag/Ga2O3/Si structure is constructed subsequently. The device exhibits excellent bipolar resistive switching characteristics, with the merits of a large storage window and long retention time. Furthermore, the devices generated excitatory postsynaptic currents (EPSC) and paired‐pulse facilitation (PPF) behaviors under light pulse stimulation, enabling the simulation of synaptic plasticity. The transformation of synaptic behavior from short‐term memory (STM) to long‐term memory (LTM) is achieved by observing the spike‐duration dependent plasticity (SDDP), spike‐intensity dependent plasticity (SIDP), spike‐number dependent plasticity (SNDP) and spike‐rate dependent plasticity (SRDP) characteristics of photonic synapses under different conditions. The device also simulates the process of successive “learning‐forgotten‐remembering”, revealing that RRAM‐based photonic synapses have great potential in the fields of artificial visual perception and memory storage.

Reset-Voltage Controlled Resistance-State and Applications of Forming-Free Fe-Doped SrTiO3 Thin-Film Memristor.

In this study, we prepared a strontium ferrite titanate (STF) thin film using a sol-gel process to insulate resistive random-access memory (RRAM) applications. Compared to the typical strontium titanate (STO) RRAM, the improvement in the resistive switching characteristics in STF RRAM is obvious. The Al/STO/ITO/Glass RRAM set/reset voltages of -1.4 V/+3.3 V and the Al/STF/ITO/Glass RRAM set/reset voltages of -0.45 V/+1.55 V presented a memory window larger than 103, a low operating voltage and device stability of more than 104 s. In this study, the influence of Fe on the conducting paths and the bipolar resistive switching properties of Al/STF/ITO/Glass RRAM devices is investigated.

Controlling the digital-to-analog switching in HfO2-based memristors via modulating the oxide thickness

The HfO2-based memristors have been acknowledged as a highly potential candidate for nonvolatile memory and neuromorphic computing applications. However, the analog resistive switching behavior of the HfO2-based memristors still needs improvement. In this work, Cu/HfO2/Pd devices with different oxide layer thicknesses were prepared by the magnetron sputtering process. The resistance switching characteristics related to oxide layer thickness were explored, revealing significant differences in the forming, SET, and RESET characteristics depending on oxide thickness. The conduction mechanism of the Cu/HfO2/Pd memristors is mainly attributed to the space charge limited current (SCLC) model. Thinner devices tend to show a more abrupt SET behavior and larger SET voltage, while thicker devices exhibit more gradual SET behavior and a lower SET voltage. In addition, as the thickness of the oxide layer increases, the Cu/HfO2/Pd memristors transition from digital resistive switching to analog resistive switching. The formation and rupture of oxygen vacancy filaments are predominant in thicker device, and the shape of the conduction filaments has been proposed as responsible for the abrupt and gradual changes in resistive switching. This work demonstrates that the thickness of the oxide layer plays an important role in engineering digital logic and neuromorphic behavior in HfO2-based memristors.

Nanosecond Magneto‐Ionic Control of Magnetism Using a Resistive Switching HfO2 Gate Oxide

AbstractVoltage‐controlled magnetism (VCM) offers an efficient operating method for various spintronic applications, with reduced power consumption compared to conventional current‐driven technologies. Among the VCM mechanisms, magneto‐ionic control provides large modulation and non‐volatile characteristics. However, its operating speed is limited to a microsecond timescale due to slow ion migration, which must be improved for practical device applications. Here, the nanosecond operation of magneto‐ionic VCM in a Ta/CoFeB/MgO/AlOx structure by introducing an HfO2 gate oxide with resistive switching characteristics is demonstrated. By inducing soft breakdown in the HfO2 gate oxide, the coercivity of the perpendicularly magnetized CoFeB can be controlled by 20% with a 20 ns gate voltage of ≈7 MV cm−1. This nanosecond magneto‐ionic VCM performance is maintained after repeated operations up to 10 000 cycles. Further, by utilizing an HfO2 gate in a spin‐orbit torque (SOT) device, the ability to control field‐free SOT switching polarity with nanosecond gate voltages is demonstrated. These findings provide a novel pathway to realize nanosecond, non‐volatile VCM for low‐power spintronic applications.

Coexistence of digital and analog resistive switching behaviors in nanocrystalline yttrium-iron-garnet thin films achieved by electrode engineering

Memristors with digital or analog resistive switching characteristics have the potential to acquire high-performance data storage or electronic synaptic device applications, respectively. It is considerably important to realize coexistence of digital and analog resistive switching characteristics for high-performance memristor applications. In this study, the nanocrystalline yttrium-iron-garnet Y3Fe5O12 (YIG) thin film was synthesized on Pt substrates by a facile solution-processed method, and it can be used as resistive switching layer for fabricating memory devices. The digital and analog resistive switching characteristics in YIG thin film devices were achieved by employing inert platinum and active silver electrodes, respectively. Oxygen vacancy-controlled conductive filaments (CFs) with Pt/YIG/Pt structure exhibited digital resistive switching behavior which can be modulated by changing voltage polarity. The resistive switching mode with positive Set/Reset in the Pt/YIG/Pt device acquired superior resistive switching properties. Ag/YIG/Pt device exhibited analog resistive switching characteristics based on the formation of Ag-based CFs. The conductance increase during Set operation and decrease during Reset process by adjusting current gradients and voltage windows, respectively. The present study exhibits an effective path to obtain the digital and analog resistive switching properties, which opens a door for understanding and optimizing resistive switching performance.

Influence of electrodes on the resistive switching characteristics of Al/Gd2Zr2O7/E (E=Al or ITO) RRAM devices

Sol-gel derived amorphous Gd2Zr2O7 (GZO) thin films with Al/GZO/E (E = ITO or Al) structures were prepared to demonstrate the effect of electrode material on the switching behavior of resistive random access memory (RRAM) devices. All devices exhibit bipolar resistive switching (BRS) mode. After the post-metal annealing (PMA) treatment, the AlOx interfacial layer contributes to enhancing oxygen ion storage capacity and limiting the electromigration of aluminum metal. The forming-free Al/GZO/ITO device exhibits a switching cycle of 3027, a Vset/Vreset ratio of −1.83 V/0.56 V, a Ron/Roff ratio of approximately 10, and a retention time of 104 s. And the Al/GZO/Al device features a dual-layer AlOx structure, significantly improving the integrity of filament rupture to reduce leakage current and increase the switching window. Its switching cycle is 1946, with a Vset/Vreset ratio of −3.47 V/0.29 V, a Ron/Roff ratio of approximately 102, and a retention time of 104 s. The oxygen storage capacity of the dual-layer AlOx interfacial layer in the Al/GZO/Al device surpasses that of the Al/GZO/ITO device, allowing for a reduction in the reset voltage and ensuring more complete filament rupture, thereby enhancing the RHRS. Accordingly, GZO thin films can be applied to achieve the desired RS characteristics on different electrodes, making them promising candidates for RRAM applications.

Microstructure, dielectric storage, ferroelectric polarization, and non-ferroelectric resistive switching characteristics of sol–gel-derived BZT ferroelectric films with tailored Zr content

Microstructure, dielectric storage, ferroelectric polarization, and non-ferroelectric resistive switching characteristics of sol–gel-derived BZT ferroelectric films with tailored Zr content

Complementary resistive switching characteristics of solid electrolyte chalcogenide AgxTe nanoparticles for high-density crossbar random access memory

Silver telluride (AgxTe) is a member of the chalcogenide family that comprises materials extensively used as solid electrolytes. Because of its high-ionic conductivity, low-optical bandgap, and excellent thermoelectric properties, AgxTe has been studied in many research fields, including optoelectronics and energy harvesting. Herein, AgxTe is proposed as the active channel for resistive random access memory (RRAM) showing complementary resistive switching (CRS) characteristics. AgxTe-based RRAM devices with an Ag/AgxTe/Au structure are fabricated on a glass substrate. AgxTe nanoparticles are synthesized using the colloidal method, and AgxTe thin films are prepared via spin coating of the synthesized nanoparticles dispersed in deionized water. The fabricated AgxTe-based RRAM device exhibits CRS characteristics without any additional built-in selectors or antiserial arrangement. This is attributed to the formation of the inversion of CF geometry and allows the fabrication of high-density crossbar arrays. The AgxTe RRAM device annealed at 200 °C exhibits a resistance on/off ratio of approximately 102 as well as stable retention (∼104 s) and endurance (∼103 cycles). This investigation proposes a new application of AgxTe, as a solid electrolyte, and a new strategy for the development of high-density crossbar RRAM architectures, for the first time.

Study of resistive properties and neural response of ZrO2/TiO2 heterojunction nanowire array (NWA) RRAM

Nowadays, the researcher has an eye on enhancing the performance of resistive random-access memory (RRAM) and exploring new demands and applications, particularly in the field of artificial intelligence and neural computing. It is crucially important for accurately replicating the observed plasticity within synapses to achieve superior RRAM performance in low-dimensional nanomaterials. In this work, we focus on addressing certainly unsatisfactory electrical properties of RRAM based on titanium oxide nanowire array (TiO2 NWA), for example, short retention time (∼103 s). Theoretically and experimentally, a novel and solution-based RRAM structure, namely ZrO2/TiO2(NWA), has been conducted. With the help of the ZrO2 insertion layer, the RRAM device shows impressive resistive switching characteristics, including 200 current–voltage(I-V) sweeps without degradation, increased cycling endurance (>105 cycles), and a long retention time (∼9 × 104 s). These findings hold significant implications for digital computing systems. Moreover, the successful implementation of the proposed device enables the emulation of fundamental synaptic biological features such as non-linear transmission properties, learning experiences behavior, short-term potentiation (STP) and long-term potentiation (LTP). This research provides evidence of the immense potential of the Ag/ZrO2/TiO2(NWA)/FTO RRAM device in bipolar non-volatile memory (NVM) and biomimetic neuromorphic systems.

Visible light-modulated switching behavior of CZTSSe-based synaptic memristor

Developing memristors with both light-regulated resistive switching characteristic and optronic synaptic behavior is of great significance for next-generation artificial visual applications. Herein, Cu2ZnSn(S,Se)4 (CZTSSe)-based synaptic memristor is designed and its visible light-modulated resistive switching characteristics, especially the photonic synaptic behaviors, are detailedly investigated. For such Ag/CZTSSe/Mo memristor, not only its Set/Reset switching voltage and RHRS/RLRS ratio can be well regulated by visible light, either the wavelength (470 ∼ 655 nm) or intensity (40 ∼ 110 mW/cm2), but also its photonic synaptic behavior like “learning-forgetting” can be achieved and modulated by light stimuli. The results are of significance for designing artificial photonic synaptic devices in neuromorphic vision application.

Improved resistive switching characteristics observed in amorphous boron nitride-based RRAM device via oxygen doping: A study based on bulk and interface traps analysis

In this paper, we propose an oxygen-doped amorphous boron nitride (O-doped a-BN)-based resistive random-access memory (RRAM) to address the low on/off ratio and stability concerns of conventional a-BN-based RRAM. Initially, comparative analysis with a-BN based RRAM device revealed that the O-doped a-BN based RRAM exhibited 2.5-fold increase in the on/off ratio, enhanced endurance with stable operation over 100 DC cycles, and improved retention stability lasting for 104 s. Then, on the basis of both interface and bulk trap analysis using impedance spectroscopy and the Terman method, the bulk trap density in the O-doped a-BN and a-BN films acts as a key parameter in RS properties, which results in an increase of its on/off ratio and its stable retention without any degradation. These findings show that O-doping in a-BN films can be a technique to improve the on/off ratio and reliability of a-BN-based RRAM devices.

HfO2-based resistive random access memory with an ultrahigh switching ratio

Resistive Random Access Memory (RRAM) is considered one of the most promising candidates for big data storage. By using atomic layer deposition and magnetron sputtering, HfO2 thin films were prepared on ITO first, which exhibited good resistive switching (RS) characteristics in the structure of Ag/HfO2/ITO. By analyzing the RS mechanism, it is found that both metal conductive filaments and oxygen vacancy conductive filaments coexisted and Sn ion in ITO can influence the retention of RRAM. Furthermore, a device in the structure of Ag/HfO2/Pt was proposed and prepared, which exhibited excellent RS characteristics, including an ultrahigh switching ratio averaging up to 108 and low operating voltage. It is concluded that the difference in the work function between the top and bottom electrodes contributes to improving the switching ratio, reducing the operating voltage. In addition, the Ag/HfO2/Pt device is similar to the Ag/HfO2-based threshold switching selector in the structure and in characteristics of high switching ratio, besides non-volatile memory. Hence, the device is functionally equivalent to the combination of an RRAM and a threshold switching selector. It is the potential way to replace the conventional 1S1R structure memory.

Understanding Atomic-Scale Dynamics of Defects in Complex Oxides for Memristive Devices

The exceptional resistive switching characteristics of complex oxides make them promising for use in memristors to mimic biological synapses in neuromorphic computing architectures in an energy-efficient manner. Specifically, the electrical conductance of complex oxides can be controllable switched across multiple orders of magnitude by either (a) electroforming a conduction channel (e.g., in tungsten oxide), or (b) inducing Mott-Hubbard transition (e.g., in rare-earth nickelates)– both via controlled migration of defects (such as oxygen vacancies) under applied bias. However, development of accurate neuromorphic computers (i.e., low variability) remains extremely challenging due to a lack of fundamental understanding of the atomic-scale processes that underlie migration and spatiotemporal evolution of oxygen vacancies (and other extended defects) over nano-to-mesoscopic length/timescales under applied electric field. Here, we demonstrate that a synergistic integration of density functional theory (DFT) calculation, ab initio/classical molecular dynamics (AIMD/CMD) simulations, precision synthesis, and multi-modal X-ray imaging experiments provides an effective route to address this knowledge gap. First, we use a combination of multi-modal X-ray imaging (with chemical/structural sensitivity) and CMD simulations to show that the intrinsic randomness of electroforming process in tungsten-oxide memristors can be minimized by careful introduction of sharp protrusions in the electrode gap. We find that sharp protrusions confine the electric field, which yields a reproducible distribution of oxygen vacancies over several switching cycles; and reliably form conduction channel in a specific location in the memristor. Next, we employed a large dataset of data from DFT calculations and short AIMD trajectories to develop deep neural network (DNN) models to accurately capture energetics, atomic forces, atomic charges, and maximally localized Wannier centers in rare-earth nickelates (RNO). CMD simulations based on these newly developed DNNs show that metal-insulator transition (MIT) in RNO occurs via a local structural distortion in the extended nickel structure, which facilitates the electronic instability and causes a sharp concomitant Ni-O bond and Ni-charge disproportion with decreasing temperature; this MIT is also strongly influenced by applied pressure and rare-earth ion. Furthermore, DFT calculations reveal the correlations the oxygen stoichiometry, OV distribution (including different NiOx motifs), OV transport, and electron-lattice coupling in perovskite nickelates. These results will be discussed in the context of controlling defect-induced transitions to design accurate and reliable memristive devices for neuromorphic computing platforms that are vital for artificial intelligence technologies.

Synthesis of Highly Oriented Nanocrystal Thin-Films and Their Applications to Electrochemical Devices

The precisely defined particle assembly at the nanoscale with simultaneously scalable patterning at the microscale is indispensable for enabling functionality and improving the performance of devices. While synthetic advances in the past decades now allow precise control of their size, shape, and composition, they must be processed from dispersion into functional films for many applications, which presents additional challenges. In this work, we introduce the self-assembly of inorganic nanoubes (Co3O4 NCs) and we also describe the fabrication of centimeter-scale NC superlattice thin-films on silicone substrate with anisotropic NC building blocks, thin-film patterning, and the supercrystal formation of superlattice structures. As a result, the Raman spectra of Co3O4 NC thin-films clearly exhibits the distinct feature of vibration modes of Co3O4 and synchrotron-based X-ray diffraction techniques showed the Co3O4 NCs order into [100] aligned arrays. Also, the uniformly prepared Co3O4 NC building blocks and thin-films were examined by field-emission scanning electron microscopy analysis and it shows well-defined nanostructures with ~13nm-side of NCs and ~3nm space between self-assembled NCs. To investigate the distinct feature of functional films, we prepared various type of device structures (memory, electrode, gas sensor) and examined the performances. In case of memristor, the device exhibits excellent bipolar resistance switching characteristics with better reliability and stability over various polycrystalline metal oxide nanostructures. Utilizing our approach enables the creation of miniaturized devices, followed by the transfer of NC thin-films onto TEM grids. Subsequent in-situ TEM analysis offers the potential to observe unique phenomena at the nano-scale, unveiling phase transitions in nanocrystalline materials or electrochemical reaction mechanisms previously unexplored by researchers.

Understanding the Reversible Transition of Unipolar and Bipolar Resistive Switching Characteristics in Solution-Derived Nanocrystalline Au-Co3O4 Thin-Film Memristors.

Nanocrystalline Au-Co3O4 thin films were fabricated through a facile solution-processing method, and voltage-polarity-dependent resistive switching (RS) characteristics including variability of Set/Reset voltages, stability of cycling endurance, and carriers transport mechanism were studied in the Pt/Au-Co3O4/Pt memory devices. The switching voltages of the Set and Reset processes in the bipolar RS memristors exhibited lower variability as compared to the unipolar resistance switching devices. Moreover, the switching performance of cycle-to-cycle endurance in bipolar mode had a smaller fluctuation than those of unipolar switching behavior. Based on the current-voltage curve fitting analysis, it was found that Ohmic-conduction behaviors dominated the carriers transport of low-resistance state regardless of unipolar or bipolar switching behaviors. The carrier transport of high resistance state was governed following Poole-Frenkel emission and Schottky emission mechanisms in the unipolar and bipolar switching modes, respectively. The physical switching mechanism of Pt/Au-Co3O4/Pt memory devices was proposed using the model by means of growth and disruption of conducting filaments, involved in memory effects of thermochemical mechanism and valence change mechanism in the unipolar and bipolar RS, respectively. The proposed model following the finite element method revealed the roles of electrical field distribution and temperature gradient to further clarify the RS mechanism. Our results open a door for understanding and optimizing oxide-based thin-film resistance switching memory devices.