Next-generation Nonvolatile Memory Research Articles

Effect of frequency on the classical and relaxor ferroelectric behavior of substituted titanate Ba0.7Er0.16Ca0.05Ti0.91Sn0.09O3.

In the present work, we synthesized the perovskite Ba0.70Er0.16Ca0.05Ti0.91Sn0.09O3 compound (BECTSO) by a solid-state reaction and sintering at 1200 °C. The effects of doping on the structural, electrical, dielectric, and ferroelectric characteristics of the material are examined in this work. X-ray powder diffraction analysis shows that BECTSO crystallizes in a tetragonal structure with space group P4mm. A detailed study of the dielectric relaxation of the BECTSO compound has been reported for the first time. Classical low-frequency ferroelectric and high-frequency relaxor ferroelectric behaviors have been studied. The study of the real part of the permittivity (ε') as a function of temperature demonstrated a high dielectric constant and identified a phase transition from the ferroelectric phase to the paraelectric phase at Tc = 360 K. The analysis of conductivity curves shows two behaviors: semiconductor behavior for f < 106 Hz and metallic behavior for f >106 Hz. The relaxation phenomenon is dominated by the short-range motion of the charge carriers. The BECTSO sample could be considered as a potential lead-free material for next-generation non-volatile memory devices and wide-temperature range capacitor applications.

A non-invasive approach to the resistive switching physical model of ultra-thin organic-inorganic dielectric-based ReRAMs.

The race to next-generation non-volatile memory is on, and ultra-thin (<5 nm) organic-inorganic hybrid dielectric-based ReRAMs are a top contender. However, their extremely small thickness hinders their processability through material characterization techniques, leaving gaps in our understanding of the resistive switching (RS) dynamics in the hybrid dielectric layer. Furthermore, the poor uniformity of key switching parameters remains a persistent issue in ReRAMs, which impedes any trends to be clearly defined through electrical characterization. This work uses electrical manipulation through a ramped-pulse series (RPS) method to improve the voltage and resistance fluctuations in the reset process of ultra-thin Al/Hf-hybrid/Ni devices. By analyzing their electrical behavior under different pulse and temperature conditions, we propose a comprehensive physical model that describes the operating mechanism of the device. Our results confirm the coexistence in the conductive filament (CF) of a hybrid metallic portion composed of Al and Hf3Al2 and an oxygen vacancy portion. The vacancies are found to play a significant role in RS, with most of them generated during the CF forming process and participating to different degrees in the filament rupture of the RPS-processed and non-RPS-processed devices via Joule heating, drift, and Fick forces. Additionally, we identify the cause of switching failure events to be based on the presence of an Al2O3 interlayer in the Al/Hf-hybrid interface.

Enhancement of Resistive Switching Performance in Hafnium Oxide (HfO2) Devices via Sol-Gel Method Stacking Tri-Layer HfO2/Al-ZnO/HfO2 Structures.

Resistive random-access memory (RRAM) is a promising candidate for next-generation non-volatile memory. However, due to the random formation and rupture of conductive filaments, RRMS still has disadvantages, such as small storage windows and poor stability. Therefore, the performance of RRAM can be improved by optimizing the formation and rupture of conductive filaments. In this study, a hafnium oxide-/aluminum-doped zinc oxide/hafnium oxide (HfO2/Al-ZnO/HfO2) tri-layer structure device was prepared using the sol-gel method. The oxygen-rich vacancy Al-ZnO layer was inserted into the HfO2 layers. The device had excellent RS properties, such as an excellent switch ratio of 104, retention of 104 s, and multi-level storage capability of six resistance states (one low-resistance state and five high-resistance states) and four resistance states (three low-resistance states and one high-resistance state) which were obtained by controlling stop voltage and compliance current, respectively. Mechanism analysis revealed that the device is dominated by ohmic conduction and space-charge-limited current (SCLC). We believe that the oxygen-rich vacancy concentration of the Al-ZnO insertion layer can improve the formation and rupture behaviors of conductive filaments, thereby enhancing the resistive switching (RS) performance of the device.

Using Machine Learning Potentials to Explore Interdiffusion at Metal–Chalcogenide Interfaces

Chalcogenide alloys are key materials for selector and memory elements used in next-generation nonvolatile memory cells. However, the high electric fields and Joule heating experienced during operation can promote interdiffusion at the interfaces that degrade device performance over time. A clear atomic scale understanding of how chalcogenide alloys interact with electrodes could aid in identifying ways to improve long-term device endurance. In this work, we develop a robust set of moment tensor potentials (MTPs) to examine interactions between Ge-Se alloys and Ti electrodes. Previous works have shown evidence of strong interactions between Ti and chalcogenide alloys. This system offers an important first test in the use of ML empirical potentials to understand the role of interfaces in endurance in memory elements and broader nanoscale devices. The empirical potentials are constructed using an active learning moment tensor potential framework that leverages a broad data set of first-principles calculations for Ti, Ge, and Se compounds. Long-term simulations (>1 ns) show significant interdiffusion at the Ti|Ge-Se interface with Ti and Se both actively moving across the original interface. The strong chemical affinity of Ti and Se leads to a well-defined Ti-Se region and a severely Se-depleted central Ge-Se region with unfavorable selector characteristics. The evolution of the Ti-Se layer can be described using a self-limited growth model. By comparing effective Ti-Se diffusion constants for simulations at different temperatures, we find a low activation energy of 0.1 eV for Ti-Se layer interdiffusion.

High-quality GaSb epitaxially grown on Si (001) through defects self-annihilation for CMOS-compatible near-IR light emitters

Direct epitaxial growth of III-V materials on complementary metal-oxide-semiconductor (CMOS)-compatible Si substrates has long been a scientific and engineering problem for next-generation light-emitters and non-volatile memories etc. The challenges arise from the lattice mismatch, thermal mismatch, and polarity mismatch between these materials. We report a detailed study of growing high-quality GaSb epilayers with low defect density on on-axis silicon substrates by interface engineering through all-molecular beam epitaxy (MBE) technology. We also systematically investigated the defect self-annihilation mechanism of GaSb epitaxially grown on on-axis Si (001) substrates. It was found that the misfit dislocation array was formed at the interface of AlSb/Si; threading dislocations and antiphase domain boundary annihilated at the initial GaSb layer promoted by the high-density AlSb islands, which was confirmed by transmission electron microscopy (TEM) results. Finally, a 2 µm GaSb epilayer with a step-flow surface, root-mean-square (RMS) roughness of 0.69 nm, and a rocking curve full width at half maximum (FWHM) of 251 arcsec was obtained. The photoluminescence in the near-infrared region of the GaSb/AlGaSb quantum well grown on Si substrate was also demonstrated. Our results highlighted the possible step towards the all-MBE direct growth of Sb-based infrared optoelectronic and microelectronic devices on CMOS-compatible Si substrates.

Study on ion dynamics of hafnium oxide RRAM by electrode thermal effect

Resistive random access memory (RRAM) has become one of the representatives of the next generation of nonvolatile memory because of its excellent performance, and the data reading function is caused by the growth and fracture of the conductive filament (CF) in the switch layer. Therefore, the study of the thermal properties of materials on the conductive mechanism is a step that cannot be ignored. In this paper, a numerical model of filament dissolution caused by the coupling effect of Joule heat and electric field in resistance elements is established. The influence of the thermal field on the electric field and ion mobility during device setup and reset is explained by solving partial differential equations. On this basis, the importance of electrode parameters for resistive switching behavior is emphasized. The results show that the best performance can be obtained at 298 K when TiN is selected as the top electrode material of the equipment. This work has deepened the understanding of RRAM resistance switch characteristics and has guiding significance for selecting memristor materials and promoting its industrialization process.

Self-compliance and high-performance GeTe-based CBRAM with Cu electrode

Conductive bridging random access memory (CBRAM) has been considered as a promising candidate for next-generation non-volatile memory. However, there are always problems of high-power consumption in CBRAM. In this study, a GeTe-based resistive random access memory (RRAM) with an active Cu electrode is fabricated, which reveals significant behavior of low forming voltage (<|-2|V). At the same time, the device exhibits the property of self-compliance, which is essential for protection of the device and reduction of the leakage current. Furthermore, the resistance switching (RS) mechanism of the device is proved to be filament-type, which helps explain the self-compliance phenomenon in detail. These properties provide an effective solution to the achievement of low-power consumption in CBRAM, and show a potential for the scalability of the device and high-density integration in the cross-bar array.

Impact of process-induced ellipticity on the RESET process of cylindrical phase change memory devices

Phase change memory (PCM) is one of the promising candidates for the next-generation high-speed non-volatile memory which exhibits excellent scalability. The cylindrical-type PCM devices entering the nano-scale regime should show their tolerance to the variation in the manufacturing process. However, it is highly challenging to fabricate cylindrical-type nano-scale devices ideally with circular cross-sections. In general, the degree of variation in circular cross-section is dictated by a geometrical parameter called aspect ratio (AR). In this study, the impact of variation in AR of heater (ARheater) and active material, Ge2Sb2Te5 (ARGST) on the RESET programming of the mushroom-type cylindrical PCM device is systematically investigated by using 3D TCAD simulations. The simulation results reveal that the RESET current (IRESET) of the reference device (100 nm heater diameter) consisting of elliptical cross-sections increases significantly to ∼67% when ARheater = 2 and 1 ≤ ARGST ≤ 2, whereas for the scaled-down devices of 20 nm and 10 nm heater diameter with elliptical cross-sections, the IRESET increases to ∼35% and ∼38% when ARheater = 2 and ARGST = 1, and further IRESET increases to ∼54% and ∼63% when ARheater = 2 and ARGST = 2 leading to high-power RESET programming. In the case of the reference device, the ARGST did not play any significant role on IRESET. However, in the scaled-down devices, both ARheater and ARGST significantly affect the IRESET. Furthermore, the device employing a vertically-oriented elliptical heater and horizontally-oriented elliptical GST (where ARheater = 2 and ARGST = 0.5) shows the peculiar re-amorphization among all the cases considered in this study. Hence, the miniaturized cylindrical PCM devices comprising elliptical cross-sections due to process-induced variability require an accurate understanding of the programming characteristics for reliable modeling and simulations.

Robustly Stable Ferroelectric Polarization States Enable Long-Term Nonvolatile Storage against Radiation in HfO2-Based Ferroelectric Field-Effect Transistors.

The ferroelectric field-effect transistors (FeFETs) with HfO2-based ferroelectric layers in the gate stacks are emerging as one of the most promising candidates for the next-generation nonvolatile memory devices due to their scalability and compatibility with conventional Si technology. Moreover, owing to the high radiation hardness of the HfO2-based ferroelectric thin films, HfO2-based FeFETs have attracted great interest in the fields of radiation-hard (rad-hard) memory. However, the reliability of their memory states under irradiation, which represents the validity of the stored information, has not been investigated. Here, we focus on the impact of the total ionizing dose (TID) on erased and programmed states of HfO2-based FeFETs. The TID radiation (X-ray) characteristics of erased and programmed HfO2-based FeFETs are characterized using an on-site read operation. Both the erased and programmed states show robust stability under irradiation at a dose rate of 90 rad(Si)/s, and even at 230 rad(Si)/s, only the erased state shows a slight variation. The possible factors contributing to memory state degradation are discussed. Through the analysis of the threshold voltage shift and subthreshold swing evolution, as well as studies of ferroelectric polarization stability under radiation, it is revealed that the erased state degradation is caused by oxide-trapped charges rather than interface degradation or polarization switching. The physical mechanism of the difference in radiation-induced oxide-trapped charges buildup in programmed and erased FeFETs is analyzed to explain different TID radiation characteristics between them. Our work suggests that the HfO2-based FeFETs have great potential in radiation environment applications.

Neutron radiation-resistant aluminum nitride memristor

A memristor is promising as an electronic synapse or next-generation nonvolatile memory, and its radiation resistance has recently received extensive attention for broader application fields. We fabricated Al/AlN/TiN crossbar arrays and investigated their resistive switching properties and neutron radiation-resistant performance. Al/AlN/TiN memory devices have many excellent features, such as operating currents down to 10 μA, memory endurance over 120 cycles, resistance window greater than 104, and retention time in excess of 104 s. More importantly, the memory devices with different resistance states still maintain remarkable resistive switching behaviors after irradiating with a dose of neutron radiation up to 1 × 1014 n/cm2, demonstrating outstanding potential application in radiation-resistant electronics.

Physically Transient, Flexible, and Resistive Random Access Memory Based on Silver Ions and Egg Albumen Composites.

Organic-resistance random access memory has high application potential in the field of next-generation green nonvolatile memory. Because of their biocompatibility and environmental friendliness, natural biomaterials are suitable for the fabrication of biodegradable and physically transient resistive switching memory devices. A flexible memory device with physically transient properties was fabricated with silver ions and egg albumen composites as active layers, which exhibited characteristics of write-once-read-many-times (WORM), and the incorporation of silver ions improved the ON/OFF current ratio of the device. The device can not only complete the logical operations of “AND gate” and “OR gate”, but its active layer film can also be dissolved in deionized water, indicating that it has the characteristics of physical transients. This biocompatible memory device is a strong candidate for a memory element for the construction of transient electronic systems.

High-Performance Flexible Polymer Memristor Based on Stable Filamentary Switching.

Polymer-based atomic switch memristors via the formation/dissolution of atomic-scale conductive filaments are considered as the leading candidate for next-generation nonvolatile memory. However, the instability of conductive filaments of incomplete bridge makes their switching performances unsatisfied. In this work, we report a flexible polymeric memristor using polyethylenimine incorporated with silver salt. The memristor device exhibited superior performances at room temperature with a favorable endurance, high ON/OFF ratio, good retention, and low operating voltage. These satisfactory performances are attributed to the pre-existing Ag ions in the polymer, guiding the formation of a robust Ag filament. In addition, the device shows stable bipolar switching behavior in bending conditions or after hundreds of bending cycles. In our work, we provide a simple and efficient method to construct robust filament-based memristors for flexible electronics.

Low-voltage ultrafast nonvolatile memory via direct charge injection through a threshold resistive-switching layer

The explosion in demand for massive data processing and storage requires revolutionary memory technologies featuring ultrahigh speed, ultralong retention, ultrahigh capacity and ultralow energy consumption. Although a breakthrough in ultrafast floating-gate memory has been achieved very recently, it still suffers a high operation voltage (tens of volts) due to the Fowler–Nordheim tunnelling mechanism. It is still a great challenge to realize ultrafast nonvolatile storage with low operation voltage. Here we propose a floating-gate memory with a structure of MoS2/hBN/MoS2/graphdiyne oxide/WSe2, in which a threshold switching layer, graphdiyne oxide, instead of a dielectric blocking layer in conventional floating-gate memories, is used to connect the floating gate and control gate. The volatile threshold switching characteristic of graphdiyne oxide allows the direct charge injection from control gate to floating gate by applying a nanosecond voltage pulse (20 ns) with low magnitude (2 V), and restricts the injected charges in floating gate for a long-term retention (10 years) after the pulse. The high operation speed and low voltage endow the device with an ultralow energy consumption of 10 fJ. These results demonstrate a new strategy to develop next-generation high-speed low-energy nonvolatile memory.

Preparation and incorporation of NiSe@MoSe2 nano arrays in PVA matrix for resistive switching memory

In the present work, we have synthesized Nickel selenide@Molybdenum diselenide (NiSe@MoSe2) heteronanostructured arrays by a one-step hydrothermal method. The structural and optical analysis of nanopowder is done through Field emission scanning electron microscopy (FESEM), elemental mapping, Energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), UV–vis, and Photoluminescence (PL) spectroscopy. XRD reveals the formation of a hexagonal structured system with a crystallite size of 166 nm. The bandgap of NiSe@MoSe2 nanosheets is found to be 1.71 eV, calculated from UV–vis spectra. FESEM showed the homogenous growth of NiSe@MoSe2 nanosheets while electronic transitions are investigated through PL. Further, the polymer nanocomposites of NiSe@MoSe2/PVA are synthesized by the ex-situ method at six different NiSe@MoSe2 concentrations in the PVA matrix. XRD shows good interaction between NiSe@MoSe2 and PVA polymer. The crystallinity increases as concentrations of NiSe@MoSe2 in PVA increases. FESEM shows the uniform distribution of NiSe@MoSe2 nanosheets in the PVA matrix. The two-terminal Ag/NiSe@MoSe2-PVA/FTO devices have been fabricated and characterized electrically by I-V measurements. I-V shows hysteresis behavior and also, bipolar switching is observed with a maximum Ion/Ioff ratio of ~103 at 2 wt. % NiSe@MoSe2 concentration in the PVA matrix. The good stability and reliability are checked through endurance and write-read-erase-read cyclic sequences. The obtained results open up the possibility of a next-generation non-volatile resistive switching memory device.

Nonvolatile resistive memory and synaptic learning using hybrid flexible memristor based on combustion synthesized Mn-ZnO

Memristors have been seen as promising candidates for next-generation nonvolatile memory and neuromorphic computing. Solution-processed memristors can offer many advantages, such as low-cost fabrication, large-area uniformity, and high versatility. However, the high annealing temperature of the solution process and unsatisfactory performance are impeding the advancement of flexible memristors. In this work, the combustion method was proposed to fabricate the Mn-ZnO-based memristor without a high-temperature process on the flexible substrate. The device demonstrates stable bipolar resistive switching behavior with ultralow switching voltage (−0.6 V/0.71 V), high on/off ratio (˃ 2 × 103), multilevel storage capability, and uniform resistance distribution (σ/µ = 18%/16%). The flexible device with a bend radius of 10 mm shows no obvious performance degeneration compared with the device in the flat state. Furthermore, tunable repetitively synaptic weight was achieved by the memristor. Synaptic plasticity functions, such as excitatory postsynaptic current, paired-pulse facilitation, and learning/forgetting behaviors, have been emulated. This work provides a simple and feasible strategy for fabricating the flexible high-performance memristor, with the expectation to facilitate the development of flexible electronics.

(Invited, Digital Presentation) 50 Ns Ultrafast Memory Operation in 2D Heterostructured Non-Volatile Memory Device

2D heterostructures have been extensively investigated as next-generation non-volatile memory (NVM) devices. In the last decade, drastic performance improvements and further advanced functionalities have been demonstrated. However, this progress is not sufficiently supported by the understanding of their operations, obscuring the material and device structure design policy. Here, detailed operation mechanisms are elucidated by exploiting the floating gate voltage (V FG) trajectory measurements [1,2]. Systematic comparisons of MoTe2, WSe2, and MoS2 channel devices revealed that the tunneling behavior between the channel and FG is controlled by three kinds of current-limiting paths, i.e., tunneling barrier, 2D/metal contact, and pn junction in the channel. Based on the understanding of the memory operation mechanism through the V FG trajectory measurement, we propose the best all 2D NVM device structure with a direct tunneling path between source/drain electrodes and floating gate for ultrafast memory operation. Indeed, a 50 ns program/erase operation is successfully achieved [3]. Moreover, we examined the dielectric breakdown strength (E BD) of h-BN under short voltage pulses for the origin for this ultrafast operation, because a high dielectric breakdown strength allows a large tunneling current. Surprisingly, an E BD = 26.1 MV/cm for h-BN is realized under short voltage pulses, largely exceeding the E BD = ~12 MV/cm from the DC measurement. This suggests that the high E BD of h-BN can be the physical origin of the ultrafast operations. In this talk, I would like to discuss the future perspective of 2D NVM application.[1] T. Sasaki, KN, et al., "Understanding the Memory Window Overestimation of 2D Materials Based Floating Gate Type Memory Devices by Measuring Floating Gate Voltage", Small, 2020, 16, 2004907.[2] T. Sasaki, KN, et al., "Material and Device Structure Designs for 2D Memory Devices Based on the Floating Gate Voltage Trajectory", ACS nano, 2021, 15, 6658.[3] T. Sasaki, KN, et al., "Ultrafast Operation of All 2D-Heterostructured Nonvolatile Memory Devices Provided by the Strong Short-Time Dielectric Breakdown Strength of h-BN", Adv. Funct. Mater. (under revision).

Stable Resistive Switching in ZnO/PVA:MoS2 Bilayer Memristor.

Reliability of nonvolatile resistive switching devices is the key point for practical applications of next-generation nonvolatile memories. Nowadays, nanostructured organic/inorganic heterojunction composites have gained wide attention due to their application potential in terms of large scalability and low-cost fabrication technique. In this study, the interaction between polyvinyl alcohol (PVA) and two-dimensional material molybdenum disulfide (MoS2) with different mixing ratios was investigated. The result confirms that the optimal ratio of PVA:MoS2 is 4:1, which presents an excellent resistive switching behavior. Moreover, we propose a resistive switching model of Ag/ZnO/PVA:MoS2/ITO bilayer structure, which inserts the ZnO as the protective layer between the electrode and the composite film. Compared with the device without ZnO layer structure, the resistive switching performance of Ag/ZnO/PVA:MoS2/ITO was improved greatly. Furthermore, a large resistive memory window up to 104 was observed in the Ag/ZnO/PVA:MoS2/ITO device, which enhanced at least three orders of magnitude more than the Ag/PVA:MoS2/ITO device. The proposed nanostructured Ag/ZnO/PVA:MoS2/ITO device has shown great application potential for the nonvolatile multilevel data storage memory.

Epitaxial growth and phase evolution of ferroelectric La-doped HfO2 films

Hafnium-oxide-based materials are considered a promising candidate for next-generation nonvolatile memory devices owing to their good CMOS compatibility and robust ferroelectricity at the nanoscale. In this work, we synthesize highly (111)-oriented La-doped HfO2 (HLO) ferroelectric thin films via pulsed laser deposition. Furthermore, the effect of La dopant concentration, thickness, and growth temperature on the ferroelectricity of HLO films is investigated in detail. A maximum remnant polarization of ∼9 μC/cm2 is achieved for only the 5-nm-thick 3 mol. % HLO films without a wake-up process. The 180° inversion of the domain, the butterfly-shaped capacitance–voltage curve, and typical ferroelectric displacement current curve further demonstrate the robust ferroelectricity at the nanoscale. Moreover, the phase evolves from the monoclinic to the orthorhombic and subsequently to the cubic phase with increasing La concentration, which is due to the combined action of oxygen vacancy, epitaxial strain, and chemical pressure. Additionally, in the interface configuration of HLO/La0.7Sr0.3MnO3 (LSMO), the MnO20.7− layer is substituted by the HLO layer on the MnO2-terminated surface of LSMO, which can be attributed to the fact that the HLO layer with higher electronegativity replaces the MnO20.7− layer with the same electronegativity in the HLO film. Therefore, this study provides a reliable pathway for directly obtaining a lightly doped HLO ferroelectric thin film, which can help to broaden the understanding of the ferroelectric physical mechanisms with element doping.

Conduction mechanisms of ferroelectric La:HfO2 ultrathin films

Recently, ion-doped HfO2 thin films are highly desirable for the next-generation nonvolatile memories due to excellent compatibility with current complementary metal-oxide-semiconductor processes and robust ferroelectricity persisted down to the nanoscale. In this work, we study conduction mechanisms of 4 and 8 nm-thick La:HfO2 ultrathin films sandwiched between Pt and (La0.67,Sr0.33)MnO3 (LSMO) electrodes based on band alignments of the Pt/La:HfO2/LSMO, measured by x-ray photoelectron spectroscopy, and temperature-dependent current-voltage curves from 50 to 300 K. In a 4 nm-thick La:HfO2 thin-film capacitor, the conduction mechanism is found to be governed by direct tunneling at 50–100 K and phonon-assisted indirect tunneling when the temperature is further increased to 300 K in which the (LaHf4+3+)′ acceptors are served as localized states, facilitating hole hopping through the La:HfO2 barrier. When the thickness is increased to 8 nm, the tunneling through a La:HfO2 layer is suppressed, and the current-voltage character becomes rectifying, which is regulated by the dominated La:HfO2/LSMO interfacial barrier. The transport for a forward bias of the La:HfO2/LSMO barrier is found to be governed by thermionic-field emission, exhibiting a temperature-independent build-in potential of ∼2.77 V. For the reverse bias, the Fowler–Nordheim tunneling is observed. The revealing of conduction mechanisms in terms of band alignments sheds light on leakage problems and facilitates the design of HfO2-based ferroelectric devices with excellent insulating character for high-performance memory applications.

Improved analog switching characteristics of Ta2O5-based memristor using indium tin oxide buffer layer for neuromorphic computing

A memristor is defined as a non-volatile memory switching two-terminal resistor, and a memristor with digital switching characteristics is widely studied as a next-generation non-volatile memory because of its simple structure, high integration density, and low power consumption. Recently, analog memristors with gradual resistance switching (RS) characteristics have garnered great attention because of their potential to implement artificial synapses that can emulate the brain functions. Transition metal oxides are thought to be strong candidate materials for the RS. In particular, tantalum oxide (TaO x )-based memristive devices provide stable and durable switching characteristics. TaO x -based memristors utilize analog switching characteristics and have excellent durability and reliability, so they can be applied as artificial synaptic device. In this study, the characteristics of analog RS using Ta2O5-based memristive devices were investigated. The current level of the Pt/Ta2O5/Pt memristors was improved by adjusting the thickness of Ta2O5. In particular, when an indium-tin-oxide (ITO) buffer layer was added to Ta2O5 forming a Pt/ITO/Ta2O5/Pt heterostructured double-layer device, it showed more symmetrical potentiation and depression characteristics under both polarities than a single-layer device without ITO layer. The symmetrical and linear potentiation and depression characteristics are essential for the development of efficient memristor-based neuromorphic systems. Insertion of the ITO buffer layer improves linearity, symmetry, and stability of the analog RS properties of Ta2O5-based memristors to artificial synapses.