Resistive Switching Random Access Memory Devices Research Articles

Vacancy associates-rich ultrathin nanosheets for high performance and flexible nonvolatile memory device.

On the road of innovation in modern information technology, resistive switching random access memory (RRAM) has been considered to be the best potential candidate to replace the conventional Si-based technologies. In fact, the key prerequisite of high storage density and low power consumption as well as flexibility for the tangible next generation of nonvolatile memories has stimulated extensive research into RRAM. Herein, we highlight an inorganic graphene analogue, ultrathin WO3·H2O nanosheets with only 2-3 nm thickness, as a promising material to construct a high performance and flexible RRAM device. The abundant vacancy associates in the ultrathin nanosheets, revealed by the positron annihilation spectra, act not only carrier reservoir to provide carriers but also capture center to trap the actived Cu(2+) for the formation of conductive filaments, which synergistically realize the resistive switching memory with low operating voltage (+1.0 V/-1.14 V) and large resistance ON/OFF ratio (>10(5)). This ultrathin-nanosheets-based RRAM device also shows long retention time (>10(5) s), good endurance (>5000 cycles), and excellent flexibility. The finding of the existence of distinct defects in ultrathin nanosheets undoubtedly leads to an atomic level deep understanding of the underlying nature of the resistive switching behavior, which may serve as a guide to improve the performances and promote the rapid development of RRAM.

Improved Bipolar Resistive Switching Memory Characteristics in Ge<SUB>0.5</SUB>Se<SUB>0.5</SUB> Solid Electrolyte by Using Dispersed Silver Nanocrystals on Bottom Electrode

Resistive switching random-access memory (ReRAM) devices based on chalcogenide solid electrolytes have recently become a promising candidate for future low-power nanoscale nonvolatile memory application. The resistive switching mechanism of ReRAM is based on the formation and rupture of conductive filament (CF) in the chalcogenide solid electrolyte layers. However, the random diffusion of metal ions makes it hard to control the CF formation, which is one of the major obstacles to improving device performance of ReRAM devices. We demonstrate the spin-coated metal nanocrystals (NCs) enhance the bipolar resistive switching (BRS) memory characteristics. Compared to the Ag/Ge0.5Se0.5/Pt structure, excellent resistive switching memory characteristics were obtained from the Ag/Ge0.5Se0.5/Ag NCs/Pt structure. Ag NCs improve the uniformity of resistance values and reduce the reset voltage and current. A stable DC endurance (> 100 cycles) and a high data retention (> 10(4) sec) were achieved by spin coating the Ag NCs on the Pt bottom electrode for ReRAMs.

Resistive switching characteristics of HfO2-based memory devices on flexible plastics.

In this study, we examine the characteristics of HfO2-based resistive switching random access memory (ReRAM) devices on flexible plastics. The Pt/HfO2/Au ReRAM devices exhibit the unipolar resistive switching behaviors caused by the conducting filaments. From the Auger depth profiles of the HfO2 thin film, it is confirmed that the relatively lower oxygen content in the interface of the bottom electrode is responsible for the resistive switching by oxygen vacancies. And the unipolar resistive switching behaviors are analyzed from the C-V characteristics in which negative and positive capacitances are measured in the low-resistance state and the high-resistance state, respectively. The devices have a high on/off ratio of 10(4) and the excellent retention properties even after a continuous bending test of two thousand cycles. The correlation between the device size and the memory characteristics is investigated as well. A relatively smaller-sized device having a higher on/off ratio operates at a higher voltage than a relatively larger-sized device.

Characteristic Evolution From Rectifier Schottky Diode to Resistive-Switching Memory With Al-Doped Zinc Tin Oxide Film

We demonstrate a metal sandwiched Al-doped zinc tin oxide (AZTO) thin-film device to exhibit a characteristic evolution process from Schottky junction diode to resistive-switching random access memory (RRAM) applications. The proposed TiN/Ti/AZTO/Pt device can initially show good rectifying characteristics and high forward-bias current for Schottky diodes. After applying with an electrically triggered forming process, the transition of electrical behavior occurs and evolves from the diode to RRAM characteristics. The RRAM device exhibits the coexistence of bipolar and unipolar resistive-switching modes through the positive-bias forming and reversed-bias forming process, respectively. In addition, the RRAM device with bipolar mode can perform the functionality of multilevel cell storage, while the one with unipolar mode shows stable resistive-switching performance. Furthermore, one-transistor and one-resistor (1T1R) architecture with an RRAM cell connected with a thin-film transistor (TFT) device is developed in this paper. The TFT device using AZTO film as an active channel layer performs good electrical characteristics for a driver in the 1T1R operation scheme. The integration of AZTO-based electronic devices has great potential for increasing the application diversity of metal oxide AZTO thin film as well as the flexibility of circuit design in the emerging optoelectronic technologies.

A Physics-Based Compact Model of Metal-Oxide-Based RRAM DC and AC Operations

A physics-based compact model of metal-oxide-based resistive-switching random access memory (RRAM) cell under dc and ac operation modes is presented. In this model, the conductive filament evolution corresponding to the resistive switching process is modeled by considering the transport behaviors of oxygen vacancies and oxygen ions together with the temperature effect. Both the metallic-like and electron hopping conduction transports are considered to model the conduction of RRAM. The model can reproduce both the typical I-V characteristics of RRAM in high-/low-resistance state (LRS) and the nonlinear characteristics in LRS. Moreover, to accurately model ac operation mode, the effects of parasitic capacitance and resistance are included in our model. The developed compact model is verified and calibrated by measured data in different HfOx-based RRAM devices under dc and ac operation modes. The excellent agreement between the model predictions and experimental results shows a promising prospect of the future implementation of this compact model in large-scale circuit simulation to optimize the design of RRAM.

Effect of Ni3+ concentration on the resistive switching behaviors of NiO memory devices

Resistive switching characteristics of NiO resistive switching random access memory (RRAM) are investigated which are affected by Ni3+ concentration. In order to obtain different concentration of Ni3+, we annealed the RRAM devices with a series of temperatures. We measured the Ni3+ concentrations and the typical I–V curves of the samples. It was found the resistive switching properties became worse when the annealing temperature was over 650 °C. Furthermore, we got an optimum performance at 550 °C, which indicated that the appropriate concentration of Ni3+ enhanced the resistive switching behaviors.

Complementary Switching in Oxide-Based Bipolar Resistive-Switching Random Memory

Resistive-switching random access memory (RRAM) devices utilizing a crossbar architecture represent a promising alternative for Flash replacement in high-density data storage applications. However, RRAM crossbar arrays require the adoption of diodelike select devices with high on-off -current ratio and with sufficient endurance. To avoid the use of select devices, one should develop passive arrays where the nonlinear characteristic of the RRAM device itself provides self-selection during read and write. This paper discusses the complementary switching (CS) in hafnium oxide RRAM, where the logic bit can be encoded in two high-resistance levels, thus being immune from leakage currents and related sneak-through effects in the crossbar array. The CS physical mechanism is described through simulation results by an ion-migration model for bipolar switching. Results from pulsed-regime characterization are shown, demonstrating that CS can be operated at least in the 10-ns time scale. The minimization of the reset current is finally discussed.

Resistive Switching by Voltage-Driven Ion Migration in Bipolar RRAM—Part I: Experimental Study

Resistive-switching random access memory (RRAM) based on the formation and the dissolution of a conductive filament (CF) through insulating materials, e.g., transition metal oxides, may find applications as novel memory and logic devices. Understanding the resistive-switching mechanism is essential for predicting and controlling the scaling and reliability performances of the RRAM. This paper addresses the set/reset characteristics of RRAM devices based on <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{HfO}_{x}$</tex></formula> . The set process is analyzed as a function of the initial high-resistance state and of the current compliance. The reset process is studied as a function of the initial low-resistance state. Finally, the intermediate set states, obtained by set at variable compliance current, and reset states, obtained by reset at variable stopping voltage, are characterized with respect to their reset voltage, allowing for a microscopic interpretation of intermediate states in terms of different filament morphologies.

Evidence for Voltage-Driven Set/Reset Processes in Bipolar Switching RRAM

Understanding the physical mechanisms for resistance change in metal oxides is a key challenge to assess the scalability of resistive-switching random access memory (RRAM) devices. From this standpoint, the time dependence of filament formation and dissolution in metal oxides can provide a useful insight into the fundamental mechanism of resistive switching. In this paper, we show an experimental study of the time-dependent filament growth and of the voltage dependence of set/reset times in HfO_x-based RRAM devices. The voltage across the device is shown to be regulated at any given time irrespective of the compliance current and the applied voltage, evidencing that voltage is the controlling parameter for the filament formation and dissolution during switching. These results are explained in terms of a thermally-activated ion migration model for filamentary switching. The model allows for an analytical calculation of the scaling dependence of set/reset times and energies in RRAM.

Defect engineering: reduction effect of hydrogen atom impurities in HfO2-based resistive-switching memory devices

In this study, we propose a new and effective methodology for improving the resistive-switching performance of memory devices by high-pressure hydrogen annealing under ambient conditions. The reduction effect results in the uniform creation of oxygen vacancies that in turn enable forming-free operation and afford uniform switching characteristics. In addition, H+ and mobile hydroxyl (OH−) ions are generated, and these induce fast switching operation due to the higher mobility compared to oxygen ions. Defect engineering, specifically, the introduction of hydrogen atom impurities, improves the device performance for metal–oxide-based resistive-switching random access memory devices.

Unipolar resistive switching and mechanism in Gd-doped-TiO2-based resistive switching memory devices

Gd-doped-TiO2-based resistive switching random-access memory (RRAM) devices with MOM structure of top metal electrode (W, Ti)/Gd-doped-TiO2/Pt/Ti/SiO2/Si were fabricated, and resistive switching characteristics were investigated. The W/Gd-doped-TiO2/Pt RRAM device showed typical unipolar resistive switching behavior irrespective of the bias polarity on the W top electrode. However, no unipolar switching was observed in the Ti/Gd-doped-TiO2/Pt devices when positive bias was applied on the Ti top electrode. A new resistive switching model was proposed to explain the unipolar switching behavior in the W/Gd-doped-TiO2/Pt RRAM device, based on the formation and destruction of conducting filaments which result from pinning and unpinning effects of surface trap states of oxide film on the Fermi level of metal electrode. The model clearly elucidates the mechanism of the low resistance state (LRS) and the high resistance state (HRS) as well as the switching behavior between LRS and HRS in the W/Gd-doped-TiO2/Pt RRAM device. The model also indicates the important role of the metal electrode material in unipolar resistive switching of oxide-based RRAM devices.

Reset Instability in Pulsed-Operated Unipolar Resistive-Switching Random Access Memory Devices

Resistive-switching random access memory (RRAM) devices are attracting increasing interest as a potential candidate for high-density nonvolatile memory devices. One of the main issues toward RRAM feasibility is the reduction of the reset current <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">reset</sub> necessary to restore the high-resistance state in the device. <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">reset</sub> can be reduced by controlling the size of the conductive filament responsible for the low-resistance state; however, available data only focus on direct-current reset analysis. This letter addresses <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">reset</sub> reduction under pulsed operation. Unstable reset behaviors, including set-reset and set instability, are shown to occur during relatively fast pulses and starting from set states with relatively large resistance values. These instability effects limit <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">reset</sub> reduction, posing a potential issue of minimum reset current achievable in RRAM devices.

Resistive Switching Characteristics of Solution-Processed Transparent TiOx for Nonvolatile Memory Application

We propose a solution-processed transparent -based resistive switching random access memory (ReRAM) device. Electronically active was prepared by sol–gel spin coating of a titanium(IV) isopropoxide precursor on an indium tin oxide-coated glass. The prepared film is completely transparent in the visible range and has an amorphous structure. The fabricated -based ReRAM device exhibits distinct resistive switching under consecutive dc voltage sweeps of . The device also exhibits good memory performance, including fast switching speed with a pulse width of , stable pulse endurance over 1000 cycles, and excellent retention characteristics at up to . In addition, based on the plot and X-ray photoelectron spectroscopy analysis, we postulate that the fabricated device is operated by the reversible formation/rupture of the conducting filament in the oxygen-deficient layer.