Bipolar Resistive Random Access Memory Research Articles

Nonvolatile bipolar resistive switching characteristics of aluminum oxide grown by thermal oxidation processes

This study proposes a bipolar resistive random-access memory (RRAM), which is fabricated using an aluminum oxide (AlO x ) resistive switching (RS) layer. The RRAM shows a large memory window of 106 at a low read voltage of 0.5 V. In addition, high switching speed, long retention time, and superior read-disturb immunity are observed. AlO x layers are prepared by a thermal oxidation growth process. Aluminum metal films deposited on n+-Si wafers are oxidized at O2/(O2 + N2) flow rate ratios of 50%–100%. Al/AlO x /n+-Si device shows no RS behavior when the AlO x is grown in a pure O2 environment. As the O2/(O2 + N2) flow rate ratio decreases to 50%, Al/AlO x :N/n+-Si device reveals stable bipolar RS characteristics. A filamentary mode based on oxygen interstitial and Al vacancy is proposed to explain the difference in electrical characteristics of AlO x devices prepared at different O2 flow rates.

The Resistance Analysis Attack and Security Enhancement of the IMC LUT Based on the Complementary Resistive Switch Cells

The resistive random access memory (RRAM) based in-memory computing (IMC) is an emerging architecture to address the challenge of the “memory wall” problem. The complementary resistive switch (CRS) cell connects two bipolar RRAM elements anti-serially to reduce the sneak current in the crossbar array. The CRS array is a generic computing platform, for the arbitrary logic functions can be implemented in it. The IMC CRS LUT consumes fewer CRS cells than the static CRS LUT. The CRS array has built-in polymorphic characteristics because the correct logic function cannot be distinguished based on the circuit layout. However, the logic state of every CRS cell can be readout after each operation. It helps the attacker to recover the correct function of the IMC CRS LUT. This work discusses the resistance analysis attack of the IMC LUT based on the CRS array. The proposed resistance analysis attack method is able to be applied to different computation styles based on the CRS array, such as the CRS IMPLY, CRS NOR-OR/NAND-AND, and so on. The attacker can recover the logic function of the LUT by tracing the states of CRS cells. Furthermore, an improved IMC CRS LUT method is proposed and discussed to enhance security. The simulation and analysis results show that the improved IMC CRS LUT can resist various attacks, and it maintains the polymorphic characteristics of the IMC CRS LUT. And the N-bit full adder circuit based on the improved IMC CRS NOR-OR LUTs achieves the best performance compared with the previous counterparts.

Sol-Gel-Processed Y2O3-Al2O3 Mixed Oxide-Based Resistive Random-Access-Memory Devices.

Herein, sol-gel-processed Y2O3-Al2O3 mixed oxide-based resistive random-access-memory (RRAM) devices with different proportions of the involved Y2O3 and Al2O3 precursors were fabricated on indium tin oxide/glass substrates. The corresponding structural, chemical, and electrical properties were investigated. The fabricated devices exhibited conventional bipolar RRAM characteristics without requiring a high-voltage forming process. With an increase in the percentage of Al2O3 precursor above 50 mol%, the crystallinity reduced, with the amorphous phase increasing owing to internal stress. Moreover, with increasing Al2O3 percentage, the lattice oxygen percentage increased and the oxygen vacancy percentage decreased. A 50% Y2O3-50% Al2O3 mixed oxide-based RRAM device exhibited the maximum high-resistance-state/low-resistance-state (HRS/LRS) ratio, as required for a large readout margin and array size. Additionally, this device demonstrated good endurance characteristics, maintaining stability for approximately 100 cycles with a high HRS/LRS ratio (>104). The HRS and LRS resistances were also retained up to 104 s without considerable degradation.

Sol-Gel-Processed Y2O3 Multilevel Resistive Random-Access Memory Cells for Neural Networks.

Yttrium oxide (Y2O3) resistive random-access memory (RRAM) devices were fabricated using the sol-gel process on indium tin oxide/glass substrates. These devices exhibited conventional bipolar RRAM characteristics without requiring a high-voltage forming process. The effect of current compliance on the Y2O3 RRAM devices was investigated, and the results revealed that the resistance values gradually decreased with increasing set current compliance values. By regulating these values, the formation of pure Ag conductive filament could be restricted. The dominant oxygen ion diffusion and migration within Y2O3 leads to the formation of oxygen vacancies and Ag metal-mixed conductive filaments between the two electrodes. The filament composition changes from pure Ag metal to Ag metal mixed with oxygen vacancies, which is crucial for realizing multilevel cell (MLC) switching. Consequently, intermediate resistance values were obtained, which were suitable for MLC switching. The fabricated Y2O3 RRAM devices could function as a MLC with a capacity of two bits in one cell, utilizing three low-resistance states and one common high-resistance state. The potential of the Y2O3 RRAM devices for neural networks was further explored through numerical simulations. Hardware neural networks based on the Y2O3 RRAM devices demonstrated effective digit image classification with a high accuracy rate of approximately 88%, comparable to the ideal software-based classification (~92%). This indicates that the proposed RRAM can be utilized as a memory component in practical neuromorphic systems.

Integration of Non-Filamentary Pr0.7Ca0.3MnO3– Based Memristor With Silicon-PN Junction

Crossbar memristor arrays are gaining interest in the semiconductor industry for high-density storage and in-memory computing applications. For efficient array operations, highly non-linear selector (1S) or transistor (1T) devices are connected in series with memristor devices to rectify sneak path leakage currents. For selector-memristor integration, process and device compatibility is a must. In this work, we have experimentally demonstrated the integration of Pr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.7</sub> Ca <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.3</sub> MnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (PCMO) based resistive random access memory (RRAM) and Silicon (Si) junction devices. We propose and evaluate various integration schemes for Si-PN junction Diode (1D) and PCMO-RRAM (1R) to identify a successful approach where both devices retain their characteristics after integration. Moreover, the electrical measurements for the integrated 1D1R structure show the SET switching of PCMO-RRAM. Lastly, we show bipolar RRAM switching analogous to 1S1R configuration using an anti-parallel diode connection. The integration approach can be used to design a scaled-1S1R or 1T1R cell for large-scale crossbar memristor arrays with PCMO-RRAM.

Effects of Top and Bottom Electrodes Materials and Operating Ambiance on the Characteristics of MgFx Based Bipolar RRAMs.

The effects of electrode materials (top and bottom) and the operating ambiances (open-air and vacuum) on the MgFx-based resistive random-access memory (RRAM) devices are studied. Experiment results show that the device's performance and stability depend on the difference between the top and bottom electrodes' work functions. Devices are robust in both environments if the work function difference between the bottom and top electrodes is greater than or equal to 0.70 eV. The operating environment-independent device performance depends on the surface roughness of the bottom electrode materials. Reducing the bottom electrodes' surface roughness will reduce moisture absorption, minimizing the impact of the operating environment. Ti/MgFx/p+-Si memory devices with the minimum surface roughness of the p+-Si bottom electrode show operating environment-independent electroforming-free stable resistive switching properties. The stable memory devices show promising data retentions of >104 s in both environments with DC endurance properties of more than 100 cycles.

Thickness dependence of resistive switching characteristics of the sol–gel processed Y2O3 RRAM devices

In this study, yttrium oxide (Y2O3)-based resistive random-access memory (RRAM) devices were fabricated using the sol–gel method. The fabricated Y2O3 RRAM devices exhibited conventional bipolar RRAM device characteristics and did not require a forming process. The Y2O3 film thickness was controlled by varying the liquid-phase precursor concentration. As the concentration increased, thicker Y2O3 films were formed. In addition, the concentration of oxygen vacancies increased. The RRAM device properties were not observed for thin Y2O3 films, which had the lowest oxygen vacancy concentration. Moreover, RRAM devices, which consisted of the thickest Y2O3 films with the largest oxygen vacancy concentration, showed poor non-volatile properties. The optimized Y2O3-based RRAM devices with a thickness of 37 nm showed conventional bipolar RRAM device characteristics, which did not require an initial forming process. The fabricated RRAM devices showed a high resistance state to low resistance state ratio of over 104, less than +1.5 V of SET voltage, and −15.0 V of RESET voltage. The RRAM devices also showed promising non-volatile memory properties, without significant degradation after 103 s retention and 102 cycle endurance tests.

Improved Environment Stability of Y2O3 RRAM Devices with Au Passivated Ag Top Electrodes.

In this study, we fabricated sol-gel-processed Y2O3-based resistive random-access memory (RRAM) devices. The fabricated Y2O3 RRAM devices exhibited conventional bipolar RRAM device characteristics and did not require the forming process. The long-term stability of the RRAM devices was investigated. The Y2O3 RRAM devices with a 20 nm thick Ag top electrode showed an increase in the low resistance state (LRS) and high resistance state (HRS) and a decrease in the HRS/LRS ratio after 30 days owing to oxidation and corrosion of the Ag electrodes. However, Y2O3 RRAM devices with inert Au-passivated Ag electrodes showed a constant RRAM device performance after 30 days. The 150 nm-thick Au passivation layer successfully suppressed the oxidation and corrosion of the Ag electrode by minimizing the chance of contact between water or oxygen molecules and Ag electrodes. The Au/Ag/Y2O3/ITO RRAM devices exhibited more than 300 switching cycles with a decent resistive window (>103). They maintained constant LRS and HRS resistances for up to 104 s, without significant degradation of nonvolatile memory properties for 30 days while stored in air.

Vacuum and Low-Temperature Characteristics of Silicon Oxynitride-Based Bipolar RRAM.

This study investigates the switching characteristics of the silicon oxynitride (SiOxNy)-based bipolar resistive random-access memory (RRAM) devices at different operating ambiances at temperatures ranging from 300 K to 77 K. The operating ambiances (open air or vacuum) and temperature affect the device’s performance. The electroforming-free multilevel bipolar Au/Ni/SiOxNy/p+-Si RRAM device (in open-air) becomes bilevel in a vacuum with an on/off ratio >104 and promising data retention properties. The device becomes more resistive with cryogenic temperatures. The experimental results indicate that the presence and absence of moisture (hydrogen and hydroxyl groups) in open air and vacuum, respectively, alter the elemental composition of the amorphous SiOxNy active layer and Ni/SiOxNy interface region. Consequently, this affects the overall device performance. Filament-type resistive switching and trap-controlled space charge limited conduction (SCLC) mechanisms in the bulk SiOxNy layer are confirmed.

Flexible Sol-Gel-Processed Y2O3 RRAM Devices Obtained via UV/Ozone-Assisted Photochemical Annealing Process.

Flexible indium tin oxide (ITO)/Y2O3/Ag resistive random access memory (RRAM) devices were successfully fabricated using a thermal-energy-free ultraviolet (UV)/ozone-assisted photochemical annealing process. Using the UV/ozone-assisted photochemical process, the organic residue can be eliminated, and thinner and smother Y2O3 films than those formed using other methods can be fabricated. The flexible UV/ozone-assisted photochemical annealing process-based ITO/Y2O3/Ag RRAM devices exhibited the properties of conventional bipolar RRAM without any forming process. Furthermore, the pure and amorphous-phase Y2O3 films formed via this process showed a decreased leakage current and an increased high-resistance status (HRS) compared with the films formed using other methods. Therefore, RRAM devices can be realized on plastic substrates using a thermal-energy-free UV/ozone-assisted photochemical annealing process. The fabricated devices exhibited a resistive window (ratio of HRS/low-resistance status (LRS)) of >104, with the HRS and LRS values remaining almost the same (i.e., limited deterioration occurred) for 104 s and up to 102 programming/erasing operation cycles.

Effects of the Operating Ambiance and Active Layer Treatments on the Performance of Magnesium Fluoride Based Bipolar RRAM.

This study investigates switching characteristics of the magnesium fluoride (MgFx)-based bipolar resistive random-access memory (RRAM) devices at different operating ambiances (open-air and vacuum). Operating ambiances alter the elemental composition of the amorphous MgFx active layer and Ti/MgFx interface region, which affects the overall device performance. The experimental results indicate that filament type resistive switching takes place at the interface of Ti/MgFx and trap-controlled space charge limited conduction (SCLC) mechanisms is dominant in both the low and high resistance states in the bulk MgFx layer. RRAM device performances at different operating ambiances are also altered by MgFx active layer treatments (air exposure and annealing). Devices show the better uniformity, stability, and a higher on/off current ratio in vacuum compared to an open-air environment. The Ti/MgFx/Pt memory devices have great potential for future vacuum electronic applications.

Switching characteristic of fabricated nonvolatile bipolar resistive switching memory (ReRAM) using PEDOT: PSS/GO

In this study, a bipolar resistive switching random access memory cell (ReRAM) by blending PEDOT: PSS into Graphene Oxide (GO) with 1:1 wt% is reported. To compare the switching mechanism, the memory devices based on GO and PEDOT: PSS are distinctly fabricated and characterized too. The spin coated active layers are sandwiched between ITO and Al as the bottom and top electrodes, respectively. The resistive switching mechanism in PEDOT: PSS/GO memory is based on filamentary type. The SET and RESET operation of the PEDOT: PSS/GO memory is relatively symmetric but GO and PEDOT: PSS samples have a significant asymmetric hysteresis curve. The carrier transport mechanism of the bistable behaviour for the fabricated non-volatile ReRAM is described based on space charge limited current (SCLC). In our experiments, the structure ITO/PEDOT:PSS-GO/Al demonstrated good switching behaviour with approximately 100 on/off current ratio, 3.5 V set/reset voltage and, a high retention time of 6000 s. In addition, the crystallinity and surface topography of the deposited thin film was characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), and scanning electron microscopy (SEM). The simple structure and low-cost fabrication offer embedding in data storage and computing application as future electronics and printed electronics.

Sol-gel-processed amorphous-phase ZrO2 based resistive random access memory

In this study, sol–gel-processed amorphous-phase ZrO2 was used as an active channel material to improve the resistive switching properties of resistive random access memories (RRAMs). ITO/ZrO2/Ag RRAM devices exhibit the properties of bipolar RRAMs. The effect of the post-annealing temperature on the electrical properties of the ZrO2 RRAM was investigated. Unlike the ZrO2 films annealed at 400 and 500 °C, those annealed at 300 °C were in amorphous phase. The RRAM based on the amorphous-phase ZrO2 exhibited an improved high-resistance state (HRS) to low-resistance state ratio (over 106) as well as promising retention and endurance characteristics without deterioration. Furthermore, its disordered nature, which causes efficient carrier scattering, resulted in low carrier mobility and the lowest leakage current, influencing the HRS values.

A bidirectional threshold switching selector with a symmetric multilayer structure

Selectors have been proposed as a highly effective tool for suppressing substantial leakage currents without sacrificing the high density of resistive random-access memory (RRAM) crossbar arrays. Among various selector types, the programmable metallization cell (PMC) selector is promising due to its simple structure and high selectivity. In this work, we demonstrate a new PMC selector that exhibits bidirectional threshold switching behavior by implementing symmetric multilayer dielectrics. The proposed Ag/SiTe/HfO$_{2}$/SiTe/Ag selector device has a low off current ($ 10^{5}$), and low threshold voltage variation ($<$ 0.05). Upon connection to a bipolar RRAM cell via a wire, the proposed selector successfully suppresses the leakage current of an unselected device below the threshold voltage.

Electroforming-free flexible organic resistive random access memory based on a nanocomposite of poly(3-hexylthiophene-2,5-diyl) and orange dye with a low threshold voltage

The charge trapping characteristics of an organic nanocomposite based on two polymers, poly(3-hexylthiophene-2,5-diyl) (P3HT) and orange dye (OD), were investigated by fabricating a sandwiched structure on a flexible polyethylene terephthalate substrate. The fabricated flexible organic device with a configuration of silver (Ag)/P3HT–OD/Ag displayed highly stable results consistent with a bipolar non-volatile resistive random access memory (RRAM) device using simple and controllable fabrication technology. It was observed that by the addition of OD, low threshold voltages (V th) of V set = 1.5 V and V reset = −1.5 V were achieved owing to the values of work function and energy bandgap of the materials used. Electrical rewritable effects of the fabricated memory device were tested for 102 voltage sweeps and 104s without any considerable degradation of its bistable resistive states. The switching mechanism was considered to be space charge limited current and this was verified by plotting the double logarithmic I–V curve. This is the first RRAM device based on P3HT polymer whose mechanical robustness has been tested for its mechanical robustness by bending it for 25 bending cycles at a bending diameter ranging from 15 cm to 5 cm without any considerable change in its properties. This finding offers important guidelines for reproducing next-generation flexible organic nanocomposite-based memory devices that are simple to fabricate and have low operating voltages and highly stable memory behavior.

Improved Intrinsic Nonlinear Characteristics of Ta2O5/Al2O3-Based Resistive Random-Access Memory for High-Density Memory Applications.

The major hindrance for high-density application of two-terminal resistive random-access memory (RRAM) array design is unintentional sneak path leakage through adjacent cells. Herein, we propose a bilayer structure of Ta2O5/Al2O3-based bipolar type RRAM by evaluating the intrinsic nonlinear characteristics without integration with an additional transistor and selector device. We conducted X-ray photoelectron spectroscopy (XPS) analysis with different etching times to verify Ta2O5/Al2O3 layers deposited on the TiN bottom electrode. The optimized nonlinear properties with current suppression are obtained by varying Al2O3 thickness. The maximum nonlinearity (~71) is achieved in a Ta2O5/Al2O3 (3 nm) sample. Furthermore, we estimated the comparative read margin based on the I-V characteristics with different thicknesses of Al2O3 film for the crossbar array applications. We expect that this study about the effect of the Al2O3 tunnel barrier thickness on Ta2O5-based memristors could provide a guideline for developing a selector-less RRAM application.

Nanoscale Side-Contact Enabled Three Terminal Pr0.7Ca0.3MnO3 Resistive Random Access Memory for In-Memory Computing

Resistive random-access memory (RRAM) devices that can execute logic are promising for in-memory computing. In state-of-the-art, a complementary resistive switch (CRS) bit-cell of two back-to-back bipolar RRAM shows a universal logic gate by multiple sequential write/compute operations using a resistance-based input/output (R-R logic) along with an extra read cycle. Here, we propose and experimentally demonstrate a three-terminal RRAM (3T-RRAM) to enable logic operations with voltage input and resistance output (V-R logic) in a single device and single write/compute cycle without a dedicated read cycle. Essentially, a nanoscale (~20nm) side-contact is added to a typical two-terminal Pt/Pr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.7</sub> Ca <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.3</sub> MnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (PCMO)/W RRAM. As the PCMO RRAM is non-filamentary, all the terminals can act as in-out terminals. Hence, we utilize the 3 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rd</sup> terminal to read the state of the device while the other two terminals simultaneously performing the logic operation. Finally, we demonstrate the Implication (IMP) and converse IMP logic operations using 3T-RRAM. Such a device can enable area and energy-efficient in-memory computation.

Realization of Self-Compliance Resistive Switching Memory via Tailoring Interfacial Oxygen

Self-compliance set switching (SCSS) offers the promise of a selector-less resistive random access memory (RRAM) implementation on flexible substrates, with great application in integrated flexible electronics. SCSS has been realized in RRAM devices with a series oxide layer incorporated into the memory stack. The series oxide acts as an in-built resistor, limiting the increase of the current during set transition. In this study, we show that SCSS can also be achieved in a bipolar RRAM cell without a series oxide layer, i.e., consisting of only a single switching oxide layer. This study reveals that oxygen pileup at the anode interface during the set evolution plays a crucial role in SCSS. The accumulation of oxygen gives rise to the increase of the switching oxide resistance, partially compensating the decrease of the filament resistance, and modulates the conduction barrier at the anode/oxide interface, which self-arrests the increase of the set switching current. Our results show interface engineering as a possible route for enabling SCSS in an RRAM device without the need for a complicated stack structure and careful thickness optimization.

Design of selector-based insulator-metal transition model for TiO2 bipolar resistive random access memory

In this study, a NbO2-based selector was designed that can change high resistance states to low resistance states due to the insulator–metal transition (IMT). A one-selector-one-resistor cell for a 3D crossbar array composed of this selector and a TiN/TiO2/TiN bipolar resistive random access memory (RRAM) in series was modeled using the COMSOL finite element multiphysics software package. First, the temperature dependencies of the electrical conductivity (σ), thermal conductivity (kth), and mass specific heat (CP) were used to compare two IMT selectors, which showed that the NbO2-based selector better matched the RRAM due to its appropriate hysteresis width (hw), threshold voltage (Vth), and low off-state current (Ioff). Second, the effect of size variations for the NbO2 on the electrical performance of the IMT selector was investigated. Vth, hold voltage (Vhold), threshold current (Ith), Ioff, and Ith/Ioff ratio of the IMT selector were sensitive to thickness (L) and filament radius (r) of NbO2. L and r were designed by comparing the electrical characteristics of the NbO2 selectors of different sizes to make a connection with the TiO2-based bipolar RRAM. Finally, a TiN/NbO2/TiN/TiO2/TiN stack structure was simulated, and the results show a significantly reduced leakage current (1 μA), high read window (15.8), and improved readout margin (&amp;gt;27 word lines), which are suitable for next-generation high-density memory devices. This study provides guidance for the design of selectors connected to the RRAM, which is beneficial for circuit designs and process manufacturing.

Chemical vapor deposition synthesis of sp2-BN film on 2-inch Si substrates for bipolar resistive random access memory device

Resistive random access memory (RRAM) devices with Au/sp2-BN/p-Si sandwich structure have been successfully fabricated. Single precursor ammonia borane (NH3BH3) chemical vapor deposition (CVD) was used to directly grow amorphous sp2-BN film on a 2-inch commercial p-type Si substrate without any extra catalyst. The sp2-BN-based RRAM device exhibit reproducible switching endurance (>1000 cycles), long retention time (>1 × 104 s), capability to operate under appropriate voltages (between 2.225 V and 1.475 V for SET process) as well as large area uniformity (2 inch). The resistance value of low resistance state (LRS) is as high as 3 × 103 Ω and the typical VSET/ISET and VRESET/IRESET are 4.292 × 10−4 A @ 1.925 V and 6.143 × 10−4 A @ −0.862 V, respectively. A boron vacancies (VB) induced mechanism similar to valence change memory (VCM) has been used to explain the parameters of noble Au/sp2-BN/Si RRAM device such as, RESET stop voltage, temperature and device area dependence of resistances (both low & high states). This work will propel the combination of low power consumption in sp2-BN based RRAM with Si based electronics devices.