Unipolar Resistive Switching Research Articles

Three-dimensional crossbar arrays of self-rectifying Si/SiO2/Si memristors

Memristors are promising building blocks for the next-generation memory and neuromorphic computing systems. Most memristors use materials that are incompatible with the silicon dominant complementary metal-oxide-semiconductor technology, and require external selectors in order for large memristor arrays to function properly. Here we demonstrate a fully foundry-compatible, all-silicon-based and self-rectifying memristor that negates the need for external selectors in large arrays. With a p-Si/SiO2/n-Si structure, our memristor exhibits repeatable unipolar resistance switching behaviour (105 rectifying ratio, 104 ON/OFF) and excellent retention at 300 °C. We further build three-dimensinal crossbar arrays (up to five layers of 100 nm memristors) using fluid-supported silicon membranes, and experimentally confirm the successful suppression of both intra- and inter-layer sneak path currents through the built-in diodes. The current work opens up opportunities for low-cost mass production of three-dimensional memristor arrays on large silicon and flexible substrates without increasing circuit complexity.

Thermal assisted reset modelling in nickel oxide based unipolar resistive switching memory

This paper utilizes an analytical and a filament dissolution model to calculate the local temperature increase in conducting filaments (CFs) of pulsed laser ablated unipolar NiO resistive switching memory devices. Electrical current voltage characteristics indicate unipolar switching. The formation of NiO phases is confirmed from the X-ray diffraction study. Transmission electron microscopy confirms the polycrystalline nature of NiO films having a thickness of ∼20 nm. Electrothermal simulations based on the filament dissolution model are performed using COMSOL Multiphysics® to model the CF rupture during the reset transition in the samples owing to the Joule heating effect. Obtained temperature profiles from the simulations are compared with the analytical model. Both the models corroborate with each other, allowing us to closely approximate the maximum temperature across the CF (Tcrit). This is the point corresponding to which the voltage applied across the cell (Vreset) drives the device into the reset state. The effect of annealing temperature on the maximum temperature, reset voltage, and CF diameter of the device is also discussed. The CF diameter and area of the filament are precisely estimated from the simulation.

Ce-doping induced enhancement of resistive switching performance of Pt / NiFe 2 O 4 / Pt memory devices

Abstract Ce-doping NiFe2O4 spinel ferrite thin films were prepared on Pt / Ti / SiO2 / Si substrates using a chemical solution deposition method. It has been observed that Ce-doping induced enhancement of unipolar resistive switching properties, such as uniform switching voltages, enlarged ON / OFF ratio, and long retention in Pt / NiFe2O4 / Pt memory devices. The dominant conduction mechanisms in the thin film devices were Ohmic conduction at low resistance state and lower voltage region of high resistance state, while Schottky emission dominated at higher voltage region in high resistance state. The physical mechanism of resistive switching is related to the formation and rupture of conducting filament. The improved stability of the switching parameters for the devices can be attributed to Ce-doping minimizing random formation and rupture of conductive filament. This study indicates that doping rare earth ions in ferrite thin films would be an effective approach for obtaining stable resistance switching memory devices.

Unipolar resistive switching properties of Pr-doped ZnO thin films

Abstract Pr-doped ZnO hexagonal structure thin films with c-axis preferred orientation were deposited on Pt/Ti/SiO 2 /Si substrates by chemical solution deposition technique. The effects of Pr doping amount on the resistive switching behavior of Pt/Zn 1−x Pr x O/Pt (x = 0, 0.01, 0.03, and 0.05) memory cells were investigated. The results showed that Pr-doping lowered the c-axis orientation degree of the ZnO thin films, but improved the resistive switching properties of Pt/Zn 1−x Pr x O/Pt devices. The resistive switching devices exhibited good endurance, long retention, and uniform switching voltages. I-V characteristics and their temperature dependence analysis indicated that the conduction mechanism of LRS was Ohmic behavior, and that of HRS at relatively higher voltage is trap-controlled space charge limited current. The physical origin of the resistive switching can be referred to the formation and rupture of the oxygen vacancies related filaments.

Bipolar switching properties and electrical conduction mechanism of manganese oxide RRAM devices

Abstract Electrical conduction mechanism and bipolar resistive switching properties of the manganese oxide thin films resistive random access memory (RRAM) devices for high resistive status and low resistive status (HRS/LRS) were investigated and discussed. The electrical and physical properties of post-treatment manganese oxide thin films for different oxygen concentration parameters were measured by x-ray diffraction (XRD) and operation current versus applied voltage ( I-V ) measurement. For I-V curves in different temperature, the ohmic conduction, space charge limited conduction (SCLC), schottky emission conduction, and hopping conduction mechanism of manganese oxide RRAM devices were discussed and verified in initial metallic filament path forming process. Finally, the excess oxygen ions, vacancy, and defect factor resulting in bipolar and unipolar resistive switching behavior of thin films RRAM devices will be explained and investigated by metallic filament path forming model.

Double‐Layer‐Stacked One Diode‐One Resistive Switching Memory Crossbar Array with an Extremely High Rectification Ratio of 109

A double‐layer‐stacked 1 diode‐1 resistor (1D1R) cross‐bar array (CBA) resistance switching random access memory is fabricated. The TiO2‐based Schottky diode and the unipolar resistance switching TiO2 comprise the cell selector and nonvolatile memory components, respectively. All the fabrication processes are performed near room temperature through physical vapor deposition processes, and the performance degradation by the thermal budget is well suppressed. As a result, a rectification ratio as high as 1.4 × 109 is achieved from the appropriately cycled diode, which can last up to 108 cycles in the integrated structure. Such highly promising performance is confirmed from both the upper and lower memory layers, which confirm the possible route for the multistacked memory structure. The two‐bit‐per‐cell operation allows an effective minimum cell area of F2, which is confirmed from the randomly accessed cells to be completely free from adverse interference through the adoption of an extremely‐high‐performance diode. The present 1D1R integration technique will eventually allow the fabrication of an extremely high‐integration‐density (above 1 Gb per block) CBA with the help of the two‐port sensing scheme. Such a high‐performance CBA device is a feasible contender for memory‐dominant computation.

Resistive switching properties and physical mechanism of europium oxide thin films

A forming-free resistive switching effect was obtained in Pt/Eu2O3/Pt devices in which the Eu2O3 thin films were fabricated by a chemical solution deposition method. The devices show unipolar resistive switching with excellent switching parameters, such as high resistance ratio (107), stable resistance values (read at 0.2 V), low reset voltage, good endurance, and long retention time (up to 104 s). On the basis of the analysis of the current–voltage (I–V) curves and the resistance-temperature dependence, it can be concluded that the dominant conducting mechanisms were ohmic behavior and Schottky emission at low resistance state and high resistance state, respectively. The resistive switching behavior could be explained by the formation and rupture of conductive filament, which is related to the abundant oxygen vacancies generated in the deposition process. This work demonstrates the great potential opportunities of Eu2O3 thin film in resistive switching memory applications, which might possess distinguished properties.

Self-Compliant Bipolar Resistive Switching in SiN-Based Resistive Switching Memory

Here, we present evidence of self-compliant and self-rectifying bipolar resistive switching behavior in Ni/SiNx/n+ Si and Ni/SiNx/n++ Si resistive-switching random access memory devices. The Ni/SiNx/n++ Si device’s Si bottom electrode had a higher dopant concentration (As ion > 1019 cm−3) than the Ni/SiNx/n+ Si device; both unipolar and bipolar resistive switching behaviors were observed for the higher dopant concentration device owing to a large current overshoot. Conversely, for the device with the lower dopant concentration (As ion < 1018 cm−3), self-rectification and self-compliance were achieved owing to the series resistance of the Si bottom electrode.

Unipolar nonvolatile memory devices based on the composites of poly(9-vinylcarbazole) and zinc oxide nanoparticles

The application of poly(9-vinylcarbazole) (PVK) to make a composite with zinc oxide (ZnO) as the active layer has been reported. Unipolar resistive switching behaviors were observed from ITO/PVK + ZnO/Al memory devices. The reset voltages were higher than the set voltages. These devices present a low resistance state (LRS)/high resistance state (HRS) current ratio of 104 when read at −0.5 V, retaining the information for a time of 105 s. The bistable resistive switching behaviors were entirely steady within 104 cycles. The fitted results of I–V curves shown that the dominant conduction mechanisms in LRS and HRS were Ohmic conductive behavior and space-charge-limited current mechanism, respectively.

Observation of the Ni2O3 phase in a NiO thin‐film resistive switching system

We propose that the Ni2O3 phase is the conducting filament in a unipolar NiO resistive switching system. The NiO film was grown by pulsed laser deposition using platinum top and bottom electrodes. Electron diffraction and high‐resolution transmission electron microscopy were used to explore the microstructure. This system had forming‐free properties, which suggested that the filaments were introduced in the pristine state. The Ni2O3‐phase nanostructures were connected and disconnected in the pristine‐ and OFF‐state samples, respectively, which implied that the phase acted as the conducting channel. X‐ray photoemission spectroscopy results showing Ni3+ bonding in the pristine‐state film support the importance of the Ni2O3 phase in the switching.Observed Ni2O3 phase (indicated by the red line) in the low resistance state of a pristine sample.

Effect of internal field on the high resistance state retention of unipolar resistance switching in ferroelectric vanadium doped ZnO

We report the effect of an internal field on the high resistance state (HRS) retention of unipolar switching in ferroelectric vanadium (V) doped ZnO thin films. ZnO thin films doped with 1%, 3%, and 5% of V were found to have an increased internal field as the V concentration increased. The effect of an internal field on resistance switching was observed from the lower set voltage and shorter high resistance state retention time. A physical model was applied to explain the relationship between the internal field and the HRS retention, and a good agreement was obtained with the experimental data. Our result suggested that the internal field can reduce the activation energy of the redox process for generating oxygen vacancies, which subsequently affect the formation of conducting filaments in the resistance switching process.

Resistive switching properties of HfxZr1−xO2 thin films for flexible memory applications

The flexible unipolar resistive switching characteristics of Au/Ni/HZO/Au devices on Polyethylene terephthalate substrates have been investigated for the RRAM applications. The devices demonstrated a bistable and reproducible unipolar RS behavior with a high OFF/ON ratio about 103, and the memory widow could be maintained in repetitive programming/erase at least 1000 cycles. The retention property has no degradation at 6.3 × 104 seconds. The current–voltage characteristics of the HZO samples show that the Ohmic contact and space charge limited current are suggested to response for the low resistance state and high resistance state, respectively. Combined with the conductance mechanism, the resistive switching behaviors can be explained by the conductive filaments model. The RS mechanism is attributed to restore and rupture caused by the joule heating and the redox reaction induced by the external electron injection. The memory devices also show good mechanical flexibility. It is believed that the HZO-based memory device has great potential to be used in high performance, flexible memory applications.

Enhanced Unipolar Resistive Switching Characteristics of Hf0.5Zr0.5O2 Thin Films with High ON/OFF Ratio.

A metal–insulator–metal structure resistive switching device based on H0.5Z0.5O2 (HZO) thin film deposited by pulse laser deposition (PLD) has been investigated for resistive random access memory (RRAM) applications. The devices demonstrated bistable and reproducible unipolar resistive switching (RS) behaviors with an extremely high OFF/ON ratio over 5400. The retention property had no degradation at 6 × 104 s. The current–voltage characteristics of the HZO samples showed a Schottky emission conduction in the high voltage region (Vreset < V < Vset), while at the low voltage region (V < Vreset), the ohmic contact and space charge limited conduction (SCLC) are suggested to be responsible for the low and high resistance states, respectively. Combined with the conductance mechanism, the RS behaviors are attributed to joule heating and redox reactions in the HZO thin film induced by the external electron injection.

Observation of AlO x material in electrical resistive switching for nonvolatile random access memory application

We fabricated an Al / AlO x / Al device by using a RF magnetron sputter system. The device showed a unipolar resistive switching process. In this study, the switching mechanism of the device followed the conductive filament model. The conduction mechanisms for the conductive filament model were explained by using Ohmic conduction for the low resistance state (LRS) and Schottky emission for the high resistance state (HRS). The average value of the resistance ratio between the HRS and the LRS was about 3.48 × 107 when the reading voltage (0.1 V) was achieved. The electrical property of the endurance was achieved under 50 switching cycles. A low switching voltage could be obtained for a low power consuming device. These results proved that the AlO x material has various possibilities for use in nonvolatile random access memory applications.

Unipolar resistive switching in cobalt titanate thin films

We report giant resistive switching of the order of 104, long-time charge retention characteristics up to 104 s, non-overlapping SET and RESET voltages, Ohmic in low-resistance state (LRS) and space charge limited current (SCLC) mechanism in high-resistance state (HRS) properties in polycrystalline perovskite cobalt titanate thin films. Impedance spectroscopy study was carried out for both LRS and HRS states which illustrates that only bulk resistance changes after resistance switching, however, there is a small change (<10% which is in the pF range) in the bulk capacitance value in both states. These results suggest that in the LRS state current filaments break the capacitor in many small capacitors in a parallel configuration which, in turn, provides the same capacitance in both states even if there was a 90-degree change in phase angle and an order of change in the tangent loss.

An All-Organic Composite System for Resistive Change Memory via the Self-Assembly of Plastic-Crystalline Molecules.

An all-organic composite system was introduced as an active component for organic resistive memory applications. The active layer was prepared by mixing a highly polar plastic-crystalline organic molecule (succinonitrile, SN) into an insulating polymer (poly(methyl methacrylate), PMMA). As increasing concentrations of SN from 0 to 3.0 wt % were added to solutions of different concentrations of PMMA, we observed distinguishable microscopic surface structures on blended films of SN and PMMA at certain concentrations after the spin-casting process. The structures were organic dormant volcanos composed of micron-scale PMMA craters and disk type SN lava. Atomic force microscopy (AFM), cross-sectional transmission electron microscopy (TEM), scanning electron microscopy (SEM), and energy dispersive X-ray spectrometer (EDX) analysis showed that these structures were located in the middle of the film. Self-assembly of the plastic-crystalline molecules resulted in the phase separation of the SN:PMMA mixture during solvent evaporation. The organic craters remained at the surface after the spin-casting process, indicative of the formation of an all-organic composite film. Because one organic crater contains one SN disk, our system has a coplanar monolayer disk composite system, indicative of the simplest composite type of organic memory system. Current-voltage (I-V) characteristics of the composite films with organic craters revealed that our all-organic composite system showed unipolar type resistive switching behavior. From logarithmic I-V characteristics, we found that the current flow was governed by space charge limited current (SCLC). From these results, we believe that a plastic-crystalline molecule-polymer composite system is one of the most reliable ways to develop organic composite systems as potential candidates for the active components of organic resistive memory applications.

Unipolar Nonvolatile Resistive Switching in Pt/MgO/Ta/Ru Structures Deposited by Magnetron Sputtering.

The recent realization of memristors, nanodevices exhibiting non-volatile resistive switching, has sparked tremendous interest for applications in fields such as nonvolatile memories. Here we report unipolar resistive switching in Pt/MgO/Ta/Ru structures, with an oxide barrier thickness of only 15 nm. No electroforming process was required to achieve resistive switching and an ohmic conduction mechanism is associated with the ON state. We observed an inverse dependence of the ON state resistance on the SET current compliance and average values of 1.61 V and 1.38 V for the SET and RESET voltages, respectively. We show the stability of the switching for over 40 cycles and a clear separation of the ON (10¹ Ω) and OFF (10² Ω) states during at least 10⁴ s.

Unipolar resistive switching behavior in sol–gel synthesized FeSrTiO3 thin films

A robust and reproducible resistance switching in iron substituted strontium titanate is reported which shows giant high to low resistance state ratio (∼105) and stable charge retention.

Permanent Data Storage in ZnO Thin Films by Filamentary Resistive Switching.

Resistive memories are considered the most promising candidates for the next generation of non-volatile memory; however, attention has so far been limited to rewritable memory features for applications in resistive random access memories (RRAM). In this article, we provide a new insight into the applicability of resistive memories. The characteristics of non-rewritable resistive memories (NRRM) were investigated. Devices with Pt/ZnO/ITO architecture were prepared using magnetron sputtering, upon which various bipolar and unipolar resistive switching tests were performed. The results showed excellent distinction between the high resistance state (HRS) and low resistance state (LRS), with RHRS/RLRS = 5.2 × 1011 for the Pt/ZnO/ITO device with deposition time of 1 h. All samples were stable for more than 104 s, indicating that the devices have excellent applicability in NRRMs.

Unipolar resistive switching characteristics and scaling behaviors in La2Mo2O9 thin films for nonvolatile memory applications

La2Mo2O9 (LMO) thin films have been deposited on Pt/Ti/SiO2/Si substrates by pulsed laser deposition and the resistive switching (RS) characteristics of the Au/LMO/Pt devices has been investigated. The Au/LMO/Pt devices show excellent unipolar RS characteristics with high resistance ratio between high resistance state and low resistance state (LRS), good endurance, and retention performances. The results of temperature dependence of resistance and x-ray photoelectron spectroscopy suggest that the observed RS characteristics can be explained by the formation and rupture of conducting filaments composed of oxygen vacancies. Furthermore, the plot of the reset current (IR) as a function of the third harmonic coefficient (B0) caused by Joule heating during the reset process shows scaling behavior with a power law of IR∝B0−δ. The IR and reset power (PR) can also be scaled to the resistance in LRS (R0), i.e., IR(PR)∝R0−α(β). The observed scaling behaviors indicate the importance of the Joule heating for the RS characteristics of Au/LMO/Pt devices. These results demonstrate the potential application of LMO thin film in a nonvolatile memory device.