Conductive-bridge Random Access Memory Devices Research Articles

Kinetic Monte Carlo simulations on electroforming in nanomanipulated conductive bridge random access memory devices.

Conductive bridge random access memory (CBRAM) devices exhibit great potential as the next-generation nonvolatile memory devices. However, they suffer from two major disadvantages, namely relatively high power consumption and large cycle-to-cycle and device-to-device variations, which hinder their more extensive commercialization. To learn how to enhance their device performance, kinetic Monte Carlo (KMC) simulations were employed to illustrate the variation of electroforming processes in nanomanipulated CBRAM devices by introducing an ion-blocking layer with scalable nanopores and tuning the microstructures of dielectric layers. Both the size of nanopores and the inhomogeneity of dielectric layers have significant impacts on the forming processes of conductive filaments. The dielectric layer with a high-content loose texture plus the scalable nanopore-containing ion-blocking layer leads to the formation of size-controlled and uniform filaments, which remarkably contributes to miniaturizable and stable CBRAM devices. Our study provides insights into nanomanipulation strategies to realize high-performance CBRAM devices, still awaiting future experimental confirmation.

Enhanced resistive switching characteristics of conductive bridging memory device by a Co–Cu alloy electrode

One of the main challenges in the development of conductive bridging random access memory (CBRAM) is the large stochastic nature of ion movement that ultimately leads to large parameter variability. In this study, the resistive switching variability of CBRAM devices is significantly improved by employing Co–Cu alloy as the active electrode. By comparing with Pt/Ta2O5/Co devices, the Co70Cu30 alloy exhibited lower forming voltage (<2 V), lower SET voltage (<0.70 V), and faster response time (∼70 ns). The filament stability indicated by the distribution of SET/RESET voltage and high resistance state/low resistance state variation was significantly improved. Our experimental results suggest the formation of Co filaments, and the proposed mechanism is governed by the galvanic effect. In addition, a comparison between Co70Cu30 and Co30Cu70 alloys highlights that the relative proportion between Co and Cu plays an essential role in the device performance. A physical model based on different electrochemical activities of the alloys has been proposed to explain the filament formation and the improved switching uniformity in the Co70Cu30 alloy. This study not only develops a CBRAM with enhanced performance but also advances the implementation of suitable alloy systems for the application of such devices.

Real-time study of imaging electron current density on metal filament evolution in SiO2 during in situ TEM

Conductive-bridging random access memory devices are a candidate for artificial synapses for neuromorphic computing. However, there is still an incomplete understanding of the fundamentals of the filament evolution process. In this work, we study the effect of three imaging electron current densities on nanoscale filament dynamics in a model Cu/SiO2/Cu structure during in situ TEM electroforming of the device. We find that the filaments grow from the anode to the cathode in the form of discontinuous precipitates for all the imaging electron current densities. However, increasing the imaging electron current density results in a larger injection of Cu into SiO2. Comparing the results of voltage ramp tests in air, in the TEM vacuum without electron irradiation and, in the TEM vacuum with electron irradiation, we suggest a possible mechanism of filament evolution in vacuum. Specifically, we postulate a vacancy defect generation enabled injection of Cu ions into the dielectric as the mechanism behind filament evolution in vacuum that reconciles differing observations found in the literature.

Resistive Switching Characteristic of Cu Electrode-Based RRAM Device

The conductive bridge random access memory (CBRAM) device has been widely studied as a promising candidate for next-generation nonvolatile memory applications, where Cu as an electrode plays an important role in the resistive switching (RS) process. However, most studies only use Cu as one electrode, either the top electrode (TE) or the bottom electrode (BE); it is rarely reported that Cu is used as both TE and BE at the same time. In this study, we fabricated CBRAM devices by using Cu as both the TE and BE, and studied the RS characteristic of these devices. With Al2O3 as the switching layer (5~15 nm), the devices showed good bipolar RS characteristics. The endurance of the device could be as high as 106 cycles and the retention time could be as long as 104 s. The Al2O3 thickness influences the bipolar RS characteristic of the devices including the initial resistance, the forming process, endurance, and retention performance. The Cu electrode-based RRAM devices also present negative bias-suppressed complementary resistive switching (CRS) characteristics, which makes it effective to prevent the sneak path current or crosstalk problem in high-density memory array circuits.

Versatile GeS-based CBRAM with compliance-current-controlled threshold and bipolar resistive switching for electronic synapses

Chalcogenide materials have promising physical and electrical characteristics for use in advanced memory and electronic synaptic devices. However, limited research has been conducted on chalcogenide materials for highly advanced and scalable memory devices. In this study, we investigate compliance current (CC)-controlled resistive switching (RS) in a monochalcogenide GeS-based conductive bridge random access memory (CBRAM) device for use in advanced memory and electronic synapses. A CBRAM device with a structure stacking sequence of Ag/GeS/Pt/Ti/SiO2 is fabricated by depositing a 13 nm GeS layer using a sputtering process. The GeS-based CBRAM device exhibits the reversible CC-controlled transition between threshold resistive switching (T-RS) and bipolar resistive switching (B-RS). Under a high CC (1 mA and above), the device exhibits B-RS behavior with excellent retention and highly reproducible endurance characteristics, illustrated by a very high ION/IOFF ratio of almost ∼2 × 108. Under a low CC (100 µA and below), the device exhibits T-RS, with the automatic transition from a low resistance state (LRS) to a high resistance state (HRS) when sweeping the voltage in the reverse direction. Under all CC conditions, the device has a very low HRS current of around ∼1 × 10−12 A. A model based on the formation and rupture of conductive filaments (CFs) is proposed to explain the coexistence of T-RS and B-RS in GeS-based CBRAM devices. In particular, the change in the size and geometry of the CFs in accordance with the applied CC is speculated to be the main reason for the coexistence of T-RS and B-RS. The retention characteristics of the device in both T-RS and B-RS mode are investigated for possible application to case-sensitive hardware-based data security and the disposal of trusted and untrusted information. The ability of the CBRAM device in T-RS mode to perform important biological synaptic functions for potential use in brain-inspired neuromorphic systems is also investigated.

Conductive Bridge Random Access Memory (CBRAM): Challenges and Opportunities for Memory and Neuromorphic Computing Applications.

Due to a rapid increase in the amount of data, there is a huge demand for the development of new memory technologies as well as emerging computing systems for high-density memory storage and efficient computing. As the conventional transistor-based storage devices and computing systems are approaching their scaling and technical limits, extensive research on emerging technologies is becoming more and more important. Among other emerging technologies, CBRAM offers excellent opportunities for future memory and neuromorphic computing applications. The principles of the CBRAM are explored in depth in this review, including the materials and issues associated with various materials, as well as the basic switching mechanisms. Furthermore, the opportunities that CBRAMs provide for memory and brain-inspired neuromorphic computing applications, as well as the challenges that CBRAMs confront in those applications, are thoroughly discussed. The emulation of biological synapses and neurons using CBRAM devices fabricated with various switching materials and device engineering and material innovation approaches are examined in depth.

Formation of a ternary oxide barrier layer and its role in switching characteristic of ZnO-based conductive bridge random access memory devices

The insertion of a metal layer between an active electrode and a switching layer leads to the formation of a ternary oxide at the interface. The properties of this self-formed oxide are found to be dependent on the Gibbs free energy of oxide formation of the metal (ΔGf°). We investigated the role of various ternary oxides in the switching behavior of conductive bridge random access memory (CBRAM) devices. The ternary oxide acts as a barrier layer that can limit the mobility of metal cations in the cell, promoting stable switching. However, too low (higher negative value) ΔGf° leads to severe trade-offs; the devices require high operation current and voltages to exhibit switching behavior and low memory window (on/off) ratio. We propose that choosing a metal layer having appropriate ΔGf° is crucial in achieving reliable CBRAM devices.

Rational Design on Controllable Cation Injection with Improved Conductive-Bridge Random Access Memory by Glancing Angle Deposition Technology toward Neuromorphic Application.

A conductive-bridge random access memory (CBRAM) has been considered a promising candidate for the next-generation nonvolatile memory technology because of its excellent performance, for which the resistive switching behavior depends on the formation/dissolution of conducting filaments in an electrolyte layer originated by the cation injection from the active electrode with electrochemical reactions. Typically, the controllability of cations into the electrolyte layer is a main issue, leading to stable switching reliability. In this work, an architecture combining spike-shaped Ag electrodes created by Al2O3 nanopillar arrays as a physical diffusion barrier by glancing angle deposition technology was proposed to localize Ag cation injection for the formation of controllable filaments inside TiOx as the switching layer. Interestingly, the dimension of the Ag plugs defined by the topography of Al2O3 nanopillar arrays can control Ag cation injection to influence the dimensionality of conductive filaments. Compared to the typical planar-Ag/TiOx/Pt device, the spiked-Ag/Al2O3 nanopillar arrays/TiOx/Pt device shows improvement of endurance and voltage disturbance. With enhanced multilevel characteristics, the spiked active-metal-based CBRAM device can be expected to serve as an analogue synapse for neuromorphic applications.

Impact of O2 plasma treatment on novel amorphous oxide InWZnO on conductive bridge random access memory

The impact of O2 plasma treatment on novel amorphous oxide InWZnO (IWZO) as conductive bridge random access memory (CBRAM) was investigated. A high-quality film on the surface of IWZO can be obtained by using remote O2 plasma treatment. The uniformity of O2 plasma sample is better than control sample, and also the set and reset voltage are more uniform and smaller to suitable for memory operation. Moreover, the O2 plasma sample shows excellent memory performance, such as high switching endurance cycles (up to 3 × 103), long retention time for 104 s at 85 °C. These results show that the surface modification with O2 plasma on IWZO CBRAM device is a critical technique for next generation memory applications.

Multi‐Level Control of Conductive Filament Evolution and Enhanced Resistance Controllability of the Cu‐Cone Structure Embedded Conductive Bridge Random Access Memory

AbstractMemristor is considered as one of the key components of neuromorphic hardware aiming to overcome the limitations of classical von Neumann computers, but the continuous control of the device conduction remains a challenge. Random and stochastic dynamics of weak conducting filaments (CF) hinder the precise control of the resistance states, particularly in the conductive bridge random access memory (CBRAM). The problem of random and stochastic resistive switching of the Cu‐based CBRAM is largely mitigated by inserting the Cu‐cone as a cation source into the memory cell (Cu‐cone device). The electric field concentrated on the Cu‐cone induces stronger CF with a limited number near the tip region, making the device feature distinctive from the planar Cu CBRAM. The Cu‐cone device exhibits enhanced resistance controllability attributed to the well‐controlled CF, improved switching reliability improvement, and multiple intermediate resistance states. Such features are used to explore its usefulness as an artificial synapse, where fluent potentiation and depression can be achieved using voltage pulses. Further research is still required to emulate the artificial synapses fully, but the feasibility is presented in this work by exploiting the similarities between the Cu cations in the CBRAM device and the Ca2+ dynamics in a biological synapse.

Stochastic based compact model to predict highly variable electrical characteristics of organic CBRAM devices

A compact model is proposed using a stochastic approach to capture the resistive switching behavior of conductive-bridge random access memory (CBRAM) device featuring a solid polymer electrolyte consisting of Polyethylene Oxide (PEO). This model considers the statistical distribution of five electrical parameters used to describe the resistive switching observed in experimental data. A switching probability is defined to control the change of resistive state. This approach gives the model the stochastic behavior of current–voltage characteristics observed in this kind of devices. A good agreement between the simulation and the experimental curves is observed despite the unusual variability cycle-to-cycle for this type of ReRAM.

Impact of annealing environment on performance of InWZnO conductive bridge random access memory

In this paper, the characteristics of conductive bridge random access memory (CBRAM) with novel material tungsten doped indium zinc oxide (W doped InZnO, IWZO) acting as switching material in different annealing atmosphere have been investigated. The endurance cycles of annealed IWZO CBRAM devices are obviously increased to 104 cycles, while even longer retention time can be achieved at 85 °C, compared with the as-deposited IWZO CBRAM device. Besides, after annealing in nitrogen atmosphere, the uniformity of the resistance state has been greatly improved. By detailed X-ray photoelectron spectroscopy analysis, the better electrical properties can be attributed to the generated oxygen vacancies during nitrogen annealing. As a result, that notable improvement in IWZO CBRAM may show its potential for future integrated display circuit applications, just by utilizing simple annealing process.

Understanding of Selector‐Less 1S1R Type Cu‐Based CBRAM Devices by Controlling Sub‐Quantum Filament

AbstractThis study demonstrates a systematic approach to design a sub‐quantum selector‐less conductive bridge random access memory (CBRAM) which can work as one‐selector‐one‐resistor device in Cu/Ti/HfO2/TiO2/TiN material stack, can work nicely from sub‐µA to sub‐nA range. Optimized high thermal forming scheme is investigated to control the anatomy of filament formation at sub‐nA current level. The thickness of HfO2 layer plays a crucial role in determining such behavior. Hence, in this study the precise stack engineering is proposed and the selector‐less device design which can show highly stable one‐selector‐one‐resistor type performance at sub‐quantum level is identified. The TiO2‐based selector device is verified experimentally and theoretically. The presence of 1 eV deep level traps by CuO defect sites in HfO2 matrix with calculated nearest neighbor of 0.7 nm along with the presence of TiO2 selector layer, is the origin of highly nonlinear behavior. The devices show ultra‐low leakage current of 300 fA (system limitation) and operated with ultra‐low power of 5 pW, very high resistance ratio of 3 × 103, with high nonlinearity of 3 × 103. This work establishes the possibility to design ultra‐low power nonvolatile sub‐quantum CBRAM device which can fulfill the needs of internet of things applications after optimization.

Annealing effects on resistive switching of IGZO-based CBRAM devices

In this study, the impact of post-oxide deposition annealing on performance of IGZO-based conductive-bridging random access memory (CBRAM) is reported. It is found that besides the distinct reduction in resistive switching parameters of SET/RESET voltages, their dispersions, and the resistance ratio of high-resistance state to low-resistance state can be improved after N2 annealing. The annealing effects on enhancing of the resistive switching properties are investigated by x-ray photoelectron spectra. It can be considered that the formation of the filament is better controlled by the increase of oxygen vacancies in the switching layer, which is the main reason for the improvement of resistive switching characteristics.

Dependence of InGaZnO and SnO2 thin film stacking sequence for the resistive switching characteristics of conductive bridge memory devices

We have investigated the switching mechanism of conductive bridge random access memory (CBRAM) with Ag/SnO2/Pt, Ag/InGaZnO(IGZO)/Pt and their hybrid oxide devices with different stacking sequence (Ag/SnO2/IGZO/Pt and Ag/IGZO/SnO2/Pt). Typical bipolar resistive switching is observed in single layered devices and an Ag/SnO2/IGZO/Pt hybrid device. Interestingly, a stable and reproducible unipolar resistive switching is observed for a hybrid device with a stacking sequence of Ag/IGZO/SnO2/Pt. This result suggests that the staking sequence of dielectrics in the IGZO and SnO2 electrolyte determines unipolar or bipolar switching. The different switching types in the hybrid electrolyte are based on different migration or diffusion rates of Ag ions in the solid electrolyte and redox reaction rates at the electrodes. And as compared to single layered devices, the hybrid structured devices exhibit low operation voltages, higher ION/IOFF ratio, uniform switching cycles and better endurance and retention characteristics. The results and switching mechanisms demonstrated here in hybrid devices can be extended to other hybrid devices based on CBRAM device.

Highly durable and flexible gallium-based oxide conductive-bridging random access memory

The flexible conductive-bridging random access memory (CBRAM) device using a Cu/TiW/Ga2O3/Pt stack is fabricated on polyimide substrate with low thermal budget process. The CBRAM devices exhibit good memory-resistance characteristics, such as good memory window (>105), low operation voltage, high endurance (>1.4 × 102 cycles), and large retention memory window (>105). The temperature coefficient of resistance in the filament confirms that the conduction mechanism observed in the Ga2O3 layer is similar with the phenomenon of electrochemical metallization (ECM). Moreover, the performance of CBRAM device will not be impacted during the flexibility test. Considering the excellent performance of the CBRAM device fabricated by low-temperature process, it may provide a promising potential for the applications of flexible integrated electronic circuits.

Indium Diffusion Behavior and Application in HfO2‐Based Conductive Bridge Random Access Memory

This study proposes the design of conductive bridge random access memory (CBRAM) using an indium electrode, which diffuses into hafnium oxide. The device is found to exhibit good operating characteristics, maintaining a large operation window, low set and reset voltages, and advantageous operation speed, thereby reducing operating power consumption. Electrical experiments, transmission electron microscopy, and energy‐dispersive spectroscopy confirm that the device exhibits the characteristics of a CBRAM device. In addition, due to the large operation window of this In electrode device, a combination of two of these CBRAM can act as a complementary resistive switching (CRS) device with a large memory window.

Controllable growth of conductive filaments in sandwiched CBRAM cells using self-assembled Cu/W multilayer thin films as the electrodes

Self-assembled Cu/W multilayer thin films are fabricated and used as the top electrodes (TEs) in conductive-bridge random access memory (CBRAM) devices. The modulation period (λ) of the Cu/W multilayer structure can be well controlled by changing the substrate rotation speed during magnetron sputtering. For the λ value of 3.2 nm, the Al2O3 based CBRAM devices exhibits significantly improved resistive switching (RS) performances with effectively narrowed switching parameters and substantially increased OFF/ON ratio up to 104 at 0.10 V. Good reliability after 600 cycles is demonstrated and the retention time of 10 years at 85 °C is estimated through Arrhenius plot. In essential, the growth of conductive filaments (CFs) in Al2O3 layers is well controlled owing to the W-rich layer as the diffusion barrier. As a result, both the number and the size of CFs in Al2O3 layer are reduced if the λ value is increased. Especially, volatile RS behavior is evidenced due to the extremely slow diffusion of Cu ions across the TE/Al2O3 interface and thus the formation of tiny CFs if the λ value is up to 7.0 nm. A physical model is proposed to describe the controllable formation of CFs by using Cu/W multilayer TEs.

Failure Thresholds in CBRAM Due to Total Ionizing Dose and Displacement Damage Effects

With the growing interest to explore Jupiter’s moons, technologies with +10 Mrad(Si) tolerance are now needed, to survive the Jovian environment. Conductive-bridging random access memory (CBRAM) is a nonvolatile memory that has shown a high tolerance to total ionizing dose (TID). However, it is not well understood how CBRAM behaves in an energetic ion environment where displacement damage (DD) effects may also be an issue. In this paper, the response of CBRAM to 100-keV Li, 1-MeV Ta, and 200-keV Si ion irradiations is examined. Ion bombardment was performed with increasing fluence steps until the CBRAM devices failed to hold their programed state. The TID and DD dose (DDD) at the fluence of failure were calculated and compared against tested ion species. Results indicate that failures are more highly correlated with TID than DDD. DC cycling tests were performed during 100-keV Li irradiations and evidence was found that the mobile Ag ion supply diminished with increasing fluence. The cycling results, in addition to prior 14-MeV neutron work, suggest that DD may play a role in the eventual failure of a CBRAM device in a combined radiation environment.

Role of Al2O3 thin layer on improving the resistive switching properties of Ta5Si3-based conductive bridge random accesses memory device

Ta5Si3-based conductive bridge random access memory (CBRAM) devices have been investigated to improve their resistive switching characteristics for their application in future nonvolatile memory technology. Changes in the switching characteristics by the addition of a thin Al2O3 layer of different thicknesses at the bottom electrode interface of a Ta5Si3-based CBRAM devices have been studied. The double-layer device with a 1 nm Al2O3 layer has shown improved resistive switching characteristics over the single layer one with a high on/off resistance ratio of 102, high endurance of more than 104 cycles, and good retention for more than 105 s at the temperature of 130 °C. The higher thermal conductivity of Al2O3 over Ta5Si3 has been attributed to the enhanced switching properties of the double-layer devices.