Conductive Bridge Random Access Memory Research Articles

Enhanced analog synaptic behavior of SiNx/a-Si bilayer memristors through Ge implantation

Conductive bridging random access memory (CBRAM) has been considered to be a promising emerging device for artificial synapses in neuromorphic computing systems. Good analog synaptic behaviors, such as linear and symmetric synapse updates, are desirable to provide high learning accuracy. Although numerous efforts have been made to develop analog CBRAM for years, the stochastic and abrupt formation of conductive filaments hinders its adoption. In this study, we propose a novel approach to enhance the synaptic behavior of a SiNx/a-Si bilayer memristor through Ge implantation. The SiNx and a-Si layers serve as switching and internal current limiting layers, respectively. Ge implantation induces structural defects in the bulk and surface regions of the a-Si layer, enabling spatially uniform Ag migration and nanocluster formation in the upper SiNx layer and increasing the conductance of the a-Si layer. As a result, the analog synaptic behavior of the SiNx/a-Si bilayer memristor, such as the nonlinearity, on/off ratio, and retention time, is remarkably improved. An artificial neural network simulation shows that the neuromorphic system with the implanted SiNx/a-Si memristor provides a 91.3% learning accuracy mainly due to the improved linearity.

Improvement of Data Retention Characteristics By Inserting Ta Adhesion Layer to Ionic Liquid-Supplied Conducting-Bridge Memory

Previously, we reported on the memory characteristics of ionic liquid-supplied conducting-bridge random access memory (IL-CBRAM)1. IL-CBRAM is a kind of CBRAM devices in which ionic liquid (IL) is confined in an artificially formed pore in the SiO2 layer of Cu/SiO2/Pt stuck structure. The resistive switching phenomenon is caused by formation and rupture of Cu filaments consisting of electrochemically eluted Cu from the Cu electrode (EL). Although IL-CBRAM has excellent features such as reduction of operating voltages and their variations, there is a serious problem that IL addition makes data retention in low resistance state (LRS) poor. When two metals with different EL potentials (EPs) are in contact in the presence of solvents, it is known that the corrosion of the metal with lower EP proceeds. Therefore, we considered that the abovementioned problem of poor data retention might come from the configuration of LRS itself, that is, the configuration that Cu filaments with low EP directly contact to Pt-EL with high EP.In this study, we demonstrated that data retention characteristics in LRS were drastically improved by inserting the Ta adhesion layer between the SiO2 and Pt-EL. It was suggested that the Ta layer that had lower EP than those of both Cu and Pt provided electrons to Pt-EL, suppressing the corrosion of Cu filaments.Figure 1(a) shows the schematic diagram of IL-CBRAM device. IL-CBRAM has a stack structure of Cu (50 nm)/Cu- or Ag-IL (30 nm)/Pt (20 nm), and IL was supplied to an artificially introduced pore with the diameter of 1 μm in SiO2 layer. Here, Cu- and Ag-IL are 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) amide ([bmim][Tf2N]) solution containing 0.4 M Cu(Tf2N)2 and 0.4 M Ag(Tf2N), respectively2. The Ta layer with the thickness, d, of 1 nm or 5 nm is inserted as an adhesion layer between the SiO2 and Pt layers. Figures 1(b) and (c) show schematics when filaments are formed in IL-CBRAM at d = 1 and 5 nm, respectively. Cu filaments were observed to be formed on the wall surface of the pore that was introduced in the SiO2 layer1. Since the hole penetrates not only the SiO2 layer, but also the Ta layer, the Cu filament contacts both the Pt and Ta layers.Figure 2 shows data retention characteristics of IL-CBRAM in LRS, for d = 1 and 5 nm. The compliance current to prevent excessive growth of filament was set to 200 μA. Data retention time is less than 103 s for d = 1 nm, whereas more than 104 s for d = 5 nm (still working). After formation of Cu filaments connecting the Cu- and Pt-ELs, the corrosion of Cu filaments with lower standard EP (SEP) should proceed, because of large difference of SEPs between Cu (0.34 V) and Pt (1.2 V). The drastic improvement of the retention characteristics for d = 5 nm suggests that the insertion of the Ta layer that has lowest SEP of -0.81 V provides electrons to Pt instead of Cu, suppressing the corrosion of Cu filaments. On the other hand, the Ta layer with d = 1 nm is considered to be oxidized completely during the SiO2 deposition (Fig. 1(b)) in contrast to a case of thicker d of 5 nm (Fig. 1(c)). The completely oxidized Ta layer no longer provide electrons and cannot contribute to an improvement of data retention. It is also important that the surface of Ta layer except for the interface with Pt is oxidized for the 5 nm thick Ta layer. This prevents corrosion of the Ta layer itself. Acknowledgement This work was supported by NAGASE & CO., LTD. Colors & Advanced Processing Dept. Reference [1] H. Sato, et. al., 14th IEEE NMDC, Stockholm, Sweden, program booklet, pp.171-172, 1698674, Oct. 27-30, 2019.[2] A. Harada, et. al., J. Mater. Chem., 4, pp. 7215-7222 (2016). Figure 1

Effect of tungsten doping on the variability of InZnO conductive-bridging random access memory

The characteristics of conductive-bridging random access memory (CBRAM) with amorphous indium-tungsten-zinc-oxide (a-InWZnO) switching layer and copper (Cu) ion—supply layer were prepared by sputtering. It was found that the doping ratio of tungsten has a significant effect on the memory characteristics of the CBRAM, and the doping of tungsten acts as a suppressor of oxygen vacancies in the InWZnO film. The O 1s binding energy associated with the oxygen-deficient regions in the α-InWZnO thin film decreases with increasing tungsten doping ratio, which can be demonstrated by x-ray photoelectron spectroscopy. When the tungsten doping ratio is 15%, the a-InWZnO CBRAM can achieve the excellent memory characteristics, such as high switching endurance (up to 9.7 × 103 cycling endurance), low operating voltage, and good retention capability. Moreover, the electrical uniformity and switching behavior of InWZnO device are evidently improved as the doping ratio of tungsten in the switching layer increases. These results suggest that CBRAM based on novel material InWZnO have great potential to be used in high-performance memory devices.

Electronically Rewritable Chipless RFID Tags Fabricated Through Thermal Transfer Printing on Flexible PET Substrates

We present an electronically rewritable chipless RFID tag based on integrated nonvolatile Conductive Bridging Random Access Memory (CBRAM) RF switches. CBRAM is a nonvolatile memory technology, which is identified as an innovative RF switching solution in this decade. Reconfigurable resonator or RF encoding particle (REP) used in this tag is a closed-loop resonator integrated with two switches each to tune its electrical length, and encodes three states each, to represent more than one bit per REP. Presented tag is composed of three such REPs to represent a total of 27 unique states. This chipless RFID tag with CBRAM switches could be written/rewritten electronically using DC voltage pulses, to encode a desired code from a given combination. Thermal impression transfer printing is utilized for printing metallic layers of the presented chipless tag. Complete fabrication steps of presented tag with integrated CBRAM switches are carried out without any “clean room” processes. An insight to working mechanism of presented rewritable REPs using electrical models is included in this article. Proof of concept of a potential hybrid data encoding technique combining frequency shift coding and Radar Cross Section (RCS) magnitude level coding, through experimental studies, for presented REPs, is also given herewith.

Impact of La–OH bonds on the retention of Co/LaSiO CBRAM

Co/LaSiO conducting bridge random access memory is a promising candidate for low power storage class memories due to its high endurance and short switching pulse width. Moisture has been hypothesized to be an important parameter in determining device retention. In this study, we anneal the LaSiO layers at different temperatures and pressures in order to reduce the absorbed moisture and improve retention. We evidence by Fourier transform infrared and x-ray reflectivity that the moisture content does not change with these anneal conditions. However, we find that increasing the amount of La–OH bonds is an effective way to improve the low resistance state retention.

Emulation of Biological Synapse Characteristics from Cu/AlN/TiN Conductive Bridge Random Access Memory.

Here, we present the synaptic characteristics of AlN-based conductive bridge random access memory (CBRAM) as a synaptic device for neuromorphic systems. Both non-volatile and volatile memory are observed by simply controlling the strength of the Cu filament inside the AlN film. For non-volatile switching induced by high compliance current (CC), good retention with a strong Cu metallic filament is verified. Low-resistance state (LRS) and high-resistance state (HRS) conduction follow metallic Ohmic and trap-assisted tunneling (TAT), respectively, which are supported by I–V fitting and temperature dependence. The transition from long-term plasticity (LTP) to short-term plasticity (STP) is demonstrated by increasing the pulse interval time for synaptic device application. Also, paired-pulse facilitation (PPF) in the nervous system is mimicked by sending two identical pulses to the CBRAM device to induce STP. Finally, potentiation and depression are achieved by gradually increasing the set and reset voltage in pulse transient mode.

Investigating the origins of ultra-short relaxation times of silver filaments in forming-free SiO2-based conductive bridge memristors

The threshold switching effect is considered of outmost importance for a variety of applications ranging from the reliable operation of crossbar architectures to emulating neuromorphic properties with artificial neural networks. This property is strongly believed to be associated with the rich inherit dynamics of a metallic conductive filament (CF) formation and its respective relaxation processes. Understanding the origin of these dynamics is very important in order to control the degree of volatility and design novel electronic devices. Here, we present a synergistic numerical and experimental approach in order to deal with that issue. The distribution of relaxation time is addressed through time-resolved pulse measurements whereas the entire switching behavior is modeled through a 2D dynamical model by taking into account the destructive interference of the drift/diffusion transport mechanisms and the Soret diffusion flux due to the intense local Joule heating. The proposed mechanism interprets successfully both the threshold to bipolar switching transition as well as the self-rectifying effects in SiO2-based memories. The model incorporates the effect of electrode materials on the switching pattern and provides a different perception of the ionic transport processes, shading light into the ultra-small lifetimes of the CF and explaining the different behavior of the silver or copper active materials in a conductive bridge random access memory architecture.

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.

Cluster-Type Filaments Induced by Doping in Low-Operation-Current Conductive Bridge Random Access Memory.

Conductive bridge random access memory (CBRAM) is one of the most representative emerging nonvolatile memories in virtue of its excellent performance on speed, high-density integration, and power efficiency. Resistive switching behaviors in CBRAM involving the formation/rupture of metallic conductive filaments are dominated by cation migration and redox processes. It is all in the pursuit to decrease the operation current for low-power consumption and to enhance the current compliance-dependent reliability. Here, we propose a novel structure of Pt/TaOx:Ag/TaOx/Pt with nonvolatile switching at ∼1 μA and achieve a five-resistance-state multilevel cell operation under different compliance currents. Different from the nanocone-shaped filaments reported in traditional Ag top electrode devices, cluster-type filaments were captured in our memory devices, explaining the low-operation current-resistive switching behaviors. Meanwhile, Cu-doped counterpart devices also display similar operations. Such memory devices are more inclined to achieve low-power consumption and offer feasibility to large-scale memory crossbar integration.

Structure and charge transport of amorphous Cu-doped Ta2O5 : An ab initio study

In this paper, we present ab initio computer models of Cu-doped amorphous Ta2O5 , a promising candidate for Conducting Bridge Random Access Memory (CBRAM) memory devices, and study the structural, electronic, charge transport and vibrational properties based on plane-wave density functional methods. We offer an atomistic picture of the process of phase segregation/separation between Cu and Ta2O5 subnetworks. Electronic calculations show that the models are conducting with extended Kohn-Sham orbitals around the Fermi level. In addition to that, we also characterize the electronic transport using the Kubo-Greenwood formula modified suitably to calculate the space-projected conductivity (SPC). Our SPC calculations show that Cu clusters and under-coordinated Ta adjoining the Cu are the conduction-active parts of the network. We also report information about the dependence of the electrical conductivity on the connectivity of the Cu sub-matrix. Vibrational calculations for one of the models has been undertaken with an emphasis on localization and animation of representative modes.

Conductive Filament Localization Within Crossbar Resistive Memories by Scanning Joule Expansion Microscopy

Control of filament growth within conductive bridge random access memories (CBRAM) is of crucial interest in order to ensure the reliability of such emerging devices. Here, we demonstrate that Scanning Joule Expansion Microscopy (SJEM) can be used to detect and precisely localize conductive filaments within operating crossbar CBRAM devices. Flexible memory devices based on Pd/Al2O3/Ag stacks are first elaborated at low temperature on polyimide substrate. These devices show set and reset operations at low voltage ( $4.3~\mu \text{m}$ is extracted at 50kHz and it is also demonstrated that the thermal expansion signal is proportional to the dissipated Joule power. We believe that the proposed procedure opens the way to reliability studies that can be applied to any family of memory device based on filamentary conduction.

Spatial Confinement Effects of Embedded Nanocrystals on Multibit and Synaptic Properties of Forming Free SiO2-Based Conductive Bridge Random Access Memory

In this work we examine the position effect of embedded Pt nanocrystals (NCs) within a conductive bridge resistive switching memory (CBRAM) based on SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> which results in self-rectifying properties, low-voltage and more reliable operation. With this material configuration, we derive a large memory window, good spatial and temporal variability, robust pulse endurance and charge retention capabilities. Depending on the applied pulse characteristics we also exhibit low power 4-bit multilevel capabilities along with synaptic properties. The results are compared against a similar device without embedded NCs, gaining insights regarding the spatial confinement of silver nanofilaments by the presence of metallic NCs and its beneficial impact on device operation.

Effect of Annealing Treatment on Performance of Ga2O3 Conductive-Bridging Random-Access Memory

The effect of annealing on the performance of conductive-bridging random-access memory (CBRAM) with an amorphous gallium oxide (a-Ga2O3) switching layer has been studied. After annealing at 200°C in N2, a significant improvement of the bipolar resistive switching characteristics was observed. The a-Ga2O3 CBRAM showed good memory performance, with high switching endurance (up to 7.5 × 102 cycles) and high thermal stability. The resistive switching stability and electrical uniformity were also obviously improved. The memory window remained above 105 over 7.5 × 102 endurance cycles at 85°C. Material characterization by x-ray photoelectron spectroscopy suggested that the improved uniformity of the resistive switching in the N2-annealed Ga2O3 films may result from a higher density of oxygen vacancies. These results indicate great potential for such devices in future high-density nonvolatile memory applications.

Performance Improvement of Conductive Bridging Random Access Memory by Electrode Alloying

Large parameter variability is identified as one of the main problems in conductive bridging random access memory (CBRAM) at the device level impeding large-scale commercial applications. Here, we ...

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.

Controlling Cu Migration on Resistive Switching, Artificial Synapse, and Glucose/Saliva Detection by Using an Optimized AlO x Interfacial Layer in a-CO x -Based Conductive Bridge Random Access Memory.

The Cu migration is controlled by using an optimized AlOx interfacial layer, and effects on resistive switching performance, artificial synapse, and human saliva detection in an amorphous-oxygenated-carbon (a-COx)-based CBRAM platform have been investigated for the first time. The 4 nm-thick AlOx layer in the Cu/AlOx/a-COx/TiNxOy/TiN structure shows consecutive >2000 DC switching, tight distribution of SET/RESET voltages, a long program/erase (P/E) endurance of >109 cycles at a low operation current of 300 μA, and artificial synaptic characteristics under a small pulse width of 100 ns. After a P/E endurance of >108 cycles, the Cu migration is observed by both ex situ high-resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy mapping images. Furthermore, the optimized Cu/AlOx/a-COx/TiNxOy/TiN CBRAM detects glucose with a low concentration of 1 pM, and real-time measurement of human saliva with a small sample volume of 1 μL is also detected repeatedly in vitro. This is owing to oxidation–reduction of Cu electrode, and the switching mechanism is explored. Therefore, this CBRAM device is beneficial for future artificial intelligence application.

Lower Power, Better Uniformity, and Stability CBRAM Enabled by Graphene Nanohole Interface Engineering

Conductive-bridge random access memory (CBRAM) shows great potential as next-generation nonvolatile memory. However, the relatively high-power consumption, cycle-to-cycle and device-to-device variations are still drawbacks for industrial applications. A method of Monte Carlo simulation of CBRAM combined with a model to calculate the ${I}$ – ${V}$ curves has been developed, which investigate the formation and dissipation of filaments inside CBRAM cells. Through simulations, we conclude that the multi-filament structure is responsible for high-power consumption and the changing of regrowth filament location is responsible for the variations. We have enabled lower power, better uniformity, and stability CBRAM using graphene nanohole interface layer. Compared to CBRAM without graphene, CBRAM with inserted graphene with 0.66 nm hole shows better uniformity (60 times improvement), stability (5 times improvement), and lower power dissipation (10 times reduction). Moreover, our simulation results are matched with the experimental results of 45-nm graphene nanohole CBRAM. The reasons for better characters have been explained by the confining of the filament location with a single filament. This article shows great potential of integrating CBRAM with graphene nanohole structure.

Electro-thermal transport in disordered nanostructures: a modeling perspective.

Following the emergence of novel classes of atomic systems with amorphous active regions, device simulations had to rapidly evolve to devise strategies to account for the influence of disordered phases, defects, and interfaces into its core physical models. We review here how molecular dynamics and quantum transport can be combined to shed light on the performance of, for example, conductive bridging random access memories (CBRAM), a type of non-volatile memory. In particular, we show that electro-thermal effects play a critical role in such devices and therefore present a method based on density functional theory and the non-equilibrium Green’s function formalism to accurately describe them. Three CBRAM configurations are investigated to illustrate the functionality of the proposed modeling approach.

An electroforming-free mechanism for Cu2O solid-electrolyte-based conductive-bridge random access memory (CBRAM)

In a CuxO solid-electrolyte-based CBRAM cell using an Ag top electrode, electroforming-free and electro-reset processes could be achieved at a specific ex situ annealing temperature of the solid electrolyte.