Electrochemical Metallization Memory Research Articles

Stabilization of Ni conductive filaments using NH3 plasma treatment for electrochemical metallization memory

In this study, NH3 plasma treatment was utilized to enhance the resistive switching (RS) properties.

Understanding filamentary growth in electrochemical metallization memory cells using kinetic Monte Carlo simulations.

We report on a 2D kinetic Monte Carlo model that describes the resistive switching in electrochemical metallization cells. To simulate the switching process, we consider several different processes on the atomic scale: electron-transfer reactions at the boundaries, ion migration, adsorption/desorption from/to interfaces, surface diffusion and nucleation. These processes result in a growth/dissolution of a metallic filament within an insulating matrix. In addition, the model includes electron tunneling between the growing filament and the counter electrode, which allows for simulating multilevel switching. It is shown that the simulation model can reproduce the reported switching kinetics, switching variability and multilevel capabilities of ECM devices. As a major result, the influence of mechanical stress working on the host matrix due to the filamentary growth is investigated. It is demonstrated that the size and shape of the filament depend on the Young's modulus of the insulating matrix. For high values a wire-like structure evolves, whereas the shape is dendritic if the Young's modulus is negligible.

Electroforming-Free and Multilevel Resistance Switching Properties in Al/TiOx/Cu Structure

Resistance random access memory (RRAM) in various oxide materials,especiallybinaryoxidematerialssuchasTiO2, 1 ZrO2, 2 HfO2, 3 and ZnO, 4 has attracted considerable attention as one of the promising applications in next-generation nonvolatile semiconductor memory devices. Although the resistance switching (RS) materials can be classified into several groups according to switching mechanisms, this study focuses on combined different mechanisms effects on the RS performance in Al/TiOx/Cu. As Cu is an active metal in the electrochemical metallization memory (ECM), 5 adopting Cu as electrode may induce Cu conductive filaments (CFs). Meanwhile, because of the higher oxidation potential of Al compared with Ti, 6 adopting Al onTiOx mayinducetheoxygenvacancy (Vo)relatedRSmechanisms, such as electronic switching related with the trapping and detrapping of carriers along the Vo channels. 7 Particularly, when RRAM devices is programmed, a filament with trap-controlled space charge limited current (SCLC) could be formed, while a metallic CFs could also be formed. Therefore, multilevel resistance states may be obtained by forming different filaments inside the device. However, research on combining these two mechanisms remains to be clarified. During the resistance switching process, the “electroforming” process is usually needed to obtain the resistance switching. However, this process often results in the random fluctuations of switching parameters and needs a much higher bias, which are generally unfavorable for the device fabrication and operation. 8 In this paper, the characteristics of multilevel resistance switching, electroforming-free and low Ireset (the maximum current level changes from LRS to HRS) were investigated through Al/TiOx/Cu structure. The relationship between electroforming-free and low Ireset will be elucidated in this paper, as well as the multilevel resistance switching mechanism.

Volatile resistance states in electrochemical metallization cells enabling non-destructive readout of complementary resistive switches

Redox-based resistive memory cells exhibit changes of OFF or intermediate resistance values over time and even ON states can be completely lost in certain cases. The stability of these resistance states and the time until resistance loss strongly depends on the materials system. On the basis of electrical measurements and chemical analysis we found a viable explanation for these volatile resistance states (VRSs) in Ag-GeSx-based electrochemical metallization memory cells and identified a technological application in the field of crossbar memories. Complementary resistive switches usually suffer from the necessity of a destructive read-out procedure increasing wear and reducing read-out speed. From our analysis we deduced a solution to use the VRS as an inherent selector mechanism without the need for additional selector devices.

Tristate electrochemical metallization memory based in the hydrogenated nanocrystalline silicon films

The hydrogenated nanocrystalline silicon (nc-Si:H) films have been fabricated as resistive switching medium by radio frequency plasma enhanced chemical vapor deposition technology. The constructed Ag/nc-Si:H/Pt structure exhibits stable three nonvolatile resistance states. Tristate resistive states with large ratio 102 and 105, less variation of resistance, and long retention exceeding 2.3 × 105 s are observed in Ag/nc-Si:H/Pt stack. The temperature dependence of high resistance state (HRS) and intermediate resistance state (IRS) both show semiconductor behavior, and the temperature dependence of low resistance state (LRS) represents metallic property. Fitting results demonstrated that the conduction mechanism of HRS, IRS, and LRS showed space charge limited conduction (SCLC), tunneling, and ohmic characteristics, respectively. The discrete Ag filament with Si nanocrystalline and complete Ag filament is proposed to be responsible for the performance IRS and LRS. We supposed that the Ag+ ions prefer to be reduced to Ag atoms near the Si nanocrystalline location. Si nanocrystalline between Ag nanoparticles contribute to the current transport at IRS.

(Invited) Resistive Switching Characteristics and Controllable Quantized Conductance in Single-Crystal Anatase TiO2 on Si (001)

Resistive switching in single crystal anatase TiO2 grown by atomic layer deposition on Si (001) with an epitaxial single crystal SrTiO3 buffer layer is characterized and analyzed. Although switching occurs via a valence-change mechanism involving motion of oxygen vacancies, electrical characteristics highly reminiscent of electrochemical metallization memories – low leakage current in the high-resistance state, highly linear current-voltage characteristics in the low-resistance state, and a very high on/off current ratio – are observed. We postulate that these characteristics may be associated with formation of Ti n O2n-1 Magnéli or Magnéli-like nanophases in the low-resistance state. In addition, quantized conductance is observed in the low-resistance state, with the number of conducting ballistic channels being highly controllable via application of different compliance currents during the memory SET process. For compliance currents of 10-200µA, low resistance state conductances corresponding to 1 to 4 conductance channels, controllable to within a single quantum of conductance, are observed.

Highly Controllable and Stable Quantized Conductance and Resistive Switching Mechanism in Single-Crystal TiO2 Resistive Memory on Silicon

TiO2 is being widely explored as an active resistive switching (RS) material for resistive random access memory. We report a detailed analysis of the RS characteristics of single-crystal anatase-TiO2 thin films epitaxially grown on silicon by atomic layer deposition. We demonstrate that although the valence change mechanism is responsible for the observed RS, single-crystal anatase-TiO2 thin films show electrical characteristics that are very different from the usual switching behaviors observed for polycrystalline or amorphous TiO2 and instead very similar to those found in electrochemical metallization memory. In addition, we demonstrate highly stable and reproducible quantized conductance that is well controlled by application of a compliance current and that suggests the localized formation of conducting Magnéli-like nanophases. The quantized conductance observed results in multiple well-defined resistance states suitable for implementation of multilevel memory cells.

Effects of electrode materials on bipolar and unipolar switching in NiO resistive switching device

NiO resistive switching access memory (RRAM) devices were grown by DC-sputtering with two different top electrodes. When selected Au as the top electrode, the unipolar switching were observed in both positive and negative bias voltage range. However, when changed the top electrode to Co, the bipolar and unipolar resistive switching in negative bias voltage were achieved. Comparing these two top electrodes, the observations provide evidence that the migration of Co cations in insulation layer result in bipolar switching and the reason was attributed to electrochemical metallization memory (ECM) effect. Furthermore, it also confirmed that bipolar resistive switching tended to form in the electrodes with high activity.

Electrically and optically readable light emitting memories.

Electrochemical metallization memories based on redox-induced resistance switching have been considered as the next-generation electronic storage devices. However, the electronic signals suffer from the interconnect delay and the limited reading speed, which are the major obstacles for memory performance. To solve this problem, here we demonstrate the first attempt of light-emitting memory (LEM) that uses SiO2 as the resistive switching material in tandem with graphene-insulator-semiconductor (GIS) light-emitting diode (LED). By utilizing the excellent properties of graphene, such as high conductivity, high robustness and high transparency, our proposed LEM enables data communication via electronic and optical signals simultaneously. Both the bistable light-emission state and the resistance switching properties can be attributed to the conducting filament mechanism. Moreover, on the analysis of current-voltage characteristics, we further confirm that the electroluminescence signal originates from the carrier tunneling, which is quite different from the standard p-n junction model. We stress here that the newly developed LEM device possesses a simple structure with mature fabrication processes, which integrates advantages of all composed materials and can be extended to many other material systems. It should be able to attract academic interest as well as stimulate industrial application.

Nanoscale electrochemical metallization memories based on amorphous (La, Sr)MnO3 using ultrathin porous alumina masks

Nanoscale electrochemical metallization (ECM) memories based on amorphous La1−xSrxMnO3 (a-LSMO) were fabricated using ultrathin porous alumina masks. The ultrathin alumina masks, with thicknesses of about 200 nm and pore diameters of about 80 nm, were fabricated through a typical two-step anodization electrochemical procedure and transferred onto conductive Pt/Ti/SiO2/Si substrates. Resistive switching (RS) properties of the individual Ag/a-LSMO/Pt ECM cell were directly measured using a conductive atomic force microscope. The cells exhibited typical RS characteristics and the OFF/ON resistance ratio is as high as 102. Reproducible RS behaviours on the same ECM cell and the I–V cycles obtained from different ECM cells ensured that the RS properties in nanoscale Ag/a-LSMO/Pt cells are reproducible and reliable. This work provides an effective approach for the preparation of nanostructured large-scale ordered ECM memories or memristors.

On the SET/RESET current asymmetry in electrochemical metallization memory cells

Abstractmagnified imageIn this Letter we discuss the SET/RESET current asymmetry of the bipolar resistive switching in electrochemical metallization cells based on an analytical switching model. The model is based on the growth and dissolution of a metallic filament. It accounts for the controllability of the low resistive state and the RESET current by tuning the SET current. Based on this model a possible physical explanation for the SET/RESET asymmetry is presented. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Mechanism of resistive switching in Cu/AlOx/W nonvolatile memory structures

The mechanism for resistive switching in aluminum oxide (AlOx) based electrochemical metallization memory cells is presented. Copper/AlOx/tungsten (Cu/AlOx/W) cells show reproducible resistive switching with an ON/OFF ratio of about 5 × 102 at a reading voltage of 0.1 V and reliable retention characteristics. Resistive switching occurs due to the formation and rupture of a Cu filament between the active electrode (Cu) and the counter electrode (W). The conduction of the devices was explained through back-to-back Schottky contacts in the OFF state, while it exhibits ohmic behavior in the ON state. Thermionic emission model was used to calculate the barrier heights of the Schottky contacts. The rupture of the Cu filament proved to occur at the weakest point of the filament inside the AlOx. Using Ohms Law, the slope of the linear I-V characteristics in the ON state was used to extract the Cu filament resistance and its diameter was estimated to be between 6 and 23 nm.

Switching kinetics of electrochemical metallization memory cells

The strongly nonlinear switching kinetics of electrochemical metallization memory (ECM) cells are investigated using an advanced 1D simulation model. It is based on the electrochemical growth and dissolution of a Ag or Cu filament within a solid thin film and accounts for nucleation effects, charge transfer, and cation drift. The model predictions are consistent with experimental switching results of a time range of 12 orders of magnitude obtained from silver iodide (AgI) based ECM cells. By analyzing the simulation results the electrochemical processes limiting the switching kinetics are revealed. This study provides new insights into the understanding of the limiting electrochemical processes determining the switching kinetics of ECM cells.

Analytical analysis of the generic SET and RESET characteristics of electrochemical metallization memory cells

We report on an analytical model which describes the bipolar resistive switching in electrochemical metallization cells. To simulate the resistive switching, we modeled the growth and dissolution of a metallic filament together with electron tunneling between the growing filament and the counter electrode. The model accounts for the controllability of the low resistive state and the RESET current by tuning the SET current. By analytical analysis the relevant conditions for these generic characteristics are identified. In addition, an explanation for the asymmetry in the SET and RESET switching characteristics is presented. The results of the analytical analysis is generalized to all types of ReRAMs.

Bond nature of active metal ions in SiO2-based electrochemical metallization memory cells

Electrochemical metallization cells are candidates for the next-generation non-volatile memory devices based on resistive switching. Despite the intensive studies in recent years a microscopic model of the processes in these nanoscale electrochemical systems is still missing and the physicochemical properties of the active metal ions have been rarely reported. We examined the bonding characteristics of Cu(z+) and Ag(+) ions in SiO(2)-based cells using soft X-ray absorption spectroscopy. Whereas the Ag/SiO(2) interfaces showed no chemical interaction of Ag ions, the Cu/SiO(2) showed clear signatures of partial oxidation into two ionic species of Cu(2+) and Cu(+). The analyses on the orbital hybridization strength evidently showed that the Cu(2+)-O(2-) bonds in SiO(2) are much weaker than the Cu(+)-O(2-) bonds, analogous to the case of bulk CuO and Cu(2)O. This suggests that the Cu(2+) ions should be more mobile and with a dominating role in the process of resistive switching.

Comment on Real‐Time Observation on Dynamic Growth/Dissolution of Conductive Filaments in Oxide‐Electrolyte‐ Based ReRAM

Filament formation and dissolution in the system Ag(Cu)/ZrO(2) /Pt were observed by Liu et al. [Adv. Mater. 2012, 24, 1844]. Their explanation of the phenomena is shown here to be inappropriate. Various situations, including the "bipolar electrode" shown in the figure, are considered and the difference between the behavior in electrochemical metallization memories (ECMs) and valence change memories (VCMs) outlined. Of crucial importance for distinguishing ECM from VCM behavior, that is, the effects of cation and anion migration, is the choice of the solid materials used to transport metal cations. A possible explanation of the phenomena is proposed.

Nucleation and growth phenomena in nanosized electrochemical systems for resistive switching memories

The impact of the electrochemical nucleation on the switching kinetics in many nanoscaled redox-based resistive switching memories is critically discussed. In the case of the atomic switch, the system is site invariant and the nucleation process is strictly localized below the STM tip. Using RbAg4I5 solid electrolyte, nucleation was found to be rate limiting. The electrochemical metallization memory cells (gapless type atomic switch) operate at conditions closer to the conventional nucleation. They introduce additional difficulties for interpretation of the experimental results due to formation of hillocks, of surface oxide barrier films, induction of strain, and the influence of the high electric field. In valence change memories, the nucleation seems to be less important because of the higher applied voltages. The results are discussed in the context of the atomistic theory of electrochemical nucleation. We believe that re-analysis of the experimental data for many systems will reveal that the nucleation is limiting the switching time.

Superior TID Hardness in TiN/HfO$_{2}$/TiN ReRAMs After Proton Radiation

Resistive switching properties of valence change memory (VCM) resistive-random-access-memory (ReRAM) devices (TiN/HfO2 /TiN) are investigated after exposure to proton radiation with total ionizing doses (TID) of 1.5, 3, and 5 Grad(Si) and compared to similar measurements from electrochemical metallization memory (ECM) ReRAM devices (Pt/HfO2:Cu/Cu). The TiN/HfO2/TiN ReRAMs show significantly superior TID radiation-hardness compared to Pt/HfO2:Cu/Cu ReRAMs: 1) All devices remained functional after radiation; 2) switching parameters including average Vset, Vreset, Ron, Roff showed minimal or no degradation; and 3) TID radiation enhanced the uniformity of resistive switching among all VCM devices. The superior radiation responses of the VCM ReRAM devices relative to ECM ReRAMs result from the distinct conduction filament (CF) formation mechanisms. For the VCM ReRAM system, the radiation-induced vacancy density does not serve to inhibit the trap-assisted tunneling associated with the Hf-rich CF formation kinetics. On the contrary, vacancy-promoted charge trapping promotes VCM CF stability. In strong contrast, proton-induced vacancies for the ECM ReRAMs inhibit the formation of the metallic filament through internal field reduction due to charge trapping. The comparison of TID effects suggests that HfO2-based VCM ReRAMs can be made radiation immune to a TID up to 5 Grad(Si) and may be highly suitable for rad-hard electronics applications.

Direct Observation of Charge Transfer in Solid Electrolyte for Electrochemical Metallization Memory

X-ray absorption spectroscopy study on an electrochemical metallization cell of GeS(x) :Ag shows clear experimental evidence of chemical ionization of the active metal atoms (Ag) and consequent transfer of charge to the electrolyte (GeS(x) ). The valence electron density and its change upon the Ag intercalation are depicted schematically as transparent waves on the Ge-S bond structure in amorphous GeS(x) .

Mechanism for resistive switching in an oxide-based electrochemical metallization memory

A comparison of the asymmetric OFF-state current-voltage characteristics between Cu/ZnO/Pt and Cu/ZnO/Al-doped ZnO (AZO) electrochemical metallization memory (ECM) cells demonstrates that the Cu filament rupture and rejuvenation occur at the ZnO/Pt (or AZO) interface, i.e., the cathodic interface. Therefore, the filament is most likely to have a conical shape, with wider and narrower diameters formed at the anodic and cathodic interfaces, respectively. It is inferred that the filament growth starts at the anode surface and stops at the cathode surface. Our results indicate that oxide-based ECM cells strongly differ from sulfide- and selenide-based ones in the resistive switching mechanism.