By modification of the electrode-solid-electrolyte interface with graphene, transit from valence change memories (VCM) to electrochemical metallization memories (ECM) in the cell Ta(C)/Ta2 O5 /Pt is demonstrated, thus, bridging both mechanisms. The ECM operation is discussed in the light of Ta-cation mobility in TaOx . The crucial role of electrochemical processes and moisture in the resistive switching process is also highlighted.

No abstract

atoms of the active metal, electrically connecting both electrodes through the solid electrolyte. [11] The formation of this filament is preceded by (partial) oxidation of the active electrode under positive bias. Because of field accelerated transport, the so-formed cations are migrating through the solid electrolyte thin film (e.g., Ta 2 O 5 , HfO 2 , or SiO 2 ) toward the counter electrode. The cations are reduced at the negatively charged counter electrode and form a nucleus facilitating further fast filament growth. Filament growth continues until short circuit conditions are reached and the driving forces for the electrochemical redox processes and ion migration are diminished. The transition from the high resistive state (HRS) into a low resistive state (LRS) is the SET process. When a negative bias is applied to the active electrode, the filament is disrupted and driven by same forces, but in opposite direction it is assumed to be completely dissolved. The device is switched from LRS to HRS that is the RESET.Since electrochemical electrode processes are spatially separated, SET event requires a counter electrode reaction at the CE to keep overall electroneutrality of the system. For most oxide-based solid electrolytes, this counter reaction involves a reduction of moisture, incorporated in the films from the ambient or during preparation steps (e.g., lithography, rinsing, atomic layer deposition (ALD) deposition, etc.). Reduction of water is kinetically favorable compared to reduction (and decomposition) of the oxide itself [12] and therefore the redox processes related to moisture can be a limiting factor for the switching kinetics. [13] Furthermore, incorporated moisture can also enhance the cation mobility. [14] In ECM devices, typically silver or copper electrodes are used. In addition, metals such as Ti [15] or alloys/compounds have been suggested as active electrodes. [16,17] The advantage of these alternative systems is considered to be the direct supply of mobile ions, avoiding the necessity of direct electrode oxidation and related energy consumption. However, in most of the cases the choice of active electrode materials is based mainly on empirical observations and the electrochemical properties of the materials toward oxidation and reduction have not been considered.In this work, we investigate the electrochemical redox behavior of several metals covering a range from noble to transition metals (Ag, Al, Au, Cu, Fe, Ni, Ta Ti, V, and Zr) as active electrode materials using Me/SiO 2 /Pt system. Cyclic voltammetry (CV) measurements are used to provide information on the redox processes occurring prior to the switching events. In addition, we performed I-V switching experiments with three different thicknesses of the solid electrolyte down to 5 nm Electrochemical metallization cells rely on oxidation, reduction, and migration of metal cations in nanoscale thin films. The cations are typically provided by the oxidation of the active electrode. Commonly, Cu, Ag, or their compounds are used as electro...

The counter-electrode material in resistively switching electrochemical metallization cells (ECMs) is a crucial factor influencing the nucleation of conductive filaments, the equilibrium electrode potentials, and kinetics in the devices, and hence the overall switching characteristics. Here, we demonstrate the influence of the counter-electrode (CE) material on the SET events and the importance of appropriate choice and combination of materials. The counter-electrode material influences the counter-electrode processes at the CE/insulator interface and consequently determines the metal ion concentration in the cells. We measured the switching kinetics for SiO/Ag based ECM cells using different counter-electrode materials with different electrocatalytic activities towards water reduction, namely platinum, ruthenium, and iridium oxide, as well as titanium nitride and tantalum. The experimental results are fitted using a physical simulation model and are analysed for the limiting factors for fast SET kinetics.

but also for being building units for neuromorphic applications and non-von Neumann computing. [1,2] ReRAMs consist of a simple metal-solid electrolyte-metal stack where the information is stored as electronic resistance state of the electrochemical cell. [3] By applying a voltage of different polarity and/or magnitude, formation or rupture of a conductive filament is induced, leading to a nonvolatile low-resistive ON state (also denoted as LRS) or high-resistive OFF state (HRS), respectively. ReRAMs have the advantages of showing low power consumption, fast switching times (down to hundreds of picoseconds), and high endurance and retention. Depending on the working principle, ReRAMs are classified into valence change memory (VCM), electrochemical metallization memory (ECM), or thermochemical memory (TCM). While TCM devices are not of current interest for applications, ECM and VCM show particular promises. The functionality of both types relies on redox reactions and movement of ions and electrons within the solid electrolyte. [3,4] In the case of ECM, the active electrode is (partially) oxidized and metal cations drift within the electrolyte toward the counter electrode where they are reduced back to metal. Due to the high electric fields and high current densities the reduced metallic phase is formed as a tiny (few nm in diameter) filament. [2] The mechanism of VCM devices is considered to be based on a localized partial reduction of the solid electrolyte leading to an oxygen deficient, electrically conductive, filament. Originally the reduction was explained solely by loss of oxygen ions, i.e., formation of additional oxygen vacancies V O •• with a charge, compensated by electrons. However, new studies have demonstrated that the cations in such oxides are often mobile and can participate in the switching process as well. [5,6] Applying a voltage of opposite polarity, the formed filament is partially reoxidized within a thin region facing the high work function metal electrode, defining the OFF state. By modulation of the Schottky barrier at this interface reversible switching between the LRS and HRS is possible. [7] The mechanistic details on the electroforming and switching are still under discussion and despite many studies were performed on this topic, no consensus is reached till today. One of the most recent achievements was the experimental verification that redox processes are preceding and directly responsible for the forming itself. [6] Foreign components such as dopants and impurities in molecular or ionic form may significantly influence forming/switching processes in redox-based memories. This work presents a systematic study and discussion on effects of oxygen and moisture in Ta 2 O 5 and HfO 2 thin films, being two of the most used materials for redox-based resistively switching random access memories. Whereas oxygen is found to not affect the device behavior, the presence of moisture profoundly influences it. It plays a crucial role for the counter electrode reaction, providing additional charg...