Resistive switching (RS) devices are emerging electronic components that could have applications in multiple types of integrated circuits, including electronic memories, true random number generators, radiofrequency switches, neuromorphic vision sensors, and artificial neural networks. The main factor hindering the massive employment of RS devices in commercial circuits is related to variability and reliability issues, which are usually evaluated through switching endurance tests. However, we note that most studies that claimed high endurances >106 cycles were based on resistance versus cycle plots that contain very few data points (in many cases even <20), and which are collected in only one device. We recommend not to use such a characterization method because it is highly inaccurate and unreliable (i.e., it cannot reliably demonstrate that the device effectively switches in every cycle and it ignores cycle-to-cycle and device-to-device variability). This has created a blurry vision of the real performance of RS devices and in many cases has exaggerated their potential. This article proposes and describes a method for the correct characterization of switching endurance in RS devices; this method aims to construct endurance plots showing one data point per cycle and resistive state and combine data from multiple devices. Adopting this recommended method should result in more reliable literature in the field of RS technologies, which should accelerate their integration in commercial products.

Metal oxides are commonly used as electrolytes for redox-based resistive switching memories. In most cases, non-noble metals are directly deposited as ohmic electrodes. We demonstrate that irrespective of bulk thermodynamics predictions an intermediate oxide film a few nanometers in thickness is always formed at the metal/insulator interface, and this layer significantly contributes to the development of reliable switching characteristics. We have tested metal electrodes and metal oxides mostly used for memristive devices, that is, Ta, Hf, and Ti and TaO, HfO, and SiO. Intermediate oxide layers are always formed at the interfaces, whereas only the rate of the electrode oxidation depends on the oxygen affinity of the metal and the chemical stability of the oxide matrix. Device failure is associated with complete transition of short-range order to a more disordered main matrix structure.

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...

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