A detailed understanding of the resistive switching mechanisms that operate in redox-based resistive random-access memories (ReRAM) is key to controlling these memristive devices and formulating appropriate design rules. Based on distinct fundamental switching mechanisms, two types of ReRAM have emerged: electrochemical metallization memories, in which the mobile species is thought to be metal cations, and valence change memories, in which the mobile species is thought to be oxygen anions (or positively charged oxygen vacancies). Here we show, using scanning tunnelling microscopy and supported by potentiodynamic current-voltage measurements, that in three typical valence change memory materials (TaO(x), HfO(x) and TiO(x)) the host metal cations are mobile in films of 2 nm thickness. The cations can form metallic filaments and participate in the resistive switching process, illustrating that there is a bridge between the electrochemical metallization mechanism and the valence change mechanism. Reset/Set operations are, we suggest, driven by oxidation (passivation) and reduction reactions. For the Ta/Ta2O5 system, a rutile-type TaO2 film is believed to mediate switching, and we show that devices can be switched from a valence change mode to an electrochemical metallization mode by introducing an intermediate layer of amorphous carbon.

Resistive switching memories based on the valence change mechanism have attracted great attention due to their potential use in future nanoelectronics. The working principle relies on ion migration in an oxide matrix and subsequent nanoscale redox processes leading to a resistance change. While switching from a low resistive to a high resistive state, different intermediate resistance levels can be programmed by changing the maximum applied voltage, making resistive switches highly interesting for multibit data storage and neuromorphic applications. To date, this phenomenon, which is known as gradual reset, has been reported in various experimental studies, but a comprehensive physical understanding of this key phenomenon is missing. Here, a combined experimental and numerical modeling approach is presented to address these issues. Time‐resolved pulse measurements are performed to study the reset kinetics in TaOx‐based nano‐crossbar structures. The results are analyzed using a 2D dynamic model of nonisothermal drift–diffusion transport in the mixed electronic–ionic conducting oxide including the effect of contact potential barriers. The model accurately describes the experimental data and provides physical insights into the processes determining the gradual reset. The gradual nature can be attributed to the temperature‐accelerated oxygen‐vacancy motion being governed by drift and diffusion processes approaching an equilibrium situation.

Interface reactions constitute essential aspects of the switching mechanism in redox-based resistive random access memory (ReRAM). For example, the modulation of the electronic barrier height at the Schottky interface is considered to be responsible for the toggling of the resistance states. On the other hand, the role of the ohmic interface in the resistive switching behavior is still ambigious. In this paper, the impact of different ohmic metal-electrode (M) materials, namely W, Ta, Ti, and Hf on the characteristics of TaO ReRAM is investigated. These materials are chosen with respect to their free energy for metal oxide formation and, associated, their impact on the formation energy of oxygen vacancy defects at the M/TaO interface. The resistive switching devices with Ti and Hf electrodes that have a negative defect formation energy, show an early RESET failure during the switching cycles. This failure process with Ti and Hf electrode is attributed to the accumulation of oxygen vacancies in the TaO layer, which leads to permanent breakdown of the metal-oxide to a low resistive state. In contrast, the defect formation energy in the TaO with respect to Ta and W electrodes is positive and for those highly stable resistive switching behavior is observed. During the quasi-static and transient-pulse characterization, the ReRAM devices with the W electrode consistently show an increased high resistance state (HRS) than with the Ta electrode for all RESET stop voltages. This effect is attributed to the faster oxygen exchange reaction at the W-electrode interface during the RESET process in accordance to lower stability of WO than TaO. Based on these findings, an advanced resistive switching model, wherein also the oxygen exchange reaction at the ohmic M-electrode interface plays a vital role in determining of the resistance states, is presented.

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.