Processes and Effects of Oxygen and Moisture in Resistively Switching TaO_x and HfO_x

Abstract

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...

Recent Developments and Perspectives for Memristive Devices Based on Metal Oxide Nanowires

“…For example, electrical measurements in different atmospheres allowed to unveil the n-type conductive path responsible resistive switching in MgO core-TiO 2 shell nanowires. [200][201][202][203] Using a completely different approach, resistive switching was observed also considering conductive nanowires covered by an oxidized shell layer, in which the switching occurs along the radial direction of the NW. It is important to remark that this strategy can be adopted only in planar devices and not in widely studied capacitor-like devices where the switching mechanism is hidden in stacked sandwich structures: for these reasons, nanowires can be considered a good platform to investigate the resistive switching mechanism in a wide range of materials.…”

Electrical characteristics of interfacial barriers at metal—TiO2 contacts

“…However, this has no effect on the measured data. Finally, it is worth mentioning the role of moisture, which was reported to affect both the transport and switching properties [36,37,44] of devices based on sputtered oxide films due to their porous structure. Despite this effect having not shown significant influence in the case of TiO 2 layers [44], our exper imental procedure was performed in an environment where humidity and temperature were carefully controlled in order to minimize their influence on the measurements.…”

“…This does not apply to our work, as although the barrier formed is not proportional to the metal work function, significant variations exist for distinct electrode materials. In order to better understand the formation of the interfacial barrier we also have to bear in mind that interme diate thin oxide layers of a few nanometres may be formed at metal/oxide interfaces [37], especially on samples exposed to environmental conditions [36]. Further to this, electrode/ electrolyte interfaces may be degraded due to electrode reac tions [34], while some induction of oxygen vacancies, acting as equivalent to doping, is also expected [38].…”

“…All the above suggest that these are not conventional metal-semiconductor contacts where the metal work function dominates the pro cess. Fabrication details [27,35], environmental conditions [36] and electrochemical interactions [37] all make significant contributions as well.…”

Memristive Synapses and Neurons for Bioinspired Computing

“…The cation migrationbased memristive devices are generally referred to as atomic switch [65][66][67] or programmable metallization cell. [101][102][103] However, unlike the fruitful findings of the metal cation filaments in the ECM cells, the details of the oxygen migration and the resultant oxygendeficient conductive filaments are still elusive and controversial owing to the difficulty in directly observing the oxygen migration process. [58,[97][98][99][100] The migration of oxygen ions usually induces a redox reaction expressed by a valence change of the cation sublattice and leads to a stoichiometry change of the oxides.…”