Investigation of the Eightwise Switching Mechanism and Its Suppression in SrTiO3 Modulated by Humidity and Interchanged Top and Bottom Platinum and LaNiO3 Electrode Contacts

Abstract

DOI: 10.1002/aelm.201800566 neuromorphic computing.[5,6] The attractive properties enabling this variety of applications are the tunable time scales of their switching speeds down to subnanoseconds[7] and nonvolatility up to 10 years[8,9] combined with scalability down to a couple of nanometers and low power dissipation.[10] Changes of resistance states in the oxides are facilitated by ion transport under high local electrical fields. The investigation of the influence of small scale effects such as space charges at metal-oxide interfaces and filaments on ionic transport are important for developing this technology.[11] In general oxidebased memristive devices work on the principle of nonvolatile resistance change upon the application of an electrical potential.[12] The switching mechanism is generally considered to be facilitated by internal redistribution of oxygen vacancies (defects) coupled to a valence change in the metal ion.[1] Recently however, it has been shown that also the change in the relative humidity in the surrounding atmosphere will change the resistive switching behavior of TaOx, HfOx, SrTiO3, and TiO2. Specifically, a primary study with Pt/SrTiO3/Pt switching bits by Messerschmitt et al. investigated the influence of relative humidity on the current-voltage (I–V) properties, showing that switching can be suppressed or enabled depending on the humidity level.[15] Heisig et al. confirmed with H2O isotope labeling experiments that oxygen from water incorporates into and oxidizes the SrTiO3 switching oxide, and Tappertzhofen et al. showed that the catalytic activity toward water splitting of the electrode determines the ion incorporation extent and reaction rate.[18] Of particular interest in relation to the humidity influence is the so-called “eightwise switching polarity mechanism,” schematically depicted in Figure 1a, since it has recently been confirmed by Cooper et al. that it relies on exchange of oxygen ions in the atmosphere.[19] Using in situ transmission electron microscopy (TEM) they demonstrated that oxygen evolution and reincorporation in the top Pt electrode is responsible for the eightwise polarity switching mechanism in SrTiO3. What remains unclear is specifically the role of the top electrode material choice and catalytic activity when exposed to the atmosphere. To probe this Memristive devices are hardware components for applications in neuromorphic computing, memories, and logic computation. This work contributes to the ongoing debate on the switching mechanism of eightwise polarity in SrTiO3based resistive switches. Specifically the effect of atmospheric humidity on the materials defect chemistry and switching properties is considered. Asymmetric devices are designed by exchanging the top and bottom positions of Pt and LaNiO3 electrodes allowing for a separate analysis of the top and the bottom metal-oxide interfaces. Under dry atmospheres the switching hysteresis is enhanced with a top Pt contact and suppressed with a bottom Pt contact. It is argued that the buried position and dense microstructure of the bottom platinum impedes an oxygen vacancy driven switching mechanism. Under humid atmospheres eightwise switching occurs in both devices suggesting the presence of two switching mechanisms within the same eightwise switching polarity, namely, oxygen vacancy and hydroxide ion enabled switching. The findings help develop strategies to suppress eightwise switching by burying the active metaloxide interface and ensuring dense electrode microstructures. Suppression of switching mechanisms relying on exchange with the environment is desirable for technological implementation of resistive switches and for strategies in stacking of memristive devices for memory and for neuromorphic hardware.