(Invited) Nanoscale Probing of Field-Driven Ion Migration in TaOx for Neuromorphic Memristor Applications

Abstract

In semiconducting metal oxides, application of electric fields often causes migration of the constituent ions (oxygen ions, metal cations, and ionic vacancies) via ionic conduction and diffusion. The field-driven ion migration has been considered a key phenomenon in the recent development of oxide-based resistive random access memory (ReRAM) and memristive neuromorphic devices because their operations are generally made through the migration and resultant changes in the valence/resistance states of the oxide layers. Semiconducting amorphous TaOx is known as a practical material of switching layers in ReRAMs, and moreover, has recently been considered of critical importance in the development of neuromorphic hardware because of its variety of neuromorphic resistive switching functions. Many important functions, including multilevel (or analog) resistive switching, stochastic switching, and spike-rate/spike-timing dependent plasticity, have been demonstrated in junctions with a structure of metal/TaOx/metal, and the origin of the neuromorphic functionality has been ascribed to the high ionic conductivities (both for O2− and Ta5+) and degree of structural freedom of amorphous TaOx.Regarding the mechanisms of the neuromorphic switching, however, limited understanding has been achieved in the previous works. The mechanisms have so far been inferred from the switching behaviors in electrical measurements or numerical modeling because of the experimental difficulty in observing nanoscale ion migration during the neuromorphic resistive switching. To directly design of the neuromorphic functions and establish oxide-based neuromorphic computing, therefore, experimental probing of the ion migration processes is now importantly required for neuromorphic resistive switching of TaOx.In this study, we fabricated ultra-flat thin films of amorphous TaOx (3.0–10 nm thick) with a root-mean-square roughness of <0.2 nm by the pulsed laser deposition method, which will offer suitable conditions for ion migration analysis by probe microscopy. In the conductive tip atomic force microscopy investigations, we observed a stable resistive switching phenomenon at arbitrary positions on the TaOx surface by voltage applications from a probe. The positional independence of the switching characteristics ensures the experimental advantages of the ultra-flat films in the investigations. Additionally, this indicates that "easy spots" for resistive switching, of which the formation has been suggested in other metal oxides, are not present in the thin films of amorphous TaOx, and initial clustering of oxygen vacancies is not importantly involved in the switching. When multiple voltage stresses of >3.0 V were applied from a probe, we observed a multilevel switching phenomenon in the TaOx thin films. In the measurements, we observed that the multilevel switching appears with very small amount of structural changes of TaOx (<1.0 nm in the out-of-plane direction), and relatively large-scale structural deformations (2.0 nm in the out-of-plane direction and 150 nm in the in-plane direction) started to be observed only after breakdown-like behaviors in the resistance with formation of a 10 nm-sized conductive spot. This indicates that the intrinsic ion migration distance involved in the multilevel switching of TaOx is in a range of <1.0 nm in out-of-plane direction, which is smaller than previous predictions. From these results, the spatial extents of neuromorphic resistive switching of TaOx were revealed in this study, which will offer important insights into the process development for TaOx-based neuromorphic memristor.