Abstract
The next generation of nonvolatile memory storage may well be based on resistive switching in metal oxides. TiO2 as transition metal oxide has been widely used as active layer for the fabrication of a variety of multistate memory nanostructure devices. However, progress in their technological development has been inhibited by the lack of a thorough understanding of the underlying switching mechanisms. Here, we employed high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) combined with two-dimensional energy dispersive X-ray spectroscopy (2D EDX) to provide a novel, nanoscale view of the mechanisms involved. Our results suggest that the switching mechanism involves redistribution of both Ti and O ions within the active layer combined with an overall loss of oxygen that effectively render conductive filaments. Our study shows evidence of titanium movement in a 10 nm TiO2 thin-film through direct EDX mapping that provides a viable starting point for the improvement of the robustness and lifetime of TiO2-based resistive random access memory (RRAM).
Highlights
Nanoscale resistive random access memory (RRAM) cells are typically metal−insulator−metal (MIM) architectures where the insulator typically comprises a transition metal oxide thinfilm
This behavior, called resistive switching (RS) makes them extremely valuable for memory applications, while more recently have attracted interest from the neuromorphic community due to their “synapse-like” behavior.[1−4] The RS behavior that defines the dynamics of RRAM cells is still not well understood and remains a matter of debate despite the several mechanisms that have been suggested to date.[5,6]
The high-resolution transmission electron microscopy (HR-TEM) image of the device cross section reported in the inset of Figure 1b shows that the TiO2 film is amorphous, as expected for a thin film deposited by reactive sputtering at room temperature
Summary
Nanoscale resistive random access memory (RRAM) cells are typically metal−insulator−metal (MIM) architectures where the insulator typically comprises a transition metal oxide thinfilm. Memristive devices have the ability to switch the electrical resistance between high-resistive states (HRS) and low-resistive states (LRS) by application of an appropriate voltage This behavior, called resistive switching (RS) makes them extremely valuable for memory applications, while more recently have attracted interest from the neuromorphic community due to their “synapse-like” behavior.[1−4] The RS behavior that defines the dynamics of RRAM cells is still not well understood and remains a matter of debate despite the several mechanisms that have been suggested to date.[5,6] One of the most frequently invoked is a filament-type mechanism, which is based on the formation of highly conductive reduced oxide phases perpendicular to the electrodes (conductive nanofilaments, CFs).[7−9] Formation of CFs is believed to occur as a consequence of internal redox reactions, due to migration of defects (oxygen vacancies or cation interstitials) or ions along the metal oxide thin film.[10] Commonly, it is the motion of oxygen vacancies that is considered to be responsible for the CFs formation and RS in transition metal oxides.[11] very recently, it was shown that metal cations play an important role in the RS process of memristive systems based on TiOx, TaOx, and HfOx thin films.[12] Until now, the identification of species responsible for RS process and formation of CFs is still a point of controversy in the scientific community.
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