Camilla La Torre

Anomalous Resistance Hysteresis in Oxide ReRAM: Oxygen Evolution and Reincorporation Revealed by In Situ TEM

The control and rational design of redox-based memristive devices, which are highly attractive candidates for next-generation nonvolatile memory and logic applications, is complicated by competing and poorly understood switching mechanisms, which can result in two coexisting resistance hystereses that have opposite voltage polarity. These competing processes can be defined as regular and anomalous resistive switching. Despite significant characterization efforts, the complex nanoscale redox processes that drive anomalous resistive switching and their implications for current transport remain poorly understood. Here, lateral and vertical mapping of O vacancy concentrations is used during the operation of such devices in situ in an aberration corrected transmission electron microscope to explain the anomalous switching mechanism. It is found that an increase (decrease) in the overall O vacancy concentration within the device after positive (negative) biasing of the Schottky-type electrode is associated with the electrocatalytic release and reincorporation of oxygen at the electrode/oxide interface and is responsible for the resistance change. This fundamental insight presents a novel perspective on resistive switching processes and opens up new technological opportunities for the implementation of memristive devices, as anomalous switching can now be suppressed selectively or used deliberately to achieve the desirable so-called deep Reset.

Oxygen Exchange Processes between Oxide Memristive Devices and Water Molecules

Resistive switching based on transition metal oxide memristive devices is suspected to be caused by the electric-field-driven motion and internal redistribution of oxygen vacancies. Deriving the detailed mechanistic picture of the switching process is complicated, however, by the frequently observed influence of the surrounding atmosphere. Specifically, the presence or absence of water vapor in the atmosphere has a strong impact on the switching properties, but the redox reactions between water and the active layer have yet to be clarified. To investigate the role of oxygen and water species during resistive switching in greater detail, isotope labeling experiments in a N2 /H218 O tracer gas atmosphere combined with time-of-flight secondary-ion mass spectrometry are used. It is explicitly demonstrated that during the RESET operation in resistive switching SrTiO3 -based memristive devices, oxygen is incorporated directly from water molecules or oxygen molecules into the active layer. In humid atmospheres, the reaction pathway via water molecules predominates. These findings clearly resolve the role of humidity as both oxidizing agent and source of protonic defects during the RESET operation.

Internal Cell Resistance as the Origin of Abrupt Reset Behavior in HfO2-Based Devices Determined from Current Compliance Series

The resistive switching behavior in different HfO2/TiO2 nano crossbar structures of 100 x 100 nm2 size is analyzed by means of DC voltage sweeps. The devices fabricated from 3 nm thin ALD layers of HfO2 and TiO2 sandwiched between Pt and Hf or Ti electrodes show VCM-type bipolar resistive switching after electroforming. For increased compliance current (cc) during set from 50 μA to 800 μA, the set current runs into self- limitation while the reset behavior changes from gradual to abrupt. A model is defined with an internal resistance being in series with the local resistive switch. A recursive algorithm is applied to the cc series for calculation of the series resistor and evaluation of the intrinsic switching characteristic of HfO2-based cells. The intrinsic LRS turns out to be current compliance controlled and to follow the universal switching rule. Supported by compact modelling, we show that an abrupt reset behavior might arise even for materials with a gradual intrinsic reset characteristic in consequence of an internal series resistor.

Resistive Switching: Resistive Switching of Individual, Chemically Synthesized TiO2 Nanoparticles (Small 48/2015)

Resistive switching of individual, spherical TiO2 nanoparticles is studied by U. Simon and co-workers. Bottom-up synthesized TiO2 particles are immobilized on a conducting substrate and individually electrically addressed by an electrode tip in an in-situ scanning electron microscope, as shown in this image by T. Pössinger. On page 6444, two switching behaviors are described, bipolar and complementary, which are associated with the inner structure of the individual particles. This opens up new opportunities for materials design.