Examining Oxygen Exchange Kinetics of Anomalous Resistive Switching By Optical Tracking in Redox Based Memristive Devices
Authors: Defferriere Thomas , Tuller Harry L. , Rupp Jennifer L.M. , Kalaev Dmitri 
Published: February 2020

Anomalous resistive switching in redox based memristive devices has recently gained increased attention due to compelling demonstrations of its origin related to homogenous oxygen exchange with device electrodes [1]. Insights into this mechanism are expected to improve understanding of the origins of the competing switching mechanism in mixed valent metal oxide memristive devices. Previously, open systems were shown to be susceptible to oxygen exchange at triple phase boundaries[2] and to the incorporation of humidity,[3,4] under applied voltages, from the gas environment. These environmental exchanges can result in modified space charge layers and surface defect chemistries that in turn, can lead to modified resistive switching properties. Strategies to mitigate these effects have relied on the addition of an interlayer acting as a source of oxygen defects, also known as an exchange layer [5–7]. While exchange between the switching layer and the interlayer has been identified as the origin of resistance change, fundamental aspects of the exchange kinetics remain unclear. Vacuum based techniques, relying on compositional analysis such as EDS in TEM, have been used to demonstrate changes in chemistry, but provided limited information about the details of the exchange kinetics. Being able to track the dynamics of oxygen migration through the metal oxide semiconductor volume, and exchange at the interface over the entire area of the device, could provide insights into the kinetics and thermodynamic limitations responsible for switching speed and device retention. Mastering these two properties is fundamental for the development of anomalous resistive switching elements with increased stability, faster switching speeds and larger hysteresis. To gain further insight into the oxygen exchange kinetics, we investigated the resistive switching properties of a bilayer oxide system based on PrxCe1-xO2/CeO2 stacks, sandwiched between transparent electrodes. Our study focuses on the model material, PrxCe1-xO2 (PCO), whose defect chemical and transport properties have been previously studied both in bulk and thin film forms[8,9]. We leverage the existence of an interband gap optical absorption, related to the oxidation state of Pr ions, to monitor the changes in the film’s non-stoichiometry. By carefully processing the sandwiched PCO, it is possible to quench in different degrees of oxygen non-stoichiometry to room temperature, as confirmed by optical absorption and Raman spectroscopy. By engineering the defect chemistry of PCO and using CeO2 as an exchange layer, we investigate and correlate the switching dynamics via electrical and optical measurements to gain insight into the speed and the amount of oxygen exchanged between the two layers.
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