Electrochemical Control of Resistance States in Metal Oxide Bilayer Stack

Abstract

Electrochemical control of metal oxide based devices has emerged recently as an attractive approach to achieving reprogrammable microelectronic devices that move beyond simple semiconductor device physics principles. The operational concept of such devices relies on the shuttling of ionic species from an ionic conducting oxide towards either an active interface or active oxide layer where a chemical reaction or exchange of ionic species occurs, changing the functional properties of that layer. Devices exhibiting electronic/magnetic/optical and thermal switching have been demonstrated to offer unique advantages over their more traditional electronic counterparts. For example, electronic switching devices, either in two-terminal device configurations typical of memristive devices or three-terminal configurations akin to a transistor device, have the ability to directly emulate neuronal behavior potentially leading to more efficient neuromorphic computational hardware. Beyond purely electronic applications, the advantages of using ionic species to modulate the functional properties of metal oxides (mechanical, ferroelectric, magnetic, optical, and thermal) offer unique perspectives to redesign traditional applications considering the ubiquitous use of these metal oxide materials for their functional properties. While electrochemically gated microelectronic devices have been demonstrated in many materials systems, open questions still remain about how to best optimize their device performance. While ionic mobility within and interfacial ion transfer between the two solid layers are key device parameters, little reliable information exists regarding these key parameters given the predominantly electronic character of metal oxide material near room temperature. Such knowledge is vital for mastering overall device performance such as speed, retention, and predictability. To address these challenges, we have investigated the reversible exchange of ions between two adjacent solid oxide layers. Specifically, we have measured the reversible ionic transport and interfacial transfer kinetics in a thin film bilayer oxide system based on a $ Pr_xCe_{1-x}O_2/La_{2-x}Ce_xCuO_4 $ stack that uses model materials for which the defect chemical and transport properties have already been carefully characterized. These materials offer high ionic mobilities and can accommodate large levels of non-stoichiometry. By studying the characteristics of our devices with a combination of electrochemical impedance spectroscopy and potentiostatic measurements, we demonstrate the ability to reversibly modulate the bilayer device resistance near room temperature and isolate the resistance contributions of both layers as a function of the total device resistive state. We show that these changes can be directly correlated with the respective materials’ defect chemistries, typically investigated at elevated temperatures under varying gas atmospheres. Dynamic current-voltage studies enable us to separate the rate-limiting kinetics related to the defect mobilities and the interfacial exchange kinetics of the system. The findings in this work can be expected to aid in developing material selection and design criteria for similar bilayer systems, and be used to achieve faster resistance switching speeds, larger resistance switching ranges, and longer device retentions.