Thermodynamically Introduced Oxygen Vacancies in Epitaxial Pt-CeO2 System and Their Influence on Resistive Switching
Authors: Chundak Mykhailo , Yoshitake Michiko , Vaclavu Michal , Matolin Vladimir , Chikyow Toyohiro 
Published: February 2020

There is a growing need for a new type of nonvolatile memory technologies which would allow further downscale the size of devices and would have lower power consumption. One of the candidates for such technology is resistive random access memory (ReRAM). ReRAM is one of the most promising emerging nonvolatile memories in which both electronic and electrochemical effects play an important role. The principle of ReRAM cell is based on reversible change of resistance states induced by current and/or voltage. Typically, the memory cell consists of a metal-oxide-metal (MOM) structure and needs an initial electroforming step as a current-limited electric breakdown. After this process, the memory cell can be reversibly set-switched between a conductive low resistance state (ON) and a less conductive reset high-resistance state (OFF). The proposed mechanisms for such resistivity change can be divided into two models: the filament type and interface type [1]. The filament model suggests creation of conductive paths between two metal electrodes, which are changing the resistance state of the MOM structure. The interface model implies that the resistance change is caused by a type of electrochemical reaction to form or dissolve a Schottky barrier at the interface and is promoted by the electric field gradient.
In our work we investigate CeO2 as a candidate material for ReRAM. Ceria is the high-k dielectric material [2] well known for its oxygen vacancy diffusion [3] and readiness for oxidation state change between Ce3+/Ce4+ [4]. These properties play a major role in resistive switching behavior of ceria as previous investigations suggest that oxygen diffusion is assisting in formation of conductive filaments responsible for such switching. The vacancies can be generated in different ways including applying voltage, incorporation of reducing agent, nonstoichiometric deposition.
In this research we performed a set of in situ experiments with a model epitaxial CeO2 thin films on Cu(111) single crystal. Epitaxial films were chosen to avoid grain boundary diffusion which can influence the resistive switching mechanism [5]. The oxygen vacancies were introduced into the thin film trough thermodynamic interaction of Pt with ceria. Pt was deposited onto CeO2 surface, causing reduction of the oxide.
Interaction of Pt with ceria was investigated with X-ray photoelectron spectroscopy and conductive atomic force microscopy. We found out that reduction of ceria was occurring not only at the Pt/CeO2 interface, but also on the surface of the ceria film which is not covered by Pt, after Pt deposition and annealing. A different distribution of oxygen vacancies in the film proves to have an influence on the resistance switching process of the film. Finally, the proper balance between the reduced and the unreduced species in order to obtain relatively stable repeatable resistance switch with clear resistance window is discussed.
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