Epitaxial CeO 2 thin films for a mechanism study of resistive random access memory (ReRAM)
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ORIGINAL PAPER
Epitaxial CeO2 thin films for a mechanism study of resistive random access memory (ReRAM) Michiko Yoshitake & Michal Vaclavu & Mykhailo Chundak & Vladimir Matolin & Toyohiro Chikyow
Received: 6 May 2013 / Revised: 10 July 2013 / Accepted: 22 July 2013 / Published online: 23 August 2013 # Springer-Verlag Berlin Heidelberg 2013
Abstract A thin epitaxial CeO2 film was grown on a Cu(111) single crystal in order to investigate the mechanism of resistive memory/switching devices with an ultimately thin high-k dielectric film. A small amount of Pt was deposited on the CeO2 film and the Pt/CeO2/Cu structure was characterized by conductive atomic force microscopy and X-ray photoelectron spectroscopy. It was found that the grown epitaxial CeO2 film was fully oxidized, i.e., the valence of Ce atoms in the film was completely Ce4+. However, after the deposition of a small amount of Pt, it was revealed that Ce atoms were partially reduced to Ce3+ in full thickness of the film. The Pt/CeO2/Cu structure did not show switching behavior in resistance. The observed reduction of CeO2 film is considered to be responsible to the non-switching behavior. The thermodynamics of the reduction of the CeO2 film and the kinetics of oxygen diffusion in the reduced CeO2 film are discussed. Keywords Ceria . Reduction . ReRAM . Memory . I–V curve . XPS
Introduction Thin metal oxide films sandwiched with metal electrodes have been attracting attention as candidates for resistive random access memory (ReRAM) or resistive switching devices [1–5]. It is known that the voltage required for the so-called forming process (soft breakdown) shows a linear relationship with the oxide layer thickness and the forming voltage decreases with a M. Yoshitake (*) : M. Vaclavu : M. Chundak : T. Chikyow MANA Nano-Electronics Materials Unit, National Institute for Materials Science, 3-13, Sakura, Tsukuba 305-0003, Japan e-mail: [email protected] M. Vaclavu : M. Chundak : V. Matolin Faculty of Mathematics and Physics, Charles University in Prague, V Holesovickach 2, 18000 Prague 8, Czech Republic
decrease in the thickness [6]. Speaking of thin oxide films, one may immediately think of dielectric films in complementary metal oxide semiconductor (CMOS) devices. In addition, due to good compatibility with the CMOS processes, high-k materials for the gate dielectric films in CMOS devices have a high potential for ReRAM devices. A HfOx-based memory with a TiN/Ti/HfOx/TiN structure exhibited good performances [7]. ReRAM devices using other high-k materials such as ZrO2 and TiO2 were reported in reviews [3, 5, 6, 8]. CeO2, one of high-k materials, was also studied [9, 10]. These studies reported that the mechanism of ReRAM function was the formation of conductive filament. Resistive switching behavior includes several elementary processes from a viewpoint of chemical reaction: oxygen vacancy or cation formation at a metal/oxide interface (thermodynamic or electrochemical equilibrium), migration of oxygen vacancy or cation (kinetics) causing low re
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