Journal of The Electrochemical Society, 164 (2) H21-H24 (2017) H21 0013-4651/2017/164(2)/H21/4/$33.00 © The Electrochemical Society Polymer-Assisted Solution Processing of TiO 2 Thin Films for Resistive-Switching Random Access Memory Sujaya Kumar Vishwanath, a Sanghun Jeon, b and Jihoon Kim a, z a Division of Advanced Materials Engineering, Kongju National University, Cheonan, Chungchungnam-do 32588, Korea b Department of Applied Physics, Korea University, Sejongro, Sejong 339-700, Korea In this study, we present a polymer-assisted solution (PAS) process to prepare TiO 2 electrolyte layers for resistive-switching random access memory (ReRAM). The PAS process utilizes the stability of metal-polymer complexes in the coating solution to form uniform and dense films. In addition, the viscosity of the PAS coating solution can easily be adapted for any currently used coating technique. The electrochemical-metallization-based (ECM-based) ReRAM devices were prepared by spin-coating the PAS coating solution on an indium tin oxide (ITO) glass substrate that is used as the bottom electrode. Cu was deposited on the PAS-TiO 2 electrolyte as an electrochemically active metal electrode used as the top electrode. The ECM-based ReRAM with the PAS-TiO 2 electrolyte layer demonstrated bipolar resistive-switching behavior with a memory window wider than 13, cycle endurance over 500 cycles, and retention time longer than 10 4 s. Analysis of the conduction mechanism in high and low resistive states indicates that the resistive switching is attributed to the formation and rupture of Cu conducting filaments (CFs) in the PAS-TiO 2 electrolyte layer. © 2016 The Electrochemical Society. [DOI: 10.1149/2.0121702jes] All rights reserved. Manuscript submitted October 10, 2016; revised manuscript received November 17, 2016. Published December 2, 2016. New nonvolatile memory devices with various material character- istics based on ferroelectric polarization, magnetic polarization, and material phase change have been investigated to overcome the scal- ability issue of modern flash memory devices. 1–3 However, recent research efforts have focused on resistive-switching random access memory (ReRAM), also known as conductive-bridge random access memory (CBRAM) or electrochemical-metallization-based (ECM- based) ReRAM, due to its outstanding advantages including a simple structure, low switching power consumption, fast switching speed, and capability of both unipolar and bipolar switching modes. 4–6 ECM- based ReRAM consists of a solid electrolyte thin film in between two electrodes, an electrochemically active metal (EAM) electrode such as Ag or Cu and an electrochemically inert metal (EIM) electrode such as Pt, Au, or W. The EAM, biased with a sufficiently positive-polarity voltage with respect to the counter electrode (EIM), is electrochemi- cally dissolved and migrates through the solid electrolyte toward the EIM electrode. Consequently, a conducting bridge forms between the two electrodes, which is also known as a conducting filament (CF). The resistive switching in the ECM-based ReRAM relies on the for- mation (SET) and rupture (RESET) of the CFs in the solid electrolyte, which depend on the polarity of the applied voltage. 7,8 Various solid electrolyte materials such as perovskites, chalcogenides and metal oxides have been explored for the fabrication of ReRAM. 9–21 Most solid electrolyte films have been deposited using vacuum based pro- cesses such as thermal oxidation, atomic layer deposition, pulsed laser deposition, sputtering, and thermal evaporation. 15,19,22–24 There are also other efforts devoted to growing metal oxide films as the solid electrolyte layers using chemical solution deposition such as sol-gel process. 25,26 However, the preparation of electronics-quality oxide films by chemical solution deposition is generally considered enormously challenging since control of stoichiometry and chemical reactivity of the metals in the functional films is not always straightfor- ward in solution. To improve such difficulties in the sol-gel process, in this study, we employed a polymer-assisted solution (PAS) pro- cess that utilizes the inherent stability of metal-polymer complexes in solution 27–32 to grow a solid electrolyte film for ReRAM. The poly- mer sequesters the metal ions from undesired chemical reactions such as pre-formation of the metal oxide in the solution, assuring an even coating of metal ions over the entire film and thus a uniform metal oxide film after the polymer is decomposed at elevated temperature. In addition, the viscosity of the PAS can be adjusted simply by the amount of the polymer used, making the PAS process applicable to various types of coating techniques such as spin-coating, dip-coating, z E-mail: jihoon.kim@kongju.ac.kr inkjet-printing, spray-coating, and bar-coating, which require differ- ent viscosities for the coating solution. Here, TiO 2 thin films were grown using the PAS process and used as a solid electrolyte for ReRAM devices with a Cu/PAS- TiO 2 /In 2 O 3 :Sn (ITO) glass structure to demonstrate the resistive switching of the PAS-TiO 2 thin films. The ITO glass was selected as an EIM bottom electrode since it provides reasonable electrical conductivity as well as chemical stability during the PAS process in- cluding a heat-treatment under oxygen environment at an elevated temperature. For a EMA electrode for ECM-based ReRAM, Cu was evaporated on to the PAS-TiO 2 by thermal evaporation in vacuum with a metal mask. The ECM-based ReRAM with the PAS-TiO 2 elec- trolyte layer exhibits bipolar resistive switching characteristics with reasonable write endurance and a long retention time. Experimental Ti-PAS was prepared by directly coordinating an anionic complex, titanium (IV) bis (ammonium lactate) dihydroxide (TALH; Aldrich) to polyethyleneimine (PEI, Mw 25000, Aldrich). Initially, 3 grams of PEI was dissolved in 20 mL of deionized water followed by the addition of 3 grams of TALH. The pH of the solution was controlled by HCl to protonate the PEI. Then, the total solution was placed in an Amicon membrane filter (molecular weight cut off limit of 10000) to remove non-coordinated ions from the solution. Analysis of the Ti-PAS by inductively coupled plasma-optical emission spectrometry (ICP-OES; PerkinElmer) indicated that the Ti concentration in the solution was 6.55 mg/mL. TiO 2 films were grown by spin-coating the prepared Ti-PAS on ITO glass substrates (2000 rpm for 60 s) as shown in Figure 1. Depolymerization of the spin-coated films was Figure 1. Schematic illustration of coordinating TALH to PEI and of the spin-coating process for Ti-PAS coated films on ITO glass. ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 128.138.73.68 Downloaded on 2016-12-04 to IP