DOI: 10.1002/cvde.201407122 Full Paper Atomic Layer Deposition of Transparent VO x Thin Films for Resistive Switching Applications** By Trilok Singh, Shuangzhou Wang, Nabeel Aslam, Hehe Zhang, Susanne Hoffmann-Eifert, and Sanjay Mathur* Atomic layer deposition (ALD) offers nearly pinhole-free, conformal, and with good thickness control, metal oxide nanometric thin films required for next-generation memory devices. Here we report on the ALD of VO x thin films grown at about 100°C from a vanadium tri-isopropoxide (VTIP) precursor, with water as the co-reactant, followed by their post-growth treatments, for potential applications in resistive switching (RS) devices. As-grown VO x films are amorphous, and transform into polycrystalline layers upon annealing. Capacitor structures fabricated from amorphous VO x films show current-voltage (I-V) characteristics, interesting for RS applications. Depending on the electroforming conditions, bipolar-type memory switching with a resistance ratio R OFF /R ON > 10 3 is obtained, as well as a combination of memory and threshold switching. The latter is attractive for its highly non-linear I-V characteristics, which is attributed to the temperature-induced insulator-to-metal transition (IMT) in vanadium dioxide. Keywords: ALD, Resistive switching, Thin Films, VO x 1. Introduction Nonvolatile random access memory (NV-RAM) devices promise advantages such as low energy consumption and high performance, together with cost effectiveness due to continuous scaling down. [1–3] Although Si-based flash memory devices still dominate the NV-RAM market, they suffer from a relatively high power consumption, low write speed, and moderate endurance. Recently, transition metal oxide-based resistance-switching RAM (ReRAM) has attracted considerable attention as a potential candidate for next-generation NV-RAM technology because of its low operation power, fast read and write access, high retention, and a quite simple capacitor-like metal-oxide-metal (MOM) device structure. [4–7] The group V transition metal oxide, vanadium oxide, is an interesting material for RS applica- tions due to the huge number of VO x polymorphs which obey significantly different electrical properties. While V 2 O 5 is an insulator with a band-gap of about 2.2 eV, Cr-doped V 2 O 3 shows an iso-structural Mott transition from an insulating anti-ferroelectric to a conducting metal under increasing pressure. At about 67°C under atmospheric pressure, VO 2 reveals a temperature-induced IMT which is accompanied by a structural transformation. [8–11] In addition to these configurations, mixed-valence compounds, e.g., Magnéli phases (V n O 2n-1 ), also exist, making phase-selective deposition of VO x coatings a challenging task [12,13] for both material synthesis and device fabrication. Some vanadium oxides with different oxidation states and their correspond- ing IMT temperatures are outlined in Figure 1. In the present work, we have grown thin films of VO x using ALD to obtain dense, pinhole-free thin films with large area uniformity and conformal coverage on silicon and soda- lime glass substrates. The initial results showed that as- deposited VO x thin films are able to perform non-volatile bipolar memory switching, or threshold switching, depend- ing on the electroforming treatment. 2. Results and Discussion 2.1. Thin Films Characterization The thermal ALD window for the H 2 O-based processes of VO x thin films from a VTIP precursor was determined from the temperature plot of the growth per cycle (GPC) values. These were determined from the slopes of the thickness vs. cycle number plot for each temperature. The resulting GPC vs. T graph in Figure 2 shows the ALD growth mode in the temperature window from 110 to150°C, and with a stable growth rate of 0.037 nm per cycle, which is [*] Prof. S. Mathur [#] , Dr. T. Singh, S. Wang Institute of Inorganic Chemistry, University of Cologne, Cologne-50939, (Germany) [#] International Research Center for Renewable Energy, School of Energy & Power Engineering, Xian Jiaotong University, Xian (Shaanxi) 710049, P.R. China E-mail: sanjay.mathur@uni-koeln.de N. Aslam, H. Zhang, Dr. S. Hoffmann-Eifert Peter Gruenberg Institute (PGI-7), Forschungszentrum Juelich and JARA-FIT, 52425, Juelich (Germany) [**] We acknowledge the Excellence Program of the University of Cologne and the financial support provided under the UoC-Forum (MOX-Switch project). This work has been supported in parts by the European Community’s 7th Framework Programme (FP7/2007-2013) under Grant ENHANCE-238409. Chem. Vap. Deposition 2014, 20,1–7 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 1 Final page numbers not assigned