Correlation of the resistive switching and polarization switching in zinc oxide thin films using scanning probe microsco
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Lai-Mun Wong and Shijie Wang Institute of Materials Research and Engineering (IMRE), Agency for Science, Technology and Research (A*STAR), 117602, Singapore (Received 30 March 2015; accepted 28 September 2015)
In this study, resistive switching (RS), polarization switching, and charge distribution under DC bias in undoped ZnO thin films are studied by applying scanning probe microscopy (SPM) techniques on the same location. The techniques include Piezoresponce Force Microscopy, Kelvin Probe Force Microscopy, and Conductive Atomic Force Microscopy. The effects of oxygen partial pressure during the film deposition are also investigated. The results show that high resistance state (HRS) is accompanied by the polarization switching and charges storage. By comparing the SPMs results from the same location, it is found that the oxygen partial pressure during film deposition is an important factor over the holes injection during the poling processes in the HRS. On the other hand, the low resistance state (LRS) may be dominated by the electrons injection. Based on these findings, the energy band diagrams in the Pt-tip/ZnO-film/ Pt-bottom-electrode structure with the applications of the external biases are illustrated schematically. This study also proposes a more persuasive mechanism of RS in ZnO films.
I. INTRODUCTION
Nonvolatile Random Access Memories (RAMs) have been developed for data storage applications with higher storage and readout speed; higher data density and lower power consumptions. The RAMs include several categories, such as ferroelectric RAMs (FRAMs), magnetic RAMs (MRAMs), resistive RAMs (RRAMs), and so on.1–7 Generally-speaking, FRAMs and MRAMs have limitations in overcoming the scaling problems,8,9 whereas RRAMs can overcome this limit and achieve a higher storage density. Therefore, RRAMs have been studied extensively in recent years. ZnO-based oxide materials, such as nanorods,10–12 nanowires,13 thin films,14–18 nanocolumns19, and nanoislands,20 are considered to be the promising materials for RRAMs applications. In most of those studies, Pt is used as top and/or bottom electrodes, it can form Schottky contact at the Pt/ZnO interface due to the differences in the work functions between Pt and ZnO.21 In addition, Pt can also act as catalyst for oxygen gas decomposition.22 The possible mechanisms underlying the resistive switching (RS) behavior have also been proposed. One of which is a conductive filament model in which conducting filamentary paths are formed and ruptured between the Contributing Editor: José Arana Varela a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.310 J. Mater. Res., Vol. 30, No. 22, Nov 27, 2015
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two electrodes or in a close vicinity of the electrode/film interface. Another one is the charge trap model in which the resistivity is modified by the defects engineering in film or trapped carrier in electrode/film interface under an electric field.1,2,8 For ZnO-based materials, the ferroelectriclik
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