Effect of post annealing on the resistive switching of TiO 2 thin film Wan-Gee Kim, Shi-Woo Rhee * Laboratory for Advanced Materials Processing (LAMP), Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea article info Article history: Received 17 December 2007 Received in revised form 8 January 2009 Accepted 25 February 2009 Available online 5 March 2009 Keywords: ReRAM TiO2 Resistive switching abstract The effect of annealing on the resistive switching of 35-nm-thick TiO 2 thin film deposited with magne- tron sputtering system was studied. Pt and Ag were used as a top electrode (TE), and Pt was used as a bottom electrode (BE). For Pt/as-deposited TiO 2 /Pt structure, both unipolar (URS) and bipolar resistive switching (BRS) were observed depending on the current compliance level. For Pt/400 °C annealed TiO 2 /Pt structure, only BRS was observed regardless of the current compliance level. The increase in the work function of the TiO 2 film after annealing lowers the potential barrier height and changes the electron transfer process which was also confirmed from Ag/as-deposited TiO 2 /Pt structure. Above 600 °C, the film becomes leaky with the increase in grain size and roughness and the resistive switching behavior was not observed. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction With conventional memories approaching their scaling limit, novel nonvolatile memory devices have drawn much attention. Most of these memory devices are resistance-change-based RAMs (random access memory) with phase change materials [1] or mate- rials with voltage dependent resistivity. Phase change materials are mainly chalcogenide materials and their resistivity depends on the crystallinity. Phase change RAM (PCRAM) uses this resistivity change from the transition between crystalline and amorphous phase through heating and cooling. On the other hand, resistance RAM (ReRAM) uses materials with voltage dependent resistivity. Both materials offer a possibility of high density integration and low power consumption. Generally, there are two types of resistive switching in oxide materials; unipolar resistive switching (URS) and bipolar resistive switching (BRS). URS needs one-side polarity of voltage for switch- ing while BRS needs both polarities. It was believed that these two types of switching phenomena were observed depending on the material. For example, most binary oxides such as NiO [2], TiO 2 [3], and ZrO 2 [4] belong to URS materials, and most pervoskite oxi- des such as Pr 0.7 Ca 0.3 MnO 3 [5], PbZr 0.52 Ti 0.48 O 3 [6], and oxides such as Cu 2 O [7] are among BRS materials. However, it was recently reported that in some binary transi- tion metal oxides such as TiO 2 and ZrO 2 , resistive switching type can be changed depending on the fabrication process and sweep condition. Jeong et al. [8] reported that BRS as well as URS behavior in the Pt/TiO 2 /Pt sample was observed depending on the current compliance during the electroforming process. They mentioned that with a lower current compliance (<0.1 mA) during the electro- forming, BRS was observed while with a high current compliance (1–10 mA), URS behavior was observed. In case of ZrO 2 [9], the ef- fect of the top electrode material on the switching behavior was investigated, and URS and BRS behaviors were observed depending on the top electrode material. In this paper, the effect of post annealing on the resistive switching of TiO 2 film was investigated. 2. Experimental 35-nm-thick TiO 2 binary oxide thin films were deposited with rf magnetron sputtering system on Pt/Ti/SiO 2 /Si substrate. Ti metal target (50.8 mm, 99.995%, SHENZHEN, China) and a gas mixture of Ar and O 2 at a mixing ratio of 10:1 were used to deposit TiO 2 thin films. During the deposition of TiO 2 thin films, rf power and sub- strate temperature were maintained at 150 W and room tempera- ture. The base pressure of the deposition chamber was maintained below 8  10 6 torr by a rotary vane pump and a turbo molecular pump and the working pressure was 7  10 3 torr. After deposition, TiO 2 thin films were annealed at 100 °C to 800 °C for 2 min using rapid thermal process (RTP) to minimize the molecular diffusions. Ar gas was used as a working gas to pre- vent additional change of composition. To make metal–insulator– metal (MIM) structure, Pt and Ag top electrode (TE) of 150 nm were deposited by DC (100 W) magnetron sputtering system and here, the shadow mask with opening diameter of 100 lm was used. 3. Results and discussion TiO 2 thin films were characterized with scanning electron microscopy (SEM), X-ray diffraction (XRD), atomic force microscopy 0167-9317/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2009.02.028 * Corresponding author. Tel.: +82 54 279 2265; fax: +82 54 279 8619. E-mail address: srhee@postech.ac.kr (S.-W. Rhee). Microelectronic Engineering 86 (2009) 2153–2156 Contents lists available at ScienceDirect Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee