Reproducible unipolar resistive switching behaviors in the metal-deﬁcient CoO x thin ﬁlm June Sik Kwak a , Young Ho Do a , Yoon Cheol Bae b , Hyunsik Im c , Jin Pyo Hong a, ⁎ a Novel Functional Materials and Device Lab, Department of Physics, Hanyang University, Seoul 133-791, Republic of Korea b Department of Engineering of Nano Semiconductor, Hanyang University, Seoul 133-791, Republic of Korea c Department of Semiconductor Science, Dongguk University, Seoul 100-715, Republic of Korea abstract article info Available online xxxx Keywords: CoO x Resistive switching Nonvolatile memory ReRAM Pt/CoO x /Pt tri-layers exhibited reproducible and stable unipolar switching under a dc sweeping voltage. In order to investigate the role of oxygen reduction in the metal-deﬁcient CoO x layer, resistive switching of the post-annealed CoO x thin ﬁlm was compared with those of the as-deposited CoO x thin ﬁlm. X-ray photoemission spectroscopy showed larger reproducible resistance switching and decreasing of current level in the post-annealed CoO x thin ﬁlm. This may be explained by a reduction in oxygen stoichiometry without phase transformation of the CoO x . In addition, stable switching in post-annealed CoO x layer is considered to originate from the decrease of the Co vacancies in local Co 3 O 4 region partially distributed in the whole CoO x layer, not in the dominant CoO. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Resistive switching materials for next-generation nonvolatile memory (NVM) were widely investigated in various oxide materials, such as binary transition metal oxides (TMO) [1–11], perovskite structure ternary oxides [12,13] and others. The resistive random access memory (ReRAM) is based on the resistance switching effect and is considered to be the most promising candidate for NVM due to superior properties including simple structure, high density integra- tion, high speed switching, and low power consumption. From a practical point of view, binary TMOs are some of the most compatible materials for ReRAM applications because of simply controllable stoichiometry and compatibility with standard complementary metal-oxide semiconductor technology (CMOS) [2,14–18]. Resistance switching in binary TMO thin ﬁlms, such as NiO [1–4], TiO 2 [5–8], HfO 2 [9], and CoO x [10,11], has been intensively studied in terms of device properties and modeling of the resistive switching mechanism. Although exact microscopic mechanisms are still under discussion, the conductive ﬁlamentary paths model is generally considered as a dominant switching mechanism in binary TMO ﬁlms. According to the ﬁlamentary path model, it is reasonable to assume that the electro-migration of oxygen vacancies or metal vacancies might play an important role in the generation and annihilation of conductive ﬁlamentary paths [1,2,4–7,16,18]. There- fore, the dependence of the vacancy characteristics will be an important parameter in determining the resistance switching properties. CoO is a rocksalt 3d transition metal monoxide with interesting electrical properties. Theoretical calculations of ideally stoichiometric CoO predict that it is a charge transfer insulator with a band gap of approximately 2.4 eV [10,11]. However, because of non-stoichiome- try, the CoO x thin ﬁlm can become a p-type oxide with metal deﬁciency [12]. Therefore, it is expected that the transport and switching properties of CoO x can be controlled by varying oxygen content in CoO x thin ﬁlms. In this article, we investigate the resistive switching properties of as-deposited CoO x and post-annealed CoO x ﬁlms, as well as the effect of oxygen reduction during the post- annealing process. 2. Experimental details A Pt/CoO x /Pt metal–oxide–metal (MOM) structure was fabricated as a resistance switching cell. The 100-nm-thick Pt layer and 30-nm- thick Ti layer were prepared on SiO 2 /Si (100) substrate using dc magnetron sputtering at room temperature, where the 30-nm-thick Ti layer was deposited for adhesion. Subsequently, the 50-nm-thick CoO x layer was deposited on the Pt electrode by radio frequency sputtering using a Co–O target (99.95% pure) under an argon pressure of 1 mTorr. In order to investigate the resistance switching characteristics of CoO x , 100-nm-thick Pt top electrodes were fabricated by a conven- tional photolithography and lift-off process. The size of the top electrode was patterned to be 50 μm×50 μm. Post-thermal annealing at 400 °C was performed in an argon atmosphere using a rapid thermal annealing (RTA) chamber for a few minutes. As a baseline experiment, the valence state of Co in CoO x and in the post-annealed CoO x layer was determined using KRATOS AXIS NOVA x-ray Thin Solid Films xxx (2010) xxx–xxx ⁎ Corresponding author. Tel.: +82 2 2220 0911; fax: +82 2 2295 6868. E-mail address: jphong@hanyang.ac.kr (J.P. Hong). TSF-27307; No of Pages 4 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.03.050 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf ARTICLE IN PRESS Please cite this article as: J.S. Kwak, et al., Thin Solid Films (2010), doi:10.1016/j.tsf.2010.03.050