Abnormal coexistence of unipolar, bipolar, and threshold resistive switching in an Al/NiO/ITO structure
© Yuan et al.; licensee Springer. 2014
Received: 17 February 2014
Accepted: 16 May 2014
Published: 29 May 2014
This paper reports an abnormal coexistence of different resistive switching behaviors including unipolar (URS), bipolar (BRS), and threshold switching (TRS) in an Al/NiO/indium tin oxide (ITO) structure fabricated by chemical solution deposition. The switching behaviors have been strongly dependent on compliance current (CC) and switching processes. It shows reproducible URS and BRS after electroforming with low and high CC of 1 and 3 mA, respectively, which is contrary to previous reports. Furthermore, in the case of high-forming CC, TRS is observed after several switching cycles with a low-switching CC. Analysis of current-voltage relationship demonstrates that Poole-Frenkel conduction controlled by localized traps should be responsible for the resistance switching. The unique behaviors can be dominated by Joule heating filament mechanism in the dual-oxygen reservoir structure composed of Al/NiO interfacial layer and ITO. The tunable switching properties can render it flexible for device applications.
Binary transition metal oxides like NiO, TiO2, and ZnO have attracted much attention in the field of resistive switching due to simple constituents, low deposition temperature, and compatibility with complementary metal-oxide semiconductor technology [1, 2]. Interestingly, different resistive switching behaviors have been found in metal/NiO/metal when different electrode materials were employed, such as Pt, Ag, Cu, and Al [3–6]. Lee et al. have found unipolar resistive switching (URS) in Ag(Cu)/NiO/Pt due to the formation of an oxide layer at the metal/NiO interface . Chiang et al. have demonstrated that bipolar resistive switching (BRS) in Al/NiO/indium tin oxide (ITO) as Al/NiO interfacial reaction region combined with ITO can form a dual-oxygen reservoir structure . In addition, Ni/NiO/Ni with different device structure exhibits URS and BRS modes, separately driven by electrochemical- and thermal-based mechanisms . Threshold resistive switching (TRS) and URS in NiO thin film were also found at different measuring temperatures by Chang et al.. The occurrence of TRS and BRS in Mn-doped ZnO device was found with a higher CC by Yang et al. due to Joule heating . More recently, the transition from URS to TRS can be tuned by the strong electron correlation through controlling the film stoichiometric ratio . Moreover, the coexistence of different resistive switching behaviors has been found in many materials such as BiFeO3[11, 12], HfO2[13, 14], SrTiO3, ZnO [16–18], diamond-like carbon , and TiO2. The choice of switching modes can broaden device applications and enable large flexibility in terms of memory architecture . Generally, URS was preferred under high compliance current (CC), while BRS under low CC. In this letter, we present an abnormal coexistence of URS with a low CC and BRS under high CC in the same Al/NiO/ITO device. Meanwhile, TRS was also observed by reducing the switching CC to forming CC. The Joule heating filament mechanism in a dual-oxygen reservoir structure composed of Al/NiO layer, and the ITO substrate was responsible for the abnormal resistance switching.
Results and discussions
In order to get further understanding of the mechanism of the URS and BRS behaviors, two linear fitting curves of log(I)-log(V) were separately depicted in Figure 3c,d. At LRS for both URS and BRS, the linearity curves with slopes of almost 1 indicate ohmic conduction behavior, which were typically due to the formation of conductive filaments in NiO. However, at the HRS state, the I-V characteristic was more complicated and could be divided into two parts. At low voltages, the I-V curve was linear, corresponding to the ohmic mechanism region. At high voltages, the slope was much larger than 1, indicating that the conduction mechanism was dominated by trap-limited space charge-limited current (SCLC) conduction. In addition, by fitting ln(I/V) ~ I1/2 curve in HRS as shown in the blue lines, it seems to be governed by Poole-Frenkel (PF) emission that involves thermal effect and trap sites such as oxygen vacancies. The detailed mechanism of resistive switching based on these effects will be explained later.
NiO thin films were prepared by solution route with nickel acetate as the metal source. By control forming and switching CC, URS, BRS, and TRS were found in the same Al/NiO/ITO device. URS existed at low-forming CC, while BRS at high-forming CC, which was different from previous reports. From the fitting curves of I-V, the HRS at low voltage and LRS were dominated by Ohmic conduction, and the HRS at high voltage could be attributed to the PF emission that involves thermal effects and trap sites such as oxygen vacancies. The switching mechanism was discussed based on the dual-oxygen reservoir structure model in which the ITO electrode and Al/NiO interface acts as the oxygen reservoirs. No matter what the direction of the electric field is, the dual-oxygen reservoir structure will support the oxygen vacancies to form the conductive filaments. The reset process indicates that Joule heating might be the main factor in rupturing the conductive filaments. When the forming and switching CC was equal, we found TRS after several loop tests. It was caused by spontaneous rupture of the filaments of insufficient heat dissipation at higher CC due to the Joule heating. The tunable switching properties would enable large flexibility in terms of device application.
This work has been supported by the Open Project of the State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials (No. 11zxfk26), the Fundamental Research Funds for the Central Universities (ZYGX2012J032), and the Open Foundation of the State Key Laboratory of Electronic Thin Films and Integrated Devices (KFJJ201307).
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