Unipolar resistive switching of ZnO-single-wire memristors
© Huang et al.; licensee Springer. 2014
Received: 11 April 2014
Accepted: 15 July 2014
Published: 7 August 2014
Well unipolar resistive switching (RS) behaviors were observed from Ag/ZnO single-microwire/Ag memristors. The reset voltages were larger than the set voltages, and all of them were less than 1 V. The resistance ratios of high-resistance state (HRS) to low-resistance state (LRS) reached 103. The bistable RS behaviors were entirely reversible and steady within 100 cycles. It was found that the dominant conduction mechanisms in LRS and HRS were ohmic behavior and space-charge-limited current (SCLC), respectively.
KeywordsZnO Resistive random access memory (RRAM) Resistive switching (RS) Electrical properties
Resistive random access memory (RRAM) and memristor have attracted rapidly increasing attention due to their high-speed operation, high-density storage, and low-voltage driving virtues for nonvolatile memory (NVM) applications [1, 2]. Generally, a memristor is composed of a metal-insulator-metal (MIM) cell, where the NVM effect comes from their ability of reversible resistive switching (RS) between low-resistance state (LRS or RON) and high-resistance state (HRS or ROFF) under voltage stimulus. Among the various candidate materials for RRAM and memristor, zinc oxide (ZnO) has promising advantages, such as facile synthesis, reversible and steady RS property, and low set and reset voltages [3–5]. Up to now, memristors based on ZnO thin films have been reported according to their RS behaviors from intrinsic defects (e.g., oxygen vacancies) and extrinsic impurities (e.g., Ag+ ions) [6–8]. However, several serious problems for memristors still exist. First of all, the RS mechanisms are still subjects of heated debate. Second, the operating voltages are usually too large and expected to be less than 1 V. Finally, the RS behavior in a single ZnO microwire has seldom been reported, but could have special applications due to its one-dimensional structure which include memristors, nanolasers, photodiodes, nanogenerators, gas sensors, acoustic resonators, piezoelectric gated diodes, etc. [5, 9].
In this paper, we report on a ZnO single-wire memristor with low driving voltage and high stability as well as its interesting RS behaviors. Well unipolar RS properties were observed, including the set and reset voltages less than 1 V, resistance ratio as high as 103, and strong endurance stability within 100 cycles. Abnormally, the reset voltages are observed to be larger than the set voltages, which are contrary to most previous reports and are explained by the space-charge-limited current (SCLC).
Results and discussion
Figure 1a shows the SEM image of a typical ZnO microwire, whose length is about 1.5 mm and diameter is about 20 μm. Interestingly, as clearly confirmed by the upper inset of Figure 1a, hierarchical structures can be observed in the microwire. The formation of such ZnO hierarchical microwires can be attributed to the fast growth habit in <001 > direction and second nucleation on the side surfaces.
Figure 1b presents the typical unipolar RS behaviors of the device. First, electrical stress was loaded through a 1.5-V-forming voltage to induce an LRS. The current compliance was restricted at 1 mA to prevent permanent breakdown. Subsequently, in such an LRS, when the voltage was swept from zero to positive values (1 V), the leakage current increased approximately linearly and then very abruptly dropped approaching to zero at 0.8 V (reset voltage, Vreset). Such an abrupt current drop indicated that the device had been switched into HRS, which is a nonvolatile off state and will be inherited in the early stage of the next voltage sweeping. Finally, during the second voltage sweep, a sudden current increase at about 0.2 V (set voltage, Vset) appeared. Such a sudden increase over the compliance value demonstrated that the device was switched into LRS again, which is the nonvolatile on state and can also be memorized in the following cycle. Furthermore, when sweeping the voltage to negative voltages, similar RS behaviors, including on-off switching and state memorizing, were also observed.
Besides the above typical RS, some unusual phenomena were also observed. First, Vreset was found to be always larger than Vset as shown in Figure 1c, which is entirely different from the reported unipolar RS from MIM thin films . Second, Vreset and Vset distribute in 0.62 to 0.8 V and 0.19 to 0.4 V, respectively. Both of them are less than 1 V, which will be very beneficial for the future application with low energy cost. Importantly, there is no overlap between these two ranges. Such obviously separated Vreset and Vset warrant a high reliability for device operation and, hence, also beneficial to application. Finally, the Vset distribution width is slightly larger than that of the Vreset, which demonstrates that conducting filaments (CF) dominate the RS of such ZnO microwire memristors prepared in this study. According to the CF model [3, 11, 12], the formation of filaments (set) is more random than their rupture (reset) process due to the competition of different filamentary paths during the formation process.
In summary, a memristor device with well unipolar resistive switching performances has been fabricated, for the first time, based on the single ZnO microwire and Ag electrodes. The single ZnO microwire memory is stable, rewritable, and nonvolatile with an on/off ratio over 1 × 103, operating voltages less than 1 V, and high-endurance stability. Abnormally, the reset voltages are observed to be larger than the set voltages. The resistive switching could be explained by conducting filamentary mechanism. The conduction mechanisms dominating the low- and high- resistance states are proposed to be ohmic behavior and space-charge-limited current, respectively. The simple structure, large on/off ratio, and bistable performance of the device make it very attractive for nonvolatile resistive switching memory applications.
This work was financially supported by the National Basic Research Program of China (2014CB931700), NSFC (061222403, 51072081), the Doctoral Program Foundation of China (20123218110030), the Opened Fund of the State Key Laboratory on Integrated Optoelectronics (IOSKL2012KF06), and the Scientific Foundation of Jinling Institute of Technology (jit-b-201201, jit-b-201202, and jit-b-201203).
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