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 [3]. 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.
These ZnO microwire memristors exhibited very high stability as shown in Figure 2. The on and off resistance values were read at 0.1 V in 100 DC sweeping cycles. The reading values of HRS appear to fluctuate from 1.2 to 28 MΩ in the early cycles, but become quite steady after 60 cycles, exhibiting very small fluctuations from 0.2 to 0.5 MΩ. When compared with previous reports [3], the LRS reading values here are relatively stable. Moreover, the on/off resistance ratios of HRS to LRS are as large as 103 to 104. Such high stability and large on/off ratios will greatly benefit the nonvolatile storage.
To further understand the switching mechanisms, the I-V curves were re-plotted in a log-log scale as shown in Figure 3a. The low-voltage regions in both LRS and HRS can be well fitted linearly, and all slopes are close to 1. This implies that the conduction mechanisms of both LRS and HRS in the low-electric field region are ohmic behavior. Furthermore, the fitting line can run through the whole I-V curve of the LRS, indicating that ohmic behavior is still effective for the LRS under a high-electric field, which is consistent with the typical CF model [3, 11, 12]. Therefore, only the electron transport of HRS under a high-electric field, marked by a frame in Figure 3a, is abnormal and needs more explanation.
For such nonlinear I-V characteristic of HRS under a high-electric field, there are three leakage mechanisms, namely, space-charge-limited current (SCLC) [13], Schottky emission [14], and Poole-Frenkel (PF) emission [15]. The corresponding I-V curves can be described following different relations, where e is the electronic charge, ϵ
r
is the relative dielectric constant, ϵ0 is the permittivity of free space, d is the film thickness, k is Boltzmann’s constant, and T is the temperature. Obviously, there are linear relationships of lnI vs V1/2, ln(I/V) vs V1/2, and I vs V2 for Schottky, PF, and SCLC mechanism, respectively.
(1)
(2)
(3)
The I-V curves of HRS under a high-electric field were re-plotted in these three kinds of scales as shown in Figure 3b. Very obviously, among these three re-plotted curves, the linearity degree of the I vs V2 curve is the highest, which demonstrates that the conduction mechanism of HRS in a high-electric field is dominated by SCLC mechanism.Figure 4 is the HRTEM image for a tiny part in the ZnO microwire. A number of crystal defects such as dislocations and stacking faults could be found in it. Even though a few stacking faults are terminated by partial dislocations, many of them are typically extended at about 10 nm between the two bounding partial dislocations. A plausible model for the occurrence of stacking faults is ascribed to condensation of vacancies or interstitials in the ZnO microwires thus leading to a missing or inducing additional (0002) plane. These extra stacking faults may provide good paths for electron transfer in the wires, and this fact results in a decrease of the electrical resistivity. Accordingly, the switching behaviors can be described as follows. The as-prepared ZnO microwire is insulating and contains many oxygen vacancy traps. Under the driving of a forming voltage, the abundant oxygen vacancies would be driven toward the cathode to assemble a conducting channel through the microwire’s grain boundaries, and hence, the device switches from the off to the on state. That is, the defects align to form tiny conducting filaments in the HRS and these tiny conducting filaments gather together to form stronger and more conducting filaments leading to the transition to the LRS. However, with the limit of compliance current, the loss of oxygen is not that serious that the HRS can be recovered through the redistribution of oxygen vacancies because of the passing of higher current and the Joule heating in the following voltage sweep, which corresponds to the reset process, whereas the so-called set process corresponds to the recovery of conductive filaments.