Nano Letters LETTER 200K 90K Figure 2. Temperature-dependent I-V measurements of PRM cells at ondition.(a)I-Vcharacteristics of ZnO PRM cells at 300,200, and 90K, respectively. The sweeping frequency is 0.1 Hz ( Inset)Schematic bias condition of the PRM cell when Schottky barrier at source side reversely biased. (b) Dependence of threshold voltages on temperat (in black )and Vehp (in red)increased in magnitudes almost linearly with the decreasing temperature. The hysteresis loop increased with the g temperature. Vhs=0.74+0.51 Vand Vuh. D=-068+0. 20 V for PRM cells of the current for PRM cell at very large bias(V=+10 V)were with treatment for 30, 20, and 10 min, respectively, by oxygen almost constant and independent of temperature( Figure 2a plasma( Figure 1c). The predictable electrical properties of these and Supporting Information Figure $3) nO NWs with controlled treatment process enable the repro- Although the nature of resistive switching and related charge ducible assembly of NW structures at large quantity for further transport process at microscale in M-S-M structures is still applications. The slight difference and asymmetry observed under debate, the movement of charged species that mod- between Vths and Vth. D within each group of PRM cells is ulates the current flow seems to be a dominant mechanism. 3 possibly induced by the nonuniform geometry of the Zno Drift/diffusion of defects such as positively charged oxyge NW, as indicated by the AFM image inset in Figure lal. It is vacancies under applied electrical field has been suggested to well-known that the Schottky barrier height(SBH) induced at change the electronic barrier at the metal/semiconductor inter- metal/semiconductor interface can be affected by factors such as face, which possibly results in the observed resistive switching the geometry and effective areas of the contact. Moreover, the Oxygen vacancies are known to be one of the predominant ionic interface/ surface states can also shift the SBH. In addition, the defects in ZnO and can influence the Schottky contacts morphology of the ZnO NWs has been monitored before and between ZnO and metal electrodes. On the basis of the after the plasma treatment and bvious variations are observed experimental results, (Supporting Information Figure S3). Notably, as can be seen from transport of charged dopants and electrons under applied electric the semilogarithmic plots of the data presented in Figure lal-a3 field is adopted and modified to explain the hysteretic switchi Supporting Information Figure S9), the PRM cells gradually lose behavior of the PRM cell without external deformation applied their nonvolatility as the period of oxygen plasma treatment increases. The drift /diffusion of the oxygen vacancies toward the interface The nonvolatility of PRM cells with 10 min oxygen plasma treatment effectively reduces the local SBH, while the drift/diffusion of can be observed by sweeping only at positive voltages(0-1 V) and vacancies away from the interface increases the SBH. the high-conductance state is not lost at small bias in subsequent The hysteretic switching sequences obtained at different sweeps(for 20 cycles), which indicates this kind of cells do have temperatures can be characterized by four typical regions: (1) the memory effect(Supporting Information Figure S10). The PRM O-A(2)A-B-O(3)O-C and(4)C-D-0, as labeled in cells with 30 min oxygen plasma, however, did not show the same Figure 2a as an example For easy discussion, the bias is set to be nonvolatility in current experiment. applied on the drain(d)electrode with respect to the source Temperature-dependent I-V measurements(Methods (S)side(Inset sketch in Figure 2a). The overall macroscopic were performed to obtain further insight into the switching resistance of the PRM cell is RPRM=Rs RNw +Rp, where Rs mechanism of the PRM cell without applying external deforma- and Rp are the electrical resistances contributed by Schottky tions.Representative result acquired from an Au/ZnO-NW/Au barriers at source and drain sides that may vary during the PRM cell clearly demonstrates the variations with temperature experiment and RNw is the intrinsic resistance of the ZnO Nw. in the hysteretic I-V switching characteristics(Figure 2a). The It has been previously demonstrated that for semiconductor hreshold turn-on voltage for the reversely biased Schottky NWbased M-S-Mstructure, the I-V transport characteristic barrier of the PRM cell increased almost linearly with decreas is normally dictated by the reversely biased Schottky barrier are(Fi b), and the hysteresis loop inc As the bias volt with the decreasing temperature( Figure 2a). The magnitudes drain side forward-biased, the voltage drops mainly at the 27812781 dx.doi.org/10.1021/nl201074a |Nano Lett. 2011, 11, 2779–2785 Nano Letters LETTER Vth,S = 0.74 ( 0.51 V and Vth,D =
0.68 ( 0.20 V for PRM cells with treatment for 30, 20, and 10 min, respectively, by oxygen plasma (Figure 1c). The predictable electrical properties of these ZnO NWs with controlled treatment process enable the repro￾ducible assembly of NW structures at large quantity for further applications. The slight difference and asymmetry observed between Vth,S and Vth,D within each group of PRM cells is possibly induced by the nonuniform geometry of the ZnO NW, as indicated by the AFM image inset in Figure 1a1. It is well-known that the Schottky barrier height (SBH) induced at metal/semiconductor interface can be affected by factors such as the geometry and effective areas of the contact.25 Moreover, the interface/surface states can also shift the SBH.25 In addition, the morphology of the ZnO NWs has been monitored before and after the plasma treatment and no obvious variations are observed (Supporting Information Figure S3). Notably, as can be seen from the semilogarithmic plots of the data presented in Figure 1a1
a3 (Supporting Information Figure S9), the PRM cells gradually lose their nonvolatility asthe period of oxygen plasmatreatment increases. The nonvolatility of PRM cells with 10 min oxygen plasma treatment can be observed by sweeping only at positive voltages (0
1 V) and the high-conductance state is not lost at small bias in subsequent sweeps (for 20 cycles), which indicates this kind of cells do have the memory effect (Supporting Information Figure S10). The PRM cells with 30 min oxygen plasma, however, did not show the same nonvolatility in current experiment. Temperature-dependent I
V measurements (Methods) were performed to obtain further insight into the switching mechanism of the PRM cell without applying external deforma￾tions. Representative resultacquired from an Au/ZnO-NW/Au PRM cell clearly demonstrates the variations with temperature in the hysteretic I
V switching characteristics (Figure 2a). The threshold turn-on voltage for the reversely biased Schottky barrier of the PRM cell increased almost linearly with decreas￾ing temperature (Figure 2b), and the hysteresis loop increased with the decreasing temperature (Figure 2a). The magnitudes of the current for PRM cell at very large bias (V = (10 V) were almost constant and independent of temperature (Figure 2a and Supporting Information Figure S3). Although the nature of resistive switching and related charge transport process at microscale in M
S
M structures is still under debate,1
5 the movement of charged species that mod￾ulates the current flow seems to be a dominant mechanism.3 Drift/diffusion of defects such as positively charged oxygen vacancies under applied electrical field has been suggested to change the electronic barrier at the metal/semiconductor inter￾face, which possibly results in the observed resistive switching.4 Oxygen vacancies are known to be one of the predominant ionic defects in ZnO26 and can influence the Schottky contacts between ZnO and metal electrodes.24 On the basis of the experimental results, a general model based on the coupled transport of charged dopants and electrons under applied electric field4 is adopted and modified to explain the hysteretic switching behavior of the PRM cell without external deformation applied. The drift/diffusion of the oxygen vacancies toward the interface effectively reduces the local SBH, while the drift/diffusion of vacancies away from the interface increases the SBH. The hysteretic switching sequences obtained at different temperatures can be characterized by four typical regions: (1) O
A (2) A
B
O (3) O
C and (4) C
D
O, as labeled in Figure 2a as an example. For easy discussion, the bias is set to be applied on the drain (D) electrode with respect to the source (S) side (Inset sketch in Figure 2a). The overall macroscopic resistance of the PRM cell is RPRM = RS + RNW +RD, where RS and RD are the electrical resistances contributed by Schottky barriers at source and drain sides that may vary during the experiment and RNW is the intrinsic resistance of the ZnO NW. It has been previously demonstrated that for semiconductor NW based M
S
M structure, the I
V transport characteristic is normally dictated by the reversely biased Schottky barrier side.19,27,28 As the bias voltage sweeps from O to A with the drain side forward-biased, the voltage drops mainly at the Figure 2. Temperature-dependent I
V measurements of PRM cells at strain-free condition. (a)I
V characteristics of ZnO PRM cells at 300, 200, and 90 K, respectively. The sweeping frequency is 0.1 Hz. (Inset) Schematic showing the bias condition of the PRM cell when Schottky barrier at source side is reversely biased. (b) Dependence of threshold voltages on temperature. Both Vth,S (in black) and Vth,D (in red) increased in magnitudes almost linearly with the decreasing temperature. The hysteresis loop increased with the decreasing temperature