Nano Letters LETTER positive at source and drain barriers, respectively. The I-V out by a read pulse(2 s)with a small magnitude(Vreadl Vhs +) characteristic is now dictated by the reversely biased drain side Subsequently a negative erase pulse(10 ms )with Vh.Do Verasel< in this case, and a shift of switching threshold voltage from Veh. Vth, D resets the A and B cells from the lRS to the HRS state, while Do to Vth. D+ was observed, indicating a larger bias has to be sets the C cell into the LRS state. A follow-up read pulse(2 s)with applied in order to switch the PRM cell from HRS to LRS state. a small negative value(VreadI >Vid)is applied to read the new By the same token, in the case of applying a compressive strain states of the cells. In the third step, a positive erase pulse(10 ms) to the PRM cell, the shift of switching threshold voltage from with Vth Verase2< Vth, so sets the A cell from the hRS to the Vuh, so to Vth,s and Veh, Do to Vth, D can be explained LRS state again, while keeps the B and C cells in the HRS state Under strain free condition and if the applied external bias The same Vreadi pulse is then applied to read the new states of the exceeds the threshold voltage, the device is in LRS, and the cells. Finally, a negative write pulse(10 ms)with Vah. D+<Vwritez< concentration of oxygen vacancies in the Nw can significantly Vah, Do resets the A cell into the HRS state, while sets the B and C influence the total conductance of the Nw as well as the SBHs at cells into the LRS state. The same Vread2 pulse is then applied to the source/drain(see Figure al-a3 and Supporting Information read the new states of the cells. After the above series of pulse train Figure S1). Now we consider the case that a strain is applied to is applied, the waveform of the output currents of the cells is the PRM cell. The effect of piezopotential can be equivalently monitored and analyzed( Figure 4). If the logic levels of the outp taken as applying a positive voltage at the barrier interface if the currents with positive, almost zero and negative values are labeled local piezoelectric polarization charges are positive, which in as"1,"0"and"-1", the logic pattem of"10 1 0" indicates the effect decreases the value of the external bias required to over ositive nature of the"stored"strain in A cell. The logic patterns of come the SBH at the interface. Alternatively, a negative voltage is 100-1"and"0-1 0-1"represent the zero and negative strain created to act on the interface if the polarization charges are status of the B and C cells, respectively. a quantitative analysis of negative, which increases the value of the external bias required to the magnitudes of the output currents can give the absolute values overcome the barrier at the interface. From the data shown in of the strains stored in the PRM cells. Although there have been Figure 3a, the HRS window remains almost constant regardless numerous research as well as commercial products on strain the magnitude and sign of the applied strain, indicating the shifts detection and measurements such as semiconductor MEMS, in the observed threshold voltages under different strains are NEMS piezoresistive strain gauges, 5.36 the PRM cells demon dictated by the piezoelectric polarization charges at the interfaces strated here are fundamentally different from the these devices. and the contribution from the diffusion of the oxygen vacancies Piezoresistance effect is a nonpolar and symmetric effect resulting has negligible effect. This is because the oxygen vacancies are from the band structure change, while the PRM cells are based on distributed in the entire NW, while the piezoelectric charges are the asymmetric piezotronic effect. ZnO is a polar structure along c- accumulated right at the very near barrier interface in a region less axis, straining in axial direction(c-axis) creates a polarization of than a subnanometer. The diffusion force contributed by the cations and anions in the nw growth direction, resulting in a piezoelectric charges on the oxygen vacancies is a long-range piezopotential drop from V to V along the NW, which produces nteraction force, thus a variation of the vacancy concentration at an asymmetric effect on the changes in the Schottky barrier heights the interface owing to piezoelectric effect is rather small. (SBHs) at the drain and source electrodes. The strain sensor based Furthermore, the entire I-v curves are, " translated for a on piezotronic effect has been reported to possess much higher sensitivity than previously reported devices caused by the piezoelectric charges. For the samples pretreated in In summary, by utilizing the strain-induced polarization charges oxygen plasma, the concentration of the oxygen vacancies was created at the semiconductor/metal interface under externally rgely reduced in the Nw, thus, the screening effect of the free applied deformation as a result of piezotronic effect, the switching ge carriers to the piezoelectric charges was significantly characteristics of the ZnO NW resistive switching devices can be reduced, and the effect of the piezoelectric charges is enhanced. modulated and controlled. We further demonstrated that the logi Therefore, the shifts in threshold switch voltages due to piezo- levels of the strain applied on the memory cell can be recorded and electric polarization at both drain and source sides for a fixed read out for the first time utilizing the piezotronic effect, which strain have the same magnitude but opposite polarities, provided has the potential for implementing novel nanoelectromechanical that the doping level is low. This indicates that the magnitude of memories and integrating with NEMS technology to achieve the piezopotential at the interface is as large as 1.2 V micro/nanosystems capable of intelligent and self-sufficient multi- 0.S-0.76% of strain. The oxygen plasma pretreatment to the dimensional operations. Taking advantage of the recently The fabra y also improve the output of the nanogenerator developed large-scale fabrication technique of ZnO NW arrays, nonvolatile resistive switching memories using ZnO NW array as memory, in which the write/read access can be programmed via the storage medium may be readily engineered and implemented mechanical actuation. A pulse train consisting of several write/ for applications such in flexible electronics and force/pressure d/erase pulses is applied to the PRM cell to record and read out imaging. Non-Boolean neuromorphic computing might also be the polarity/logic levels of the "stored strain in the cell, by realized by integrating arrays of high-density resistive memory monitoring the characteristic patterns in the output current cells",on flexible substrates ( Figure 4). The data shown in Figure 4 was obtained for the same PRM cell under different strain status, which is equivalent to the cases of three identical PRM cells under tensile strain(A cell),zero ■ METHODS SUMMARY strain(B cell), and compressive strain(C cell), for easy descrip- ee Supporting Information for details ion.First, a positive write pulse(10 ms)with Voh. so< Vwritel < Vth. The ZnO nanowires were synthesized via a physical vapor is applied to these three cells. This short pulse sets the a and B deposition process based on thermal evaporation of ZnO cells switch from the HRS to the LRS state, while the C cell powders without the presence of catalyst. Large-scale ZnO emains in the HRS state. The status of the three cells are then read NWs were subsequently transferred to the PET receiving 2784 dx. dolora/0.102/n201074a| Nano Lert.2011l277927852784 dx.doi.org/10.1021/nl201074a |Nano Lett. 2011, 11, 2779–2785 Nano Letters LETTER positive at source and drain barriers, respectively. The I
V characteristic is now dictated by the reversely biased drain side in this case, and a shift of switching threshold voltage from Vth, D0 to Vth,D+ was observed, indicating a larger bias has to be applied in order to switch the PRM cell from HRS to LRS state. By the same token, in the case of applying a compressive strain to the PRM cell, the shift of switching threshold voltage from Vth,S0 to Vth,S_ and Vth,D0 to Vth,D_ can be explained. Under strain free condition and if the applied external bias exceeds the threshold voltage, the device is in LRS, and the concentration of oxygen vacancies in the NW can significantly influence the total conductance of the NW as well as the SBHs at the source/drain (see Figure a1
a3 and Supporting Information Figure S1). Now we consider the case that a strain is applied to the PRM cell. The effect of piezopotential can be equivalently taken as applying a positive voltage at the barrier interface if the local piezoelectric polarization charges are positive, which in effect decreases the value of the external bias required to over￾come the SBH at the interface. Alternatively, a negative voltage is created to act on the interface if the polarization charges are negative, which increases the value of the external bias required to overcome the barrier at the interface. From the data shown in Figure 3a, the HRS window remains almost constant regardless the magnitude and sign of the applied strain, indicating the shifts in the observed threshold voltages under different strains are dictated by the piezoelectric polarization charges at the interfaces and the contribution from the diffusion of the oxygen vacancies has negligible effect. This is because the oxygen vacancies are distributed in the entire NW, while the piezoelectric charges are accumulated right at the very near barrier interface in a region less than a subnanometer. The diffusion force contributed by the piezoelectric charges on the oxygen vacancies is a long-range interaction force, thus a variation of the vacancy concentration at the interface owing to piezoelectric effect is rather small. Furthermore, the entire I
V curves are “translated” for a constant voltage that is the change of threshold switch voltage caused by the piezoelectric charges. For the samples pretreated in oxygen plasma, the concentration of the oxygen vacancies was largely reduced in the NW, thus, the screening effect of the free charge carriers to the piezoelectric charges was significantly reduced, and the effect of the piezoelectric charges is enhanced.34 Therefore, the shifts in threshold switch voltages due to piezo￾electric polarization at both drain and source sides for a fixed strain have the same magnitude but opposite polarities, provided that the doping level is low. This indicates that the magnitude of the piezopotential at the interface is as large as 1.2 V at 0.5
0.76% of strain. The oxygen plasma pretreatment to the ZnO NWs may also improve the output of the nanogenerator.34 The fabricated PRM can function as an electromechanical memory, in which the write/read access can be programmed via mechanical actuation. A pulse train consisting of several write/ read/erase pulses is applied to the PRM cell to record and read out the polarity/logic levels of the “stored” strain in the cell, by monitoring the characteristic patterns in the output current (Figure 4). The data shown in Figure 4 was obtained for the same PRM cell under different strain status, which is equivalent to the cases of three identical PRM cells under tensile strain (A cell), zero strain (B cell), and compressive strain (C cell), for easy descrip￾tion. First, a positive write pulse (10 ms) with Vth,S0 < Vwrite1 < Vth, S_ is applied to these three cells. This short pulse sets the A and B cells switch from the HRS to the LRS state, while the C cell remains in the HRS state. The status of the three cells are then read out by a read pulse (2 s) with a small magnitude (Vread1 < Vth,S+). Subsequently, a negative erase pulse (10 ms) withVth,D0 < Verase1 < Vth,D_ resets the A and B cells from the LRS to the HRS state, while sets the C cell into the LRS state. A follow-up read pulse (2 s) with a small negative value (Vread1 > Vth,D_) is applied to read the new states of the cells. In the third step, a positive erase pulse (10 ms) with Vth,S+ < Verase2 < Vth,S0 sets the A cell from the HRS to the LRS state again, while keeps the B and C cells in the HRS state. The same Vread1 pulse is then applied to read the new states of the cells. Finally, a negative write pulse (10 ms) with Vth,D+ < Vwrite2 < Vth,D0 resets the A cell into the HRS state, while sets the B and C cells into the LRS state. The same Vread2 pulse is then applied to read the new states of the cells. After the above series of pulse train is applied, the waveform of the output currents of the cells is monitored and analyzed (Figure 4). If the logic levels of the output currents with positive, almost zero and negative values are labeled as “1”, “0” and “-1”, the logic pattern of “1010” indicates the positive nature of the “stored”strain in A cell. The logic patterns of “100
1” and “0
1 0- 1” represent the zero and negative strain status of the B and C cells, respectively. A quantitative analysis of the magnitudes of the output currents can give the absolute values of the strains stored in the PRM cells. Although there have been numerous research as well as commercial products on strain detection and measurements such as semiconductor MEMS/ NEMS piezoresistive strain gauges,35,36 the PRM cells demon￾strated here are fundamentally different from the these devices. Piezoresistance effect is a nonpolar and symmetric effect resulting from the band structure change, while the PRM cells are based on the asymmetric piezotronic effect. ZnO is a polar structure along c￾axis, straining in axial direction (c-axis) creates a polarization of cations and anions in the NW growth direction, resulting in a piezopotential drop from V+ to V
along the NW, which produces an asymmetric effect on the changes in the Schottky barrier heights (SBHs) at the drain and source electrodes. The strain sensor based on piezotronic effect has been reported to possess much higher sensitivity than previously reported devices.27 In summary, by utilizing the strain-induced polarization charges created at the semiconductor/metal interface under externally applied deformation as a result of piezotronic effect, the switching characteristics of the ZnO NW resistive switching devices can be modulated and controlled. We further demonstrated that the logic levels of the strain applied on the memory cell can be recorded and read out for the first time utilizing the piezotronic effect,18 which has the potential for implementing novel nanoelectromechanical memories and integrating with NEMS technology to achieve micro/nanosystems capable of intelligent and self-sufficient multi￾dimensional operations.18,20 Taking advantage of the recently developed large-scale fabrication technique of ZnO NW arrays,37 nonvolatile resistive switching memories using ZnO NW array as the storage medium may be readily engineered and implemented for applications such in flexible electronics and force/pressure imaging. Non-Boolean neuromorphic computing might also be realized by integrating arrays of high-density resistive memory cells38,39 on flexible substrates. ’METHODS SUMMARY (See Supporting Information for details.) The ZnO nanowires were synthesized via a physical vapor deposition process21 based on thermal evaporation of ZnO powders without the presence of catalyst. Large-scale ZnO NWs were subsequently transferred to the PET receiving