Nano Letters LETTER o Fhhh 且 B cele 8=0 0 C cell: a=-07B% 之 Figure 4. Write/read access of PRM cell as an electromechanical memory. A pulse train consisting of several write/read/erase pulses was applied to the PRM cell to record and read out the logic levels of the"stored"strain in the cell. Each write/erase pulse was followed by a read pulse with positive or negative polarity respectively. The following voltage amplitudes were applied: 6.44V (write 1), 1.sv(read 1),-535v(erase 1),-15V(read 2),5.37 (erase 2), and-6.42 V(write 2). Short current spikes occur in the output reading current with the write/erase pulses if the resistance states(HRs LRS)change in the PRM cell. switching curve shifted toward higher voltage side by 1.18 V (blue modified by shifting the local Fermi level. The change in SBH line in Figure 2a).VchS*, Vth.so, Vh s-and Vth, D+, Vuh, Do, Vt, D-are induced by piezoelectric polarization is given approximately by e threshold switching voltages for the PRM cell with tensile, ApB=polD(1+1/(2gswa)), where opol is the volume ro,and compressive strains, respectively. The same hystereti density of the polarization charge and directly related to the witching curves can then be plotted in a semilogarithmic current piezoelectric polarization P vector, D is the two-dimensional scale to illustrate and highlight the characteristics of the curves density of interface states at the Fermi level at the Schottky (Supporting Information Figure S5). The ratios of conductance barrier, as is the two-dimensional screening parameter, and wa between LRS and HRS for the PRM cell remain steady at high is the width of the depletion layer. Thus the mechanical strain values(10)under different strains( Supporting Information an effectively change the local contact characteristics as well as Figure S6), demonstrating the stable performance of the cell and e charge carrier transport process. On the basis of the above discussions, the modulation effect of strain on the hystereti operations. The intrinsic rectifying behavior of the PRM cell may switching behavior of the PRM cell, as shown in Figure 3a, b, solve the sneak path problem as well as reduce the static power can then be understood and explained using the band-diagrar consumption,which allows for construction of large passive of the working device(Figure 3c). If the PRM cell is under esistive-switching device arrays. The changes in threshold switching tensile strain with the Schottky barrier at drain side being ages of the PRM cell with different strains have been plotted in forward-biased(V>0 in Figure 3a), the positive piezoelectric Figure 3b. It can be seen that the change in both the Vuhs and Vad potential resulting from the positive strain-induced polariza almost linearly depends on strain applied to the PRM cell, while the tion charges reduced the SBH at the reverse-biased source width of the HRS window(Vohs-Voh D, where i=+, 0, - )remains barrier; while the negative piezoelectric potential resulting almost constant for different strain values. This strain-modulated from the negative strain-induced polarization charge hange in the threshold switching voltages was also observed for creased the SBH at the forward-biased drain barrier(red line other PRM cells with oxygen plasma treatment. in Figure 3c1). Since the I-V characteristic in this situation is It is well-known that ionic polarization in ZnO can be dictated by the reversely biased source barrier, the existence of induced by strain owing to the lacking of center symmetry in strain-induced piezoelectric potential results in the shift ZnO, which can strongly affect the charge transpor Novel switching threshold voltage from Vuh. so to Vth. + indicatin effects and applications9, 27, 32 have been observed and im- only a smaller bias is required to switch the PRM cell from HRS plemented utilizing the piezotronic effect in ZnO. The to LRS state. Alternatively, if the Schottky barrier at drain side ndamental concept of the piezotronic effect is that the is reverse-biased(V <0 in Figure 3a), the SBH is still reduced SBH at the metal-semiconductor contact can be effectively at the source barrier while it is increased at the drain barrier tuned by the strain-induced piezoelectric polarization charges Figure 3c2)since the polarity of the strain did not change, at the interface. The local conduction band profile can then be and hence the piezoelectric potential remained negative and 2783 dx. dolora/0.102/n201074a| Nano Lert.2011l277927852783 dx.doi.org/10.1021/nl201074a |Nano Lett. 2011, 11, 2779–2785 Nano Letters LETTER switching curve shifted toward higher voltage side by 1.18 V (blue line in Figure 2a). Vth,S+, Vth,S0, Vth,S
and Vth,D+, Vth,D0, Vth,D
are the threshold switching voltages for the PRM cell with tensile, zero, and compressive strains, respectively. The same hysteretic switching curves can then be plotted in a semilogarithmic current scale to illustrate and highlight the characteristics of the curves (Supporting Information Figure S5). The ratios of conductance between LRS and HRS for the PRM cell remain steady at high values (∼105 ) under different strains (Supporting Information Figure S6), demonstrating the stable performance of the cell and its potential feasibility for applications in flexible memory and logic operations.19The intrinsic rectifying behavior of the PRM cell may solve the sneak path problem as well as reduce the static power consumption,23 which allows for construction of large passive resistive-switching device arrays. The changes in threshold switching voltages of the PRM cell with different strains have been plotted in Figure 3b. It can be seen that the change in both the Vth,S and Vth,D almost linearly depends on strain applied to the PRM cell, while the width of the HRS window (Vth,Si
Vth,Di , where i = +, 0,
) remains almost constant for different strain values. This strain-modulated change in the threshold switching voltages was also observed for other PRM cells with oxygen plasma treatment. It is well-known that ionic polarization in ZnO can be induced by strain owing to the lacking of center symmetry in ZnO, which can strongly affect the charge transport.18 Novel effects31 and applications19,27,32 have been observed and im￾plemented utilizing the piezotronic effect in ZnO.18 The fundamental concept of the piezotronic effect is that the SBH at the metal
semiconductor contact can be effectively tuned by the strain-induced piezoelectric polarization charges at the interface. The local conduction band profile can then be modified by shifting the local Fermi level. The change in SBH induced by piezoelectric polarization is given approximately by ΔϕB = σpolD
1 (1 + 1/ (2qswd))
1 , where σpol is the volume density of the polarization charge and directly related to the piezoelectric polarization P vector, D is the two-dimensional density of interface states at the Fermi level at the Schottky barrier, qs is the two-dimensional screening parameter, and wd is the width of the depletion layer.33 Thus the mechanical strain can effectively change the local contact characteristics as well as the charge carrier transport process. On the basis of the above discussions, the modulation effect of strain on the hysteretic switching behavior of the PRM cell, as shown in Figure 3a,b, can then be understood and explained using the band-diagram of the working device (Figure 3c). If the PRM cell is under tensile strain with the Schottky barrier at drain side being forward-biased (V > 0 in Figure 3a), the positive piezoelectric potential resulting from the positive strain-induced polariza￾tion charges reduced the SBH at the reverse-biased source barrier; while the negative piezoelectric potential resulting from the negative strain-induced polarization charges in￾creased the SBH at the forward-biased drain barrier (red line in Figure 3c1). Since the I
V characteristic in this situation is dictated by the reversely biased source barrier, the existence of strain-induced piezoelectric potential results in the shift of switching threshold voltage from Vth,S0 to Vth,S+, indicating only a smaller bias is required to switch the PRM cell from HRS to LRS state. Alternatively, if the Schottky barrier at drain side is reverse-biased (V < 0 in Figure 3a), the SBH is still reduced at the source barrier while it is increased at the drain barrier (Figure 3c2) since the polarity of the strain did not change, and hence the piezoelectric potential remained negative and Figure 4. Write/read access of PRM cell as an electromechanical memory. A pulse train consisting of several write/read/erase pulses was applied to the PRM cell to record and read out the logic levels of the “stored” strain in the cell. Each write/erase pulse was followed by a read pulse with positive or negative polarity respectively. The following voltage amplitudes were applied: 6.44 V (write 1), 1.5 V (read 1),
5.35 V (erase 1),
1.5 V (read 2), 5.37 V (erase 2), and
6.42 V (write 2). Short current spikes occur in the output reading current with the write/erase pulses if the resistance states (HRS to LRS) change in the PRM cell