Nano Letters LETTER g=1.17% E=0% E=0.76% -1.6 Strain ( %) Drain Source F thee ctively. (b)Dependence of threshold volans of PRM cell. (a)I-V characteristics of ZnO PRM cells under tensile, zero, and compressive strains Figure 3. Stain-modulated hysteret the PRM cell, while the width of the hrS window remains almost constant for different strain values. (c) Schematic of band-diagram of PRM cell under sile strain. (cl)Schottky barrier at drain side is forward biased. ( c2)Schottky barrier at drain side is reversely biased. Red solid lines represent band- diagrams after tensile strain is applied. Black dashed lines represent band-diagrams under strain free condition. The color gradient represents the distribution of piezopotential field. reversely biased source side. The total resistance of PRM cell is It can also be observed that both Vth.s and Vuh. D and hence the RPRM"Rs(with Rs RNw, RD), which is the HRS state. The width of the HRS window increased as the temperature de- lower voltage at the source side attracts oxygen vacancies creased from 350 to 90 K( Figure 2b and Supporting Information toward the interface to modify the contact barrier at the source. Figure S3). Qualitatively, this can be understood since the drift/ The switching from HRS to LRS state occurs at a larger bias diffusion of the charged ions/dopants and electrons are thermally beyond point A, in corresponding to a largely reduced SBH at activated processes Employing the rigid point ion model derived the source side. When the bias voltage sweeps from point A to by Mott and Gurney, the diffusion coefficient of oxygen point O through point B, the In(I)-V curve for region B-o vacancy is given by D=Do exp(-E/kT)and the drift velocity empty circle in Supporting Information Figure S4), shows that is v= af exp(-E/kT)sinh(qE,/2kT), where E, is the activation In(relates to Vi/4, as confirmed by the numerical fitting curve energy, k is the Boltzmann constant, a is the effective hopping (purple line in Supporting Information Figure $4). This distance for the ion to hop between potential wells, and f is the indicates that the thermionic emission-diffusion model dom- attempt-to-escape frequency. At decreased temperatures, larger ates the transport at the reverse-biased source barrier. An bias is required to attract sufficient oxygen vacancies toward the accelerated diffusion of the oxygen vacancies toward the source respective reversely biased barrier to switch the PRM cell from side at a large applied voltage and their accumulation are hRS to lRS state within the time scale in the experimental setup considered as the cause of the hysteresis observed in I-Vcurve. (the sweeping frequency of the bias signal was 0. 1 Hz).The As the applied bias switches the polarity from point o to point characteristic of the PRM cells at different sweeping rates has C, the source side is now forward-biased and the bias voltage drops also been investigated primarily for two frequencies(0.1 and mainly at the reversely biased drain side with the total resistance of 0.01 Hz). It is interesting to notice that the hysteresis loop PRM cell RPRM N Ro, which is the new HRS state Oxygen shrinks for larger sweeping frequency, while no significant acancies near the drain side are attracted toward and accumulated variations can be observed for the turn-on threshold voltage at the reversely biased drain barrier to modify the interface contact,(Supporting Information Figure S8). hile oxygen vacancies previously piled up at the source side are The external mechanical perturbation-induced strain(E,)acts drifting away. Similar to the case in region O-A, the switch from as the programming input for modulating the hysteretic I-V hRS to lRS state occurs only after a larger bias beyond point C is characteristics of the PRM cell. A positive/negative strain is created applied. When the bias voltage sweeps from point C to point when the ZnO NW is stretched/compressed(see Supporting through point D, the In (I)-V curve for region D-o(empty Information for calculation of the strain in the PRM cell) triangle in Supporting Information Figure $4)can again be Interesting phenomena was observed when a PRM cell experi numerically fitted using the In(nw v/ relationship(blue line enced straining(Figure 3a). When the PRM cell was tensile in Supporting Information Figure S4), indicates che transport at toward lower voltage side by 1. 49 V(red line in Figure 3a);when ing that the ther stretched(E= 1.17%), the hysteretic switching curve shifted mionic emission-diffusion model also dominates the reverse-biased drain barrier the cell was compressively deformed(E=-0.76%), the hysteretic 2782 dx. dolora/0.102/n201074a| Nano Lert.2011l277927852782 dx.doi.org/10.1021/nl201074a |Nano Lett. 2011, 11, 2779–2785 Nano Letters LETTER reversely biased source side. The total resistance of PRM cell is RPRM ∼ RS (with RS . RNW, RD), which is the HRS state. The lower voltage at the source side attracts oxygen vacancies toward the interface to modify the contact barrier at the source. The switching from HRS to LRS state occurs at a larger bias beyond point A, in corresponding to a largely reduced SBH at the source side. When the bias voltage sweeps from point A to point O through point B, the ln(I)
V curve for region B
O (empty circle in Supporting Information Figure S4), shows that ln(I) relates to V1/4, as confirmed by the numerical fitting curve (purple line in Supporting Information Figure S4). This indicates that the thermionic emission-diffusion model dom￾inates the transport at the reverse-biased source barrier.29 An accelerated diffusion of the oxygen vacancies toward the source side at a large applied voltage and their accumulation are considered as the cause of the hysteresis observed in I
V curve. As the applied bias switches the polarity from point O to point C, the source side is now forward-biased and the bias voltage drops mainly at the reversely biased drain side with the total resistance of PRM cell RPRM ∼ RD, which is the new HRS state. Oxygen vacancies near the drain side are attracted toward and accumulated at the reversely biased drain barrier to modify the interface contact, while oxygen vacancies previously piled up at the source side are drifting away. Similar to the case in region O
A, the switch from HRS to LRS state occurs only after a larger bias beyond point C is applied. When the bias voltage sweeps from point C to point O through point D, the ln(I)
V curve for region D
O (empty triangle in Supporting Information Figure S4) can again be numerically fitted using the ln(I) ∼ V1/4 relationship (blue line in Supporting Information Figure S4), indicating that the ther￾mionic emission
diffusion model also dominates the transport at the reverse-biased drain barrier. It can also be observed that both Vth,S and Vth,D and hence the width of the HRS window increased as the temperature de￾creased from 350 to 90 K (Figure 2b and Supporting Information Figure S3). Qualitatively, this can be understood since the drift/ diffusion of the charged ions/dopants and electrons are thermally activated processes. Employing the rigid point ion model derived by Mott and Gurney,30 the diffusion coefficient of oxygen vacancy is given by D = D0 3 exp(
Ea/kT) and the drift velocity is v = af exp(
Ea/kT)sinh(qEa/2kT), where Ea is the activation energy, k is the Boltzmann constant, a is the effective hopping distance for the ion to hop between potential wells, and f is the attempt-to-escape frequency. At decreased temperatures, larger bias is required to attract sufficient oxygen vacancies toward the respective reversely biased barrier to switch the PRM cell from HRS to LRS state within the time scale in the experimental setup (the sweeping frequency of the bias signal was 0.1 Hz). The characteristic of the PRM cells at different sweeping rates has also been investigated primarily for two frequencies (0.1 and 0.01 Hz). It is interesting to notice that the hysteresis loop shrinks for larger sweeping frequency, while no significant variations can be observed for the turn-on threshold voltage (Supporting Information Figure S8). The external mechanical perturbation-induced strain (εg) acts as the programming input for modulating the hysteretic I
V characteristics ofthe PRM cell. A positive/negative strain is created when the ZnO NW is stretched/compressed (see Supporting Information for calculation of the strain in the PRM cell). Interesting phenomena was observed when a PRM cell experi￾enced straining (Figure 3a). When the PRM cell was tensile stretched (ε = 1.17%), the hysteretic switching curve shifted toward lower voltage side by 1.49 V (red line in Figure 3a); when the cell was compressively deformed (ε =
0.76%), the hysteretic Figure 3. Stain-modulated hysteretic switching of PRM cell. (a) I
V characteristics of ZnO PRM cells under tensile, zero, and compressive strains respectively. (b) Dependence of threshold voltages on applied strains. Both Vth,S (in red) and Vth,D (in black) almost linearly depends on strain applied to the PRM cell, while the width of the HRS window remains almost constant for different strain values. (c) Schematic of band-diagram of PRM cell under tensile strain. (c1) Schottky barrier at drain side is forward biased. (c2) Schottky barrier at drain side is reversely biased. Red solid lines represent band￾diagrams after tensile strain is applied. Black dashed lines represent band-diagrams under strain free condition. The color gradient represents the distribution of piezopotential field