Fig. 1. (Color online) Structure and switching performances of the Ti/TaO_x/HfO_x/Pt device. (a) Schematic of the Ti/ TaO_x/HfO_x/Pt device with a crossbar structure. The inset shows the SEM result of the as-fabricated device, the feature size of the fabricated device is 5 × 5 μm². (b) The cyclic I–V curves of Ti/TaO_x/HfO_x/Pt (red) and Ti/HfO_x/Pt (gray) device, which exhibits a typical bipolar resistive switching with uniform switching property. (c) The cumulative distribution of the resistance states (HRS and LRS) and operation voltages (V_SET and V_RESET) of Ti/TaO_x/HfO_x/Pt device. The HRS and LRS of Ti/TaO_x/HfO_x/Pt device is 17 ± 0.155 MΩ and 5 ± 0.053 kΩ, respectively. The V_SET and V_RESET of the device is 2.0 ± 0.16 V and –1.25 ± 0.15 V, respectively. (d) Retention characteristics of the HRS and LRS of Ti/TaO_x/HfO_x/Pt device for more than 3.5 × 10⁴ s. The insets show the measurement results after 24 and 72 h, respectively. The result shows the device with good retention property in both HRS and LRS.

Fig. 2. (Color online) The pulse mode switching characteristics of the Ti/TaO_x/HfO_x/Pt devices. (a) The device response characteristics of the initialization pulse operation. (b) The switching cycles of the device under extreme fast pulse stimulus. (c) The distribution of HRS/LRS under pulse mode switching. The statistical results show that this distribution conform to the lognormal distribution, indicating a uniform switching property of the device under pulse stimulus. (d) The Ti/TaO_x/HfO_x/Pt device exhibits reliable endurance for more than 10⁸ cycles, each box in the figure representing 300 switching cycles. (e) The endurance property of the Ti/HfO_x/Pt device, each box in the figure representing 100 switching cycles. (f) Comparison of I–V characteristics curves of the Ti/TaO_x/HfO_x/Pt device before (black line) and after (red line) the pulse measurement.

Fig. 3. (Color online) (a) The response property of Ti/TaO_x/Pt device before and after the forming process. The initial resistance state modulated by the oxygen percentage during the spurting of the TaO_x process. Schematic switching mechanisms of the Ti/TaO_x/HfO_x/Pt device. (b) Initial state, (c) forming process, (d) RESET process and (e) SET process.

Fig. 4. (Color online) The cross-sectional HRTEM image and the element component profile of the device under different operation conditions. The cross-sectional HRTEM image of (a) intial state, (b) forming, (c) RESET, (d) SET. (e) The element component profile of the pristine device. The (f) CF and (g) out of CF element component profile of the device under the forming operation. The (h) CF and (i) out of CF element component profile of the device under the RESET operation. The (j) CF and (k) out of the CF element component profile of the device under SET operation.