High-Performance TiN/Al2O3/ZnO/Al2O3/TiN Flexible RRAM Device With High Bending Condition

The bipolar resistive switching (RS) characteristics of the ZnO-based TiN/Al<sub>2</sub>O<sub>3</sub>/ZnO/Al<sub>2</sub>O<sub>3</sub>/TiN structure are investigated for flexible nonvolatile memory applications. Using a thin Al<sub>2</sub>O<sub>3</sub> buffer layer on both sides of the ZnO device shows uniform and eminently stable bipolar resistance switching characteristics. The device exhibits good RS with more than two orders of resistance ON-OFF ratio, retention of >10<sup>4</sup> s at 120 °C, good dc endurance >10<sup>4</sup> cycles, and high ac endurance of >10<sup>8</sup> cycles with 40-ns pulsewidth without any degradation. The device shows high mechanical stability when under 10<sup>4</sup> continuous repetitive flexible bendings, indicating that high endurance with a very small bending radius of up to 3 mm. The lower Gibbs free energy of the Al<sub>2</sub>O<sub>3</sub> (−1676 kJ/mol) film compared with the ZnO (−320.4 kJ/mol) and TiO<sub>2</sub> (−994 kJ/mol) films can improve the RS properties of the TiN/Al<sub>2</sub>O<sub>3</sub>/ZnO/Al<sub>2</sub>O<sub>3</sub>/TiN device. The significant improvement in the TiN/Al<sub>2</sub>O<sub>3</sub>/ZnO/Al<sub>2</sub>O<sub>3</sub>/TiN device is due to the reason that thin Al<sub>2</sub>O<sub>3</sub> layers on both sides of ZnO would help stabilize the local oxygen migrations for the formation and rupture of the conductive filament during the continuous switching cycles, resulting in high memory switching characteristics.

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