| Summary: | Resistive switching characteristics by using the Al<sub>2</sub>O<sub>3</sub> interfacial layer in an Al/Cu/GdO<sub>x</sub>/Al<sub>2</sub>O<sub>3</sub>/TiN memristor have been enhanced as compared to the Al/Cu/GdO<sub>x</sub>/TiN structure owing to the insertion of Al<sub>2</sub>O<sub>3</sub> layer for the first time. Polycrystalline grain, chemical composition, and surface roughness of defective GdO<sub>x</sub> film have been investigated by transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and atomic force microscopy (AFM). For bipolar resistive switching (BRS) characteristics, the conduction mechanism of high resistance state (HRS) is a space-charge limited current for the Al/Cu/GdO<sub>x</sub>/TiN device while the Al/Cu/GdO<sub>x</sub>/Al<sub>2</sub>O<sub>3</sub>/TiN device shows Schottky emission. However, both devices show Ohmic at a low resistance state (LRS). After the device has been SET, the Cu filament evidences by both TEM and elemental mapping. Oxygen-rich at the Cu/GdO<sub>x</sub> interface and Al<sub>2</sub>O<sub>3</sub> layer are confirmed by energy dispersive X-ray spectroscopy (EDS) line profile. The Al/Cu/GdO<sub>x</sub>/Al<sub>2</sub>O<sub>3</sub>/TiN memristor has lower RESET current, higher speed operation of 100 ns, long read pulse endurance of > 10<sup>9</sup> cycles, good data retention, and the memristor with a large resistance ratio of > 10<sup>5</sup> is operated at a low current of 1.5 µA. The complementary resistive switching (CRS) characteristics of the Al/Cu/GdO<sub>x</sub>/Al<sub>2</sub>O<sub>3</sub>/TiN memristor show also a low current operation as compared to the Al/Cu/GdO<sub>x</sub>/TiN device (300 µA vs. 3.1 mA). The transport mechanism is the Cu ion migration and it shows Ohmic at low field and hopping at high field regions. A larger hopping distance of 1.82 nm at the Cu/GdO<sub>x</sub> interface is obtained as compared to a hopping distance of 1.14 nm in the Al<sub>2</sub>O<sub>3</sub> layer owing to a larger Cu filament length at the Cu/GdO<sub>x</sub> interface than the Al<sub>2</sub>O<sub>3</sub> layer. Similarly, the CRS mechanism is explained by using the schematic model. The CRS characteristics show a stable state with long endurance of > 1000 cycles at a pulse width of 1 µs owing to the insertion of Al<sub>2</sub>O<sub>3</sub> interfacial layer in the Al/Cu/GdO<sub>x</sub>/Al<sub>2</sub>O<sub>3</sub>/TiN structure.
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