Enhanced nanoscale resistive switching memory characteristics and switching mechanism using high- Ge-content Ge_0.5Se_0.5 solid electrolyte

We demonstrate enhanced repeatable nanoscale bipolar resistive switching memory characteristics in Al/Cu/Ge_0.5Se_0.5/W, as compared with Al/Cu/Ge_0.2Se_0.8/W structures, including stable AC endurance (>10⁵ cycles), larger average SET voltage (approximately 0.6 V), excellent data retention (>10⁵ s) at 85°C, and a high resistance ratio (>10⁴) with a current compliance of 8 μA and a small operation voltage of ±1.5 V. A small device size of 150 × 150 nm² and a Cu nanofilament with a small diameter of 30 nm are both observed by highresolution transmission electron microscope in the SET state. The Ge_xSe_{1 - x} solid electrolyte compositions are confirmed by both energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. The switching mechanism relies on the smaller barrier heights for holes rather than for electrons; the positively charged Cu^z+ ions (i.e., holes) migrate through the defects in the Ge_xSe_{1 - x} solid electrolytes during SET/RESET operations. Hence, the Cu nanofilament starts to grow at the Ge_0.5Se_0.5/W interface, and starts to dissolve at the Cu/Ge_0.5Se_0.5 interface, as illustrated in the energy band diagrams. Owing to both the higher barrier for hole injection at the Cu/Ge_0.5Se_0.5 interface than at the Cu/Ge0.2Se0.8 interface and greater thermal stability, the resistive switching memory characteristics of the Al/Cu/Ge_0.5Se_0.5/W are improved relative to the Al/Cu/Ge_0.2Se_0.8/W devices. The Al/Cu/Ge_0.5Se_0.5/W memory device can also be operated with a low current compliance of 1 nA, and hence, a low SET/RESET power of 0.61 nW/6.4 pW is achieved. In addition, a large memory size of 1,300 Pbit/in² is achieved with a small nanofilament diameter of 0.25 Å for a small current compliance of 1 nA.