Enhanced nanoscale resistive switching memory characteristics and switching mechanism using high-Ge-content Ge0.5Se0.5 solid electrolyte

We demonstrate enhanced repeatable nanoscale bipolar resistive switching memory characteristics in Al/Cu/Ge0.5Se0.5/W, as compared with Al/Cu/Ge0.2Se0.8/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 high-resolution transmission electron microscope in the SET state. The GexSe1 - 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 Cuz+ ions (i.e., holes) migrate through the defects in the GexSe1 - x solid electrolytes during SET/RESET operations. Hence, the Cu nanofilament starts to grow at the Ge0.5Se0.5/W interface, and starts to dissolve at the Cu/Ge0.5Se0.5 interface, as illustrated in the energy band diagrams. Owing to both the higher barrier for hole injection at the Cu/Ge0.5Se0.5 interface than at the Cu/Ge0.2Se0.8 interface and greater thermal stability, the resistive switching memory characteristics of the Al/Cu/Ge0.5Se0.5/W are improved relative to the Al/Cu/Ge0.2Se0.8/W devices. The Al/Cu/Ge0.5Se0.5/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.