Long-term and short-term plasticity independently mimicked in highly reliable Ru-doped Ge₂Sb₂Te₅ electronic synapses

In order to fulfill the complex cognitive behaviors in neuromorphic systems with reduced peripheral circuits, the reliable electronic synapses mimicked by single device that achieves diverse long-term and short-term plasticity are essential. Phase change random access memory (PCRAM) is of great potential for artificial synapses, which faces, however, difficulty to realize short-term plasticity due to the long-lasting resistance drift. This work reports the ruthenium-doped Ge₂Sb₂Te₅ (RuGST) based PCRAM, demonstrating a series of synaptic behaviors of short-term potentiation, pair-pulse facilitation, long-term depression, and short-term plasticity in the same single device. The optimized RuGST electronic synapse with the high transformation temperature of hexagonal phase >380°C, the outstanding endurance >10⁸ cycles, the low resistance drift factor of 0.092, as well as the extremely high linearity with correlation coefficients of 0.999 and 0.976 in parts of potentiation and depression. Further investigations also go insight to mechanisms of Ru doping according to thorough microstructure characterization, revealing that Ru dopant is able to enter GST lattices thus changing and stabilizing atomic arrangement of GST. This leads to the short-term plasticity realized by RuGST PCRAM. Eventually, the proposed RuGST electronic synapses performs a high accuracy of ~94.1% in a task of image recognition of CIFAR-100 database using ResNet 101. This work promotes the development of PCRAM platforms for large-scale neuromorphic systems.