The ultimate aim of artificial synaptic devices is to mimic the features of biological synapses as closely as possible, in particular, its ability of self-adjusting the synaptic weight responding to the external stimulus. In this work, memristors, based on trilayer oxides with a stack structure of TiN/TiON/HfOy/HfOx/TiN, are designed to function as the artificial synapses where intrinsically designed oxygen-deficient HfOx layer, less oxygen-deficient HfOy layer, and TiON layer, imitating the corresponding biological functionality of the pre-synapse, synaptic cleft, and post-synapse, respectively, resemble the features of bio-synapses most closely. Thus, diverse bio-synaptic functions and plasticity, including long-term potentiation and depression, spike-rate-dependent plasticity, spike-timing-dependent plasticity, and metaplasticity, are fulfilled in these devices. Moreover, they exhibit analogue plasticity in both potentiating and depressing, fully emulating the learning protocols of excitation and inhibition in the bio-synapses. The structure and Hf/O distribution of these devices, mimicking the structure and Ca2+ deployment of bio-synapses, are consolidated by the high-resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy, respectively. Powerful bio-realistic behavior, implemented in these simple artificial synaptic devices, make them tailored for neuromorphic hardware applications.
Keywords: analog memristors; artificial synapses; metaplasticity; neuromorphic devices; spike-timing-dependent plasticity.