Demonstration of electronic and optical synaptic properties modulation of reactively sputtered zinc-oxide-based artificial synapses

Abstract

A stable and highly controllable multistate analog memory system was developed using ZnO-based memristors. Indium–tin oxide (ITO)/ZnO/ITO memristors exhibited electrical and optical synaptic properties similar to those of biological synapses. Multilevel conductivity was achieved by controlling the current compliance and RESET stop voltage for high-density memory storage and learning applications. Results of high-resolution transmission electron microscopy indicated that polycrystalline ZnO was formed during the reactive sputtering of Zn with O2. The memristor could realize multilevel memory storage owing to the continuous formation of neutral oxygen vacancy defects. Biological synaptic rules were mimicked and implemented by modifying and analyzing the excitatory post-synaptic current of the memristor. Furthermore, short- and long-term memory applications during paired-pulse facilitation, spike-number-dependent plasticity, and pulse rate-dependent plasticity were demonstrated. The light-controlled synaptic weight was modulated through the charge carrier generation inside the ZnO switching layer below the valence band. Repeated rehearsals of the optical pulse resulted in short-term plasticity and long-term potentiation. The transition occurred depending on the pulse intensity and number, resulting from the dynamic generation and trapping of photogenerated carriers. Short- and long-term memory functions for the learning and forgetting processes of artificial visual memory systems were accomplished by varying the optical illumination intensity. The proposed ZnO-based memristors can be applied to electronic and photonic devices.