Emulation of bio-synaptic behaviours in copper-doped zinc oxide memristors: A nanoscale scanning probe microscopic study

Memristors emulating biological synapses to perform memory and learning functions are crucial for realizing bio-inspired neuromorphic systems. However, to meet the increasing demand for high-speed operation, low power consumption, and large integration density in neuromorphic architectures, the synaptic functions must be achieved at nanometer scale and with ultra-low write current. This report evidences electronic synapses realized in Cu-doped ZnO (CZO) memristors by nanoscale scanning probe microscopic techniques. In this process, the device can be downsized all the way up to the size of the probe, offering ultra-high density (>109 devices/mm2) of electronic synapses. The essential bio-synaptic functions in the memristor are accomplished with ultra-low write current (∼10 nA) and in a forming-free approach that marks their potential in building power-efficient computing systems. The modulation in device conductivity is elucidated on the basis of charged defect migration, forming conducting bridges across the thickness or locally within the CZO layer. In addition, the influence of Cu-doping in controlling nanoscale neuromorphic and memristive performances of the device is explored. The present study, therefore, demonstrates a novel approach to decipher oxide-based synaptic-memristors and highlights CZO as a promising candidate to fabricate ultra high-density power-efficient neuromorphic devices.