An Opto-Electronic HfO_x-Based Transparent Memristive Synapse for Neuromorphic Computing System

In this study, a transparent bilayer memristor showing both electrical and optical synapses along with good electrical properties after annealing is presented. In addition to 85% transparency, the device shows excellent electrical characteristics for 1000 cycles of stable LRS/HRS and more than 104 s retention at high temperatures. The annealed device also exhibits stable potentiation and depression cycles for more than 10 000 ac pulses with a low coefficient of nonlinearity. By applying consecutive ac pulses, synaptic properties of paired-pulse facilitation (PPF) and spike time-dependent plasticity (STDP) are calculated. The memristor is illuminated by a 405 nm light source in which different light intensities ranging from 20 to 40 mW/cm2 are used for achieving multilevel cell (MLC) characteristics. Learning/Forgetting curve (PSC) and optical PPF are measured to mimic optical synaptic function. An image recognition comparison of optical and electrical synaptic properties with a normalized loss rate of < 0.1 is obtained after just 100 epoch trainings. These excellent attributes of this transparent memristor make it a promising candidate for electrical/optical memory devices or for using it as an optically synaptic sensor device.