Growth-Temperature-Dependent Synaptic Plasticity in ALD-ZnO Optoelectronic Synaptic Devices on Polyimide Substrates

Flexible Al/ZnO/Al optoelectronic synaptic devices were fabricated on flexible polyimide (PI) substrates using ZnO thin films deposited by atomic layer deposition (ALD) at growth temperatures ranging from 90 to 145 °C. The effects of growth temperature on the surface morphology, optical properties, electrical transport, and synaptic behavior were systematically investigated. All ZnO films were grown at temperatures compatible with polymer substrates, while their surface roughness, photoluminescence, optical transmittance, dark current, and photocurrent varied considerably with growth temperature. Increasing the growth temperature enhanced carrier transport, photocurrent, and persistent photoconductivity, resulting in larger excitatory postsynaptic currents (EPSCs) and slower post-illumination decay. The EPSC was further modulated by the ultraviolet (UV) stimulation duration, light intensity, and pulse number. Paired-pulse facilitation decreased with increasing pulse interval, whereas higher-temperature-grown devices maintained stronger facilitation over longer intervals. Repeated optical stimulation enabled faster relearning and slower forgetting, thereby mimicking learning-experience behavior. Moreover, 5 × 5 visual-memory tests showed growth-temperature-dependent image retention, with the 145 °C device exhibiting the slowest pattern decay. These results demonstrate that ALD growth-temperature engineering provides an effective strategy for tuning synaptic plasticity and visual-memory retention in flexible ZnO-based optoelectronic devices.