Ultra-Low Power Photosynaptic Devices Based on Persistent Photoconductivity-Modulated SnO₂

Realizing visual perception performance using robust metal oxide devices is highly desirable for brain-inspired neuromorphic computing, visual prosthetics, and artificial intelligence. SnO₂, a representative transparent semiconductor, has attracted considerable attention for its potential in ultraviolet photodetectors and photosynaptic devices operating in solar-blind spectral ranges. This problem has been circumvented by lowering the electron concentration and reducing the persistent photoconductivity (PPC), one of the essential properties of photosynaptic devices. In this case, the electron concentration and PPC properties in SnO₂ must be controlled independently. This paper reports the successful control of PPC in SnO₂ by controllable addition of Zn and Sr ions, which act as acceptors to annihilate the charge carriers and as a reductant to form oxygen vacancies, respectively. Zn and Sr-added SnO₂ exhibits superior synaptic performance with low energy consumption, successfully imitating the biological synapses without a gate electrode. As a result, the PPC remains at about 40% even after 20 seconds of turning off the ultraviolet light, and energy consumption is reduced to 1–10 fJ, similar to energy consumption with biological synapses. A novel optical sensor that can receive analog signals is demonstrated using a photosynaptic device with Zn and Sr-added SnO₂ thin films.