Highly UV-responsive indium-free Zn–Sn–Al–O phototransistor for optoelectronic artificial synapses

Abstract

Conventional computing systems based on the von Neumann architecture face critical bottlenecks in power efficiency and parallel processing capabilities, significantly constraining their potential for emerging data-intensive technologies such as artificial intelligence and neuromorphic computing. Neuromorphic architectures, inspired by biological synapses, have thus emerged as promising alternatives, offering efficient signal processing and storage in a highly parallel and energy-conserving manner. Among neuromorphic devices, optoelectronic artificial synapses based on amorphous oxide semiconductors (AOS) have attracted substantial attention due to their unique persistent photoconductivity, which effectively mimics synaptic plasticity in biological neural networks. However, existing AOS-based synaptic devices predominantly rely on indium-based materials, raising concerns regarding resource scarcity and environmental sustainability. Herein, we have successfully developed indium-free Zn–Sn–Al–O (ZTAO) phototransistors through plasma-enhanced atomic layer deposition with a staggered bottom-gate architecture. The fabricated ZTAO phototransistors exhibit exceptional UV responsiveness, reaching an optimal photoresponsivity at an illumination intensity of approximately 3.5 mW cm⁻², primarily driven by direct bandgap transitions in the UV region and defect-state transitions related to oxygen vacancies under visible light. The ZTAO optoelectronic artificial synapses exhibit tunable plasticity under single-pulse stimulation, responsive to wavelength, intensity, and duration, and accurately emulate paired-pulse facilitation, demonstrating characteristic exponential decay with relaxation times (τ₁ = 0.50 s, τ₂ = 5.14 s). Furthermore, controlled modulation of optical pulse number and frequency enables a transition from short-term to long-term plasticity, effectively simulating experience-dependent learning. This research establishes indium-free ZTAO phototransistors as highly promising candidates for next-generation neuromorphic computing devices, paving the way toward sustainable, efficient, and multifunctional optoelectronic synaptic systems.