Memory-Enhanced Visible-Light Photonic Synapses in Interface-Engineered ZnON/IGZO Oxide Transistors

Zinc oxynitride (ZnON) thin-film transistors (TFTs) are attractive visible-light photoactive devices owing to their narrow and tunable optical bandgap; however, the limited population of deep-level trap states can restrict post-illumination carrier retention, resulting in weak photonic synaptic retention. Here, memory-enhanced visible-light photonic synapses are demonstrated using interface-engineered ZnON/Indium gallium zinc oxide (IGZO) double-layer oxide TFTs. A thin IGZO capping layer is integrated onto the ZnON channel to preserve ZnON-dominated red, green, and blue (RGB) photoresponse, while modifying the interfacial electronic environment and carrier-relaxation dynamics. The ZnON/IGZO TFT exhibits RGB responsivities of 2.93, 11.0, and 17.6 A W-1 and specific detectivities of 1.10 × 1012, 3.82 × 1012, and 6.08 × 1012 Jones under red, green, and blue illumination, respectively. The device also exhibits enhanced short- and long-term synaptic retention behavior under pulsed optical stimulation, wavelength-dependent synaptic plasticity, and analogue synaptic weight modulation. Spectroscopic ellipsometry and X-ray photoelectron spectroscopy reveal that the IGZO capping layer preserves ZnON-dominated visible-light absorption, while inducing changes in oxygen-related bonding environments across the bilayer structure. Photoinduced current transient spectroscopy reveals a redistribution of electrically active trap states toward both relatively shallow- and deep-level regions after IGZO capping. The shallow-level states are correlated with enhanced photoresponse, whereas the deep-level states are consistent with delayed carrier relaxation and prolonged post-illumination retention. These results demonstrate that ZnON/IGZO bilayer engineering provides an effective strategy for coupling broadband visible-light sensitivity with tunable carrier-retention dynamics in oxide photonic synapses.