High-Mobility Enhancement-Mode ITO/ZnO Thin-Film Transistors with an Asymmetric Vertical Channel Architecture

Oxide thin-film transistors (TFTs) for displays, transparent electronics, and integrated sensors require enhancement-mode operation, high carrier mobility, device-to-device reproducibility, and operational stability. In this work, a 19-nm-thick asymmetric ITO/ZnO channel was fabricated by RF magnetron sputtering to balance gate-interface control, carrier transport, and contact coupling. The channel comprises a dielectric-facing ZnO layer, a repeated ITO/ZnO internal region, and an oxygenated ITO cap. Cross-sectional transmission electron microscopy confirms channel continuity and the intended thickness, with nanoscale vertical contrast modulation consistent with the designed stack. X-ray diffraction indicates nanocrystalline ordering, while X-ray photoelectron spectroscopy identifies defect-related oxygen environments. The TFT exhibits enhancement-mode operation with a threshold voltage of +1.71 V, a mean maximum effective linear-regime field-effect mobility of 50.29 ± 1.01 cm2 V–1 s–1 across eight devices, an ION/IOFF ratio exceeding 106, a subthreshold swing of 0.42 V dec–1, and a threshold-voltage hysteresis of 0.485 V. After 3000 s of operational gate–drain bias stress at VDS,stress = 30 V with VGS,stress = +30 and –30 V, the threshold-voltage shifts were +1.36 and –0.80 V, respectively, while the on-state current remained nearly unchanged. Broadband illumination at 250 W m–2 and heating to 150 °C produced negative threshold-voltage shifts of –0.81 and –1.90 V, respectively, consistent with defect-mediated carrier redistribution. These results demonstrate that asymmetric vertical composition engineering can combine normally-off operation, high and reproducible maximum effective field-effect mobility, and operational bias stability in sputtered ITO/ZnO TFTs.