Thermally Intermixed ZnO-Rich-to-ITO-Rich Vertical Gradient Channels for Near-Zero-Threshold Oxide Thin-Film...

Balancing carrier transport, electrostatic control, and operational stability remains a central challenge in oxide thin-film transistors (TFTs). Here, a 20 nm vertically compositiongraded ITO/ZnO channel was developed with a ZnO-rich region adjacent to the gate dielectric and a progressively increasing ITO fraction toward the source/drain contacts. The channel was fabricated by sequential RF magnetron sputtering followed by annealing at 350 • C. Cross-sectional TEM and EDS analyses revealed a continuous, compositionally intermixed oxide film retaining the intended depth-dependent Zn/In distribution, while XRD and XPS revealed a nanocrystalline metal-oxygen network containing defect-related oxygen states. The resulting TFTs exhibited near-zero-threshold enhancement-mode operation with a threshold voltage of 0.18 V, an I ON /I OFF ratio exceeding 10 6 , a hysteresis of 0.2 V, and a maximum effective field-effect mobility of 37.15 ± 0.28 cm 2 V -1 s -1 across five devices. After 3600 s of positive-and negative-bias stress, the threshold-voltage shifts were +0.43 and -1.25 V, respectively, while operation at 150 • C produced a shift of -0.40 V. The preserved on-state characteristics suggest that these shifts are predominantly associated with defect-mediated charge redistribution rather than substantial degradation of the conduction pathway. These results establish vertical composition grading as an effective strategy for combining high mobility, near-zero-threshold switching, and operational stability in sputtered oxide TFTs.