Performance Improvement for Spray-Coated ZnO TFT by F Doping With Spray-Coated Zr–Al–O Gate Insulator

We report the fluorine (F) doping effect on zinc oxide (ZnO) thin-film transistor (TFT) fabricated by spray pyrolysis on spray-coated zirconia alumina oxide (ZAO) gate insulator. F doping was performed by NF3 plasma treatment on ZnO thin film. The ZnO film shows ${C}$ -axis-aligned hexagonal structure, which remains unchanged by F doping. The photoluminescence (PL) spectra and X-ray photoelectron spectroscopy (XPS) show a reduction in oxygen-related defects by F doping. Valance band edge XPS spectra show the shift of Fermi energy level from 2.46 to 3.12 eV which reveals that the more carriers are generated in the conduction band and defect states are reduced. The F-doped ZnO (F:ZnO) TFT exhibits the saturation mobility of 31.59 cm2V−1s−1, the subthreshold swing of 238mV/dec, ON/ OFF current ratio of ~108, and zero hysteresis voltage. The F:ZnO TFT also shows significant improvement of threshold voltage shift ( $\Delta \text{V}_{TH}$ ) under negative/positive gate bias stress compared to pristine ZnO. The enhancement of mobility and stability is attributed to the substitution of oxygen (O) and passivation of oxygen vacancies ( ${V}_{o}$ ) by F in ZnO. Therefore, it is expected that F doping is an effective method to improve the performance and stability of solution-processed oxide TFTs.