Suppression of oxygen vacancy and enhancement in bias stress stability of high-mobility ZnO thin-film transistors with N₂O plasma treated MgO gate dielectrics

ZnO thin-film transistors (TFTs) using MgO dielectrics achieve a high field-effect mobility of around 50 cm²/Vs. Plasma treatments with different gases applied on the MgO dielectric surface will amend the TFT's electrical stability and X-ray photoelectron spectroscopy analysis is done nearby the ZnO/MgO interface to study the change of oxygen chemical bonding states. The results show that MgO dielectric without plasma treatment causes the threshold voltage shift of the transistor, which may be attributed to the migration of oxygen ion toward the ZnO/MgO interface to induce the generation of oxygen vacancies in ZnO active layer when devices are under positive gate bias stress. This instability behavior can be eliminated with N₂O plasma treatment on MgO. N₂O plasma treatment inhibits the generation of oxygen vacancies in ZnO against gate bias stress thus reduces the effective trap state density in the subgap of ZnO and thereby enhances the TFT's stability. The detailed scenario for the plasma treatment effect is explored to conclude its mechanism on improving the electrical stability.