Low temperature, highly stable ZnO thin-film transistors

A low-temperature and straightforward fabrication process for ZnO thin-film transistors (TFTs) with near-zero aging and negligible instability enabled by using an ultrathin oxide as a top-passivation layer is demonstrated. The process features bottom-gate top-contacts ZnO TFTs with ultrathin HfO₂ or Al₂O₃ as passivation layers on top of the TFT followed by post-fabrication annealing (PFA). Devices with ultra-thin capping films of Al₂O₃ followed by a 150 °C PFA show threshold voltage shift (ΔV_TH) of <1% after bias stress and negligible shift after aging. The devices show saturation threshold voltage (V_TH-SAT) of 2.70 V, saturation mobilities larger than 10 cm²/V·s, and current I_ON/I_OFF ratios >10⁶. On the contrary, devices without nanofilm show similar performance to those with Al₂O₃ but show more considerable instability to aging and bias stress (ΔV_TH > 5%). Also, devices with HfO₂ as a capping layer shows severe instability (ΔV_TH > 40%). A degradation mechanism to explain the improved aging and reliability performance is also discussed.