Remarkable Stability Improvement of ZnO TFT with Al₂O₃ Gate Insulator by Yttrium Passivation with Spray Pyrolysis

We report the impact of yttrium oxide (YO_x) passivation on the zinc oxide (ZnO) thin film transistor (TFT) based on Al₂O₃ gate insulator (GI). The YO_x and ZnO films are both deposited by spray pyrolysis at 400 and 350 °C, respectively. The YO_x passivated ZnO TFT exhibits high device performance of field effect mobility (μ_FE) of 35.36 cm²/Vs, threshold voltage (V_TH) of 0.49 V and subthreshold swing (SS) of 128.4 mV/dec. The ZnO TFT also exhibits excellent device stabilities, such as negligible threshold voltage shift (∆V_TH) of 0.15 V under positive bias temperature stress and zero hysteresis voltage (V_H) of ~0 V. YO_x protects the channel layer from moisture absorption. On the other hand, the unpassivated ZnO TFT with Al₂O₃ GI showed inferior bias stability with a high SS when compared to the passivated one. It is found by XPS that Y diffuses into the GI interface, which can reduce the interfacial defects and eliminate the hysteresis of the transfer curve. The improvement of the stability is mainly due to the diffusion of Y into ZnO as well as the ZnO/Al₂O₃ interface.