Trilayer ZnO Thin-Film Transistors With In Situ ${\rm Al}_{2}{\rm O}_{3}$ Passivation

In this letter, we report trilayer ZnO thin-film transistors (TFTs) with in situ Al₂O₃ passivation fabricated using plasma-enhanced atomic layer deposition. The bottom-gate, top-contact TFTs use an Al₂O₃-ZnO-Al₂O₃ trilayer deposited in one deposition run at 200°C that provides protection for the active layer back surface with no extra passivation step. Compared with Al₂O₃ passivated, nontrilayer ZnO TFTs, these trilayer devices have similar field effect mobility, but more positive turn-on voltage and improved bias stability. Seven-stage trilayer ZnO TFT ring oscillators operated at 3.5 MHz at a supply voltage of 17 V, corresponding to a propagation delay of ~ 27 ns/stage.