Performance Improvement of Atomic Layer-Deposited ZnO/Al2O3 Thin-Film Transistors by Low-Temperature Annealing in Air

High-performance thin-film transistors (TFTs) with atomic layer-deposited (ALD) ZnO channel/Al₂O₃ dielectric were fabricated under a maximum processing temperature of 200 °C. The effects of postannealing temperature and time on the performance of the TFT were investigated. Under annealing at 200 °C in air, the performance of the TFT was greatly improved by increasing the annealing time to 120 min, showing a very low OFF-current of 2.98 × 10^-13 A, a small subthreshold swing (SS) of 244 mV/decade, a quite large I_ON/I_OFF ratio of 4 × 10⁸, and a high field-effect electron mobility of 21.9 cm²/V·s. Furthermore, good electrical stabilities were also demonstrated under gate-bias stress, such as a threshold voltage shift (ΔV_th) of -1.1 V and a Δ SS of 86 mV/decade under -20 V for 3000 s, a ΔV_th of 0.29 V, and a Δ SS of -44 mV/decade under +20 V for 3000 s. The above results are attributed to the gradual passivation of oxygen vacancies in the ZnO channel and interface traps at the interface of ZnO/Al₂O₃ with an increment of annealing time. Thus, the current ZnO TFT with a low thermal budget and high performance is very promising for flexible electronic applications.