Performance Optimization of Atomic Layer Deposited ZnO Thin-Film Transistors by Vacuum Annealing

In this letter, high-performance zinc oxide thin-film transistors (ZnO TFTs) have been fabricated on Si substrate by atomic layer deposition (ALD) at 190 °C. The field-effect mobility ( μ _FE ) can be improved from 18.5 cm ² / V·s to a record high 43.2 cm ² / V·s while maintaining a large I _on /I _off ratio of 5 × 10 ⁹ after rapid thermal annealing (RTA) under vacuum at 350 °C. The excellent performance can be attributed to the improvement in the ZnO channel and the interface. The oxygen vacancy ratio in ZnO increases from 36.2% to 40.0%. The interface trap density ( N _it ) between silicon oxide (SiO ₂ ) and ZnO decreases from 5.1 × 10 ¹¹ eV ^-1 cm ^-2 to 1.4 × 10 ¹¹ eV ^-1 cm ^-2 . Furthermore, the oxide trap density ( N _ot ) of the SiO ₂ dielectric reduces from 3.7 × 10 ¹⁹ eV ^-1 cm ^-3 to 1.1 × 10 ¹⁹ eV ^-1 cm ^-3 . Finally, we systematically study the electrical performance of the annealed ZnO device at measurement temperature from room temperature of 20 °C to a much higher temperature of 180 °C without device failure.