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.