Effects of excess oxygen on operation characteristics of amorphous In-Ga-Zn-O thin-film transistors

Operation characteristics of amorphous In-Ga-Zn-O (a-IGZO) thin-film transistors (TFTs) were improved to a subthreshold voltage swing (S) of 217 mV (decade)−1, a mobility of ∼11.4 cm2 (Vs)−1, and a threshold voltage (Vth) of 0.1 V by O3 annealing at a temperature as low as 150 °C. However, the O3 annealing at 300 °C caused serious deterioration and exhibited a bistable transition between a large S state and a large Vth state. This transition is attributed to incorporation of excess oxygen and associated subgap defects with a negative-U characteristic. It also explains why a-IGZO channels deposited at high oxygen pressures do not produce operating TFTs.