Effects of thermal annealing on elimination of deep defects in amorphous In-Ga-Zn-O thin-film transistors

We investigated the effects of thermal annealing for high-density subgap states in amorphous In-Ga-Zn-O (a-IGZO) films by focusing on low-quality defective films deposited without O2 supply (LQ films). It was found that most of the subgap states were thermally unstable and decreased dramatically by annealing at ≤ 400 °C in O2. These defects (but with different shapes) were further reduced by 600 °C annealing, whose subgap states appeared similar to that of a-IGZO films deposited at an optimum condition (high quality, HQ films) and annealed at 300 °C. However, electron Hall mobilities and field-effect mobilities of their thin-film transistors (TFTs) were low for the LQ films/TFTs even annealed at 600 °C compared to those for the HQ films/TFTs. It implies that not only the subgap states but also heavier structural disorder deteriorated the electron transport in the LQ films. The present results also suggest that although a-IGZO deposition without O2 supply is sometimes employed in particular for DC sputtering, supplying some O2 gas would be better to produce good TFTs at lower temperatures.