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

Abstract

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.