Oxygen Adsorption Effect of Amorphous InGaZnO Thin-Film Transistors

The effect of oxygen adsorption at the back channel of a-IGZO thin-film transistors (TFTs) is investigated. It is shown that for TFTs with the channel layer sputter-deposited at a high O2/Ar flow rate ratio ( $\text{R}_\text {O/Ar})$ , the threshold voltages in vacuum and O2 ambient do not show any difference; for devices fabricated at a low $\text{R}_{\text {O/Ar}}$ , the threshold voltages in vacuum are lower than those in O2. In addition, the devices in O2 show a more significant threshold voltage shift than those in vacuum do under a positive gate bias stress. The surface-state model is used to explain this observation. It is inferred that the oxygen adsorptions are physical and chemical, respectively, in the high- and low- $\text{R}_{\text {O/Ar}}$ cases, and the transition from physical to chemical adsorption occurs when a positive gate bias stress is applied.