Evolution of Defect States within the Band Gap of Indium–Tin–Zinc Oxide Thin Film Transistors Using the Quantitative Defect Analysis Method

To develop electronic devices and improve their performance, it is crucial to understand the causes of bias instability in thin film transistors (TFTs). Here, we examine the origin of the bias stability of Indium–Tin-Zinc Oxide (a-ITZO) TFTs after annealing in various atmospheres. The annealing process was performed in N₂ (N₂–ITZO), air (Air-ITZO), and O₂ (O₂–ITZO) after the a-ITZO was deposited by magnetron sputtering. Air-ITZO has superior bias stability under positive bias stress (PBS) despite its high defect oxygen vacancies. On the other hand, N₂–ITZO and O₂–ITZO both showed worse PBS stability despite having low oxygen vacancies and defect densities. The results of a qualitative defect investigation using X-ray photoelectron spectroscopy and spectroscopic ellipsometry failed to explain the primary cause of these phenomena. In contrast, a quantitative examination of oxygen-related defect states using photo-induced current transient spectroscopy revealed that the excellent PBS stability of Air-ITZO was mostly attributable to the low density of defect states above the Fermi level. Moreover, the negative bias stress (NBS) stability of the devices exhibits the trend of O₂–ITZO > N₂–ITZO > Air-ITZO, which is consistent with the trend found for deep-level defect densities. These results indicate that quantitative defect state analysis is key to understanding the mechanism of device performance and stress bias stability in metal oxide TFTs.