Elaboration of near‐valence band defect states leading deterioration of ambipolar operation in SnO thin‐film transistors

Abstract Ambipolar transistor operation in SnO thin‐film transistors (TFTs) is a promising character for future practical application, such as in integrated logic devices based on oxide semiconductors, because of its ability to develop them using a single material. However, there are only a few reports that demonstrate the apparent ambipolar operation for SnO TFTs owing to the insufficient knowledge on the reasons for deterioration of device performance. Although a previous study controls the operation mode of SnO TFTs [A. W. Lee et al. Adv. Electron. Mater. 6, 200742 (2020)], an additional passivating layer is required; however, it hinders the benefits of SnO usage. In this study, we provide the mechanism of deterioration of the ambipolar character of bare SnO TFTs, that is, the origin of defect states near the valence band maximum (near‐VB defect). Comprehensive spectroscopic approaches including photoemission, X‐ray emission, and X‐ray absorption spectroscopy, reveal that near‐VB defect states originate from oxygen vacancies, existing at the surface, and also in the entire SnO film. This finding is useful to improve device performance for obtaining the ambipolar operation of SnO TFTs.