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.

Characteristic Electronic Structure of SnO Film Showing High Hole Mobility

“…Angle-resolved photoemission spectroscopy (ARPES) is a powerful and unique technique for investigating the electronic structure of materials experimentally based on the well-known relationship between the kinetic energy and emission angle of photoelectron. , Owing to its surface sensitivity, a clean and well-defined surface is indispensable for observing these intrinsic electronic structures of materials. Sputtering and/or annealing processes have conventionally been applied to prepare such surfaces. − However, these processes cause compositional deviations from the original and the observed ARPES spectra do not reflect the intrinsic electronic structure. , To overcome this drawback, a compact in vacuo transferring system has been developed that enables the transfer of thin films, prepared by the thin-film deposition equipment, to the ARPES apparatus without exposing them to the atmosphere …”

“…In order to elucidate the possible changes in the local crystal structure, we focus on the Fourier transform (FT) magnitudes of the Sn K-edge extended X-ray absorption fine structure (EXAFS) and the data of SnO film and powder (Figure a). The EXAFS spectra of the SnO film and powder were analyzed using the original data reported previously. , The FT of the Sn K-edge EXAFS spectra exhibit two prominent features at approximately 1.7 and 3.2 Å, which is dominantly caused by the single-scattering from the nearest-neighbor O and the next-nearest-neighbor Sn (Sn1), respectively (see also Figure b). The FT spectra show that both the Sn–O distance ( d Sn–O ) and Sn–Sn1 distance ( d Sn–Sn1 ) are almost unchanged for the SnO film and powder.…”

“…(a) Fourier transforms of the k 3 -weighted Sn K-edge EXAFS of SnO film and powder. The EXAFS spectra of SnO film and powder were replotted from the original articles by Minohara et al . used under a Creative Commons Attribution (CC BY 4.0) license, and from Minohara et al, respectively.…”