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Effects of metal content on electrical and physical properties in solution-processed IGZO thin films

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Abstract

The on-current of indium gallium zinc oxide (IGZO) thin-film transistors (TFTs) fabricated by solution process with low-temperature post-annealing increases monotonically with the decrease in the Ga content. This trend is different from that of IGZO fabricated by a solution process, which shows the maximum electron mobility at a Ga content of 33%. To clarify its cause, effects of Ga and In contents in solution-processed IGZO post-annealed at a low-temperature on their oxygen deficiency and band structures were investigated with spectroscopic methods such as X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, photoluminescence, and ultraviolet–visible absorption. When the Ga content is low, the oxygen vacancy becomes less abundant. The increase in In content exhibits similar effects on the oxygen vacancy. We have reported that Ga is not oxidized sufficiently at low temperatures, which should cause the generation of oxygen vacancies. By this theory, the above-mentioned decrease in oxygen vacancies can be explained. Furthermore, when these films are actually used as TFT channels, the on-current increases with the decrease in Ga content. From this, in solution-processed IGZO post-annealed at a low temperature, oxygen vacancies mainly act as electron traps that decrease the on-current, rather than acting as donors. In manufacturing flexible devices, even when we decrease the process temperature low enough to suppress the damage to polymer substrates, the devices must maintain good performance. Our results suggest that an optimal metal ratio different from that of a typical vacuum process is necessary to improve the performance of IGZO TFTs fabricated by the low-temperature solution process.

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Acknowledgments

This research was partly supported by JSPS KAKENHI Grant Number 17K18177. The PL experiments were conducted mainly in the UVSOR Facility, Institute for Molecular Science, Okazaki, Japan. We are grateful to Masaki Ito of the Materials Characterization Central Laboratory, Waseda University, for his support for the XPS measurements and to Yoko Kasuya and Shintaro Ogura in AIST for their technical support.

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Author notes

  1. Takaaki Morimoto

    Present address: Department of Materials Science and Engineering, National Defense Academy, 1-10-20, Hashirimizu, Yokosuka, Kanagawa, 239-8686, Japan

Authors and Affiliations

  1. Department of Electrical Engineering and Bioscience, Waseda University, Shinjuku, Tokyo, 169-8555, Japan

    Takaaki Morimoto, Yicheng Yang, Yusuke Ochiai & Yoshimichi Ohki

  2. Sensing System Research Center (SSRC), National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, 305-8565, Japan

    Takaaki Morimoto & Nobuko Fukuda

  3. Research Institute for Materials Science and Technology, Waseda University, Shinjuku,Tokyo, 169-8555, Japan

    Yoshimichi Ohki

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Appendix

Appendix

The O1s XPS spectra measured for the IGZO films with different contents of Ga and those with different contents of In, shown in Figs. 2d and 3d, respectively, were separated into three Gaussian component peaks with their maxima at 530, 531, and 532 eV. The results are shown in Figs. 16 and 17. For each sample, the intensity integrated with respect to the binding energy was calculated for the whole O1s spectrum and each component. Then, the ratio of the integrated intensity of each component to that of the whole spectrum is plotted in Fig. 6.

Fig. 16
figure 16

(Color online) O1s XPS spectra observed in IGZO films with Ga contents of 0 (a), 10 (b), 20 (c), 40 (d), 60 (e), and 80 (f) mol  %. The curves in each figure represent the spectrum measured (solid black) and components due to oxygens bonded with metal (broken red), oxygens adjacent to oxygen vacancies (chain blue), and oxygens bonded weakly to the surface (dotted green). The broken pink curves are the spectra reproduced by the three components

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Fig. 17
figure 17

(Color online) O1s XPS spectra observed in IGZO films with In contents of 0 (a), 20 (b), 40 (c), 60 (d), and 80 (e) mol  %. The notations of curves are the same as those in Fig. 16

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Morimoto, T., Yang, Y., Ochiai, Y. et al. Effects of metal content on electrical and physical properties in solution-processed IGZO thin films. Appl. Phys. A 126, 388 (2020). https://doi.org/10.1007/s00339-020-03579-2

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