| Summary: | In this work, we demonstrate the effectiveness of nitrogen plasma treatment on the formation of low-resistive source/drain (S/D) in self-aligned (SA) oxide thin-film transistors (TFTs) using a high-mobility oxide semiconductor (OS), In-Ga-Zn-Sn-O (IGZTO). The nitrogen plasma treatment was more effective at reducing the sheet resistance (<inline-formula> <tex-math notation="LaTeX">${R} _{\mathrm{ sheet}}$ </tex-math></inline-formula>) of IGZTO films than the commonly used argon plasma treatment. Furthermore, <inline-formula> <tex-math notation="LaTeX">${R} _{\mathrm{ sheet}}$ </tex-math></inline-formula> for nitrogen-plasma-treated IGZTO films remained low, even when the RF power and radiation time during the plasma treatment were increased when the minimum <inline-formula> <tex-math notation="LaTeX">${R} _{\mathrm{ sheet}}$ </tex-math></inline-formula> was achieved. The same trends were also observed in OS films with different compositions, such as In-Ga-Zn-O and In-Sn-Zn-O. These results indicate that nitrogen plasma treatment is effective for achieving a reduction of <inline-formula> <tex-math notation="LaTeX">${R} _{\mathrm{ sheet}}$ </tex-math></inline-formula> for various OS films with a wide process window regarding plasma processing parameters. The advantages could be attributed to the smaller sputtering effect on the OS films due to the lower mass of nitrogen ions than argon ions, which was verified by X-ray reflectivity and X-ray photoelectron spectroscopy analyses. For further validation, SA IGZTO TFTs with a channel length (<inline-formula> <tex-math notation="LaTeX">${L}$ </tex-math></inline-formula>) of 3 to 100 <inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> were fabricated with nitrogen or argon plasma treatment. The width-normalized parasitic SD resistance (<inline-formula> <tex-math notation="LaTeX">$R_{\mathrm{ SD}} {W}$ </tex-math></inline-formula>) with the nitrogen plasma treatment was determined to be 11.3 <inline-formula> <tex-math notation="LaTeX">$\Omega \cdot $ </tex-math></inline-formula>cm, which was <italic>ca.</italic> 40% lower than that with the argon plasma treatment. This improvement in <inline-formula> <tex-math notation="LaTeX">$R_{\mathrm{ SD}} {W}$ </tex-math></inline-formula> resulted in higher mobility (<inline-formula> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula>) in the nitrogen-plasma-treated SA IGZTO TFTs. A nitrogen-plasma-treated SA IGZTO TFT with <inline-formula> <tex-math notation="LaTeX">$L=10\,\,\mu \text{m}$ </tex-math></inline-formula> exhibited a high <inline-formula> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula> of 27.2 cm<sup>2</sup>/Vs.
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