Eco-Friendly Fabrication of 2D a-SnO_x Thin-Film Transistors Derived from Deep Eutectic Solvents

We have fabricated amorphous tin oxide (a-SnO_x) thin-film transistors (TFTs) with Al₂O₃ gate insulator from deep eutectic solvents (DESs). DESs were formed using the chloride derivates of each precursor (SnCl₂, or AlCl₃) mixed with urea. The DESs were then used as precursors for the semiconductor and dielectric. Our target was to form extremely thin semiconductor film, and a sufficient high capacitance insulator. We characterized the physical and chemical properties of the DES-derived thin films by X-ray diffraction (XRD), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). We could evaluate that the highest content of metal-oxygen bonds was from the DES condition SnCl₂-urea = 1:3. At a low 300 °C budget temperature, we could fabricate a 3.2 nm thick a-SnO_x layer and 30 nm thick Al₂O₃, from which the TFT demonstrated a mobility of 80 ± 17 cm²/Vs, threshold voltage of -0.29 ± 0.06 V, and subthreshold swing of 88 ± 11 mV/dec. The proposed process is adequate with the back-end of the line (BEOL) process, but it is also eco-friendly because of the use of DESs.