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

We have fabricated amorphous tin oxide (a-SnOx) 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-SnOx 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.