## 2 Experiment  
The AlO\({}_{\mathrm{x}}\) precursor solution was prepared by dissolving 0.2 M Al(NO\({}_{3}\))\({}_{3}\).9H\({}_{2}\)O in 2-methoxyethanol, stirred at room temperature for 24 h, and aged for 6 h. The precursor materials, including solutes and solvents, were purchased from Aladdin. The AlO\({}_{\mathrm{x}}\) precursor films were deposited by spin-coating at 3000 rpm for 30 s and then soft-baked on a hot plate at 150 \({}^{\circ}\)C for 10 min to remove the solvents. After that, the AlO\({}_{\mathrm{x}}\) precursor films were annealed in the air at different temperature of 250, 300, and 350 \({}^{\circ}\)C for 1 h. Each sample was baked at 150 \({}^{\circ}\)C for 10 min immediately after spin-coating. Then, each sample was annealed separately at different temperature for 1 h. The thickness of AlO\({}_{\mathrm{x}}\) is about 57 \(\pm\) 3 nm.  
TFTs with AlO\({}_{\mathrm{x}}\) gate dielectric layer and InScO\({}_{\mathrm{x}}\) (In\({}_{2}\)O\({}_{3}\):Sc\({}_{2}\)O\({}_{3}\) = 98:2 wt%) semiconductor layer were constructed with a bottom-gate top-contact structure. A 200 nm indium tin oxide (ITO, In:Sn = 9:1) gate electrode was deposited onto the glass substrate by DC magnetron sputtering (70 W) under argon pressure of 0.5 Pa at room temperature through a shading mask. Then, a layer of AlO\({}_{\mathrm{x}}\) dielectric film was deposited onto the ITO gate electrode using the process described above. After that, the InScO\({}_{\mathrm{x}}\) semiconductor layer (20 nm) was deposited by RF magnetron sputtering (60 W) under argon pressure of 0.5 Pa at room temperature through a shading mask. The ITO source and drain electrodes (200 nm) were deposited onto the InScO\({}_{\mathrm{x}}\) semiconductor layer by DC magnetron sputtering with the same conditions as that of the gate deposition. The channel width (W) and length (L) were defined by a shading mask to be 800 and 200 um, respectively. Finally, the TFT devices were post-annealed at 250 \({}^{\circ}\)C for 1 h. The ITO/AlO\({}_{\mathrm{x}}\)/ITO metal-insulator-metal (MIM) devices were prepared by depositing a circular ITO top electrode (200 nm) with a diameter of 0.04 mm onto the AlO\({}_{\mathrm{x}}\) film by DC magnetron sputtering.  
To investigate the effect of hydrogen ions (H\({}^{+}\)) on the dielectric properties of the AlO\({}_{\mathrm{x}}\) layer, water treatment was performed on the surface of the ITO gate electrode before spin-coating the AlO\({}_{\mathrm{x}}\) film. Because the solution-processed AlO\({}_{\mathrm{x}}\) film is very sensitive to moisture, water treatment is a simple way to increase the density of the mobile H\({}^{+}\). The water treatment process was performed by spin-coating deionized water at a speed of 3000 rpm for 30 s onto the surface of the ITO gate electrode to introduce large amount of adsorbed H\({}^{+}\) and OH\({}^{-}\) groups at the ITO/AlO\({}_{\mathrm{x}}\) interface. The ITO surface is treated by O\({}_{2}\) plasma before water treatment. The contact angles of the plasma treated and plasma + water treated ITO surfaces are 5.03\({}^{\circ}\) and 3.93\({}^{\circ}\), respectively.  
The thermal behavior of AlO\({}_{\mathrm{x}}\) precursor was analyzed by thermogravimetric analyses (TG). The chemical composition, water adsorption, and proton quantity behavior of the AlO\({}_{\mathrm{x}}\) films were characterized by X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific Inc, ESCALAB250 Xi), infrared spectroscopy (FT-IR), and time-of-flight secondary-ion mass spectrometry (TOF-SIMS), respectively. The electrical properties of MIM and TFTs were characterized by semiconductor parameter analyzer (Keysight B1500A).