## 3 Results and Discussion  
Figure 1a shows the TG curve of AlO\({}_{\mathrm{x}}\) precursor. It reveals an initial mass loss event of \(\sim\)60%, occurring around 120-180 \({}^{\circ}\)C, which is assigned to the removal of solvent and organic residues and the dehydroxylation of AlO\({}_{\mathrm{x}}\) precursor. Then, a slow mass loss is observed around 180-350 \({}^{\circ}\)C, with weight stabilizing at close to 20% by 350 \({}^{\circ}\)C, indicating complete conversion of precursors to form the dense metal oxide. Figure 1b shows the FT-IR spectra of AlO\({}_{\mathrm{x}}\) films annealed at different temperatures. The peaks around 1700 cm\({}^{-1}\) (C=C stretching) and 1500 cm\({}^{-1}\) (N-O asymmetric stretching) are  
attributed to the residual organic elements and the undecomposed precursor metal salts, respectively [22; 23]. As the annealing temperature increases from 250 to 350 \({}^{\circ}\)C, the intensity of the peaks decreases continuously, which is consistent with the TG analysis. The peaks in the range of 3500-3800 cm\({}^{-1}\) is due to the peaks of O-H stretching plausibly resulting from surface hydroxylation [22; 23]. Figure 2 shows the XPS O 1s peaks of the AlOx films with different annealing temperature. Each O 1s spectrum was fitted by two Gaussian distributions with binding energies at 531 and 532.3 eV, corresponding to the contribution of lattice oxygen (M-O-M), and hydroxyl group (M-OH), respectively [7; 10; 18; 21; 22]. The relative concentrations of M-OH related oxygen for the untreated samples annealed at 250, 300, and 350 \({}^{\circ}\)C are 41.88%, 36.81%, and 25.40%, respectively, while the water-treated samples annealed at 250, 300, and 350 \({}^{\circ}\)C exhibit higher M-OH concentrations of 43.06%, 37.60 %, and 25.97%, respectively. The result is in accordance with the change of O-H stretching peaks in the FT-IR spectra (Figure 1b). It indicates that there are plenty of hydroxyl groups on the surface of AlOx film annealed at 250 \({}^{\circ}\)C, and the number of them can be reduced effectively when the annealing temperature reaches 350 \({}^{\circ}\)C. The concentration difference between the water-treated and untreated samples narrows as the annealing temperature increases, because the water molecules are easier to be dislodged at higher annealing temperature. Figure 1: **(a)** TG curve of the AlOx precursor from 35 to 600 \({}^{\circ}\)C. **(b)** FT-IR spectra of the AlOx films annealed at different temperatures. Figure 2: XPS O1s spectra of the untreated AlOx films annealed at **(a)** 250, **(b)** 300, and **(c)** 350 \({}^{\circ}\)C; and XPS O1s spectra of the water treated AlOx films annealed at **(d)** 250, **(e)** 300, and **(f)** 350 \({}^{\circ}\)C.  
The insulating properties of AlO\({}_{\rm x}\) films were evaluated with a MIM structure of ITO/AlO\({}_{\rm x}\)/ITO. Figure 3a shows the frequency dependence of capacitance of the MIM devices. Interestingly, the capacitance decreases as the annealing temperature increases at lower frequency regime (<10 kHz), while it increases as the annealing temperature increases at higher frequency regime (>1 MHz). In addition, the capacitance of the MIM devices annealed at 250 or 300 \({}^{\circ}\)C depends on the frequency greatly, while the AlO\({}_{\rm x}\) film annealed at 350 \({}^{\circ}\)C is almost independent of the measuring frequency (when the measuring frequency is lower than 100 kHz). The areal capacitances at 1 kHz for the MIM devices annealed at 250, 300, and 350 \({}^{\circ}\)C are 179.1, 166.3, and 148.1 nF/cm\({}^{2}\), respectively; and the areal quasistatic (QS) capacitances (see Figure 3b) for the devices annealed at 250, 300, and 350 \({}^{\circ}\)C are 288.9, 263.4, and 152.8 nF/cm\({}^{2}\), respectively.