Table 3 shows relative peak area ratio and binding energy of the IZTO thin films deposited by RF magnetron sputtering on silicon substrates at various annealing temperatures in the range of 150–350 °C. The related binding energies existed in metal-oxide (In-O, Zn-O, and Sn-O), oxygen vacancy (Oxy. Vac), and impurities such as hydroxides (O-OH) known as trapping sites on the interface of the TFT . Based on the detailed O 1s XPS spectra of IZTO films shown in Figure 3a, the oxygen vacancy peak existed at the binding energy of 530.4 eV. It was seen that the oxygen vacancy tended to increase slightly with the increase of annealing temperature. This rather unusual result could only be explained by the rearrangement of oxygen ions in the film to thermodynamically more stable positions, thereby yielding slightly more oxygen vacancies. However, as shown in Table 2, the increase of electron concentration is much higher than the increase of oxygen vacancy, and thereby is not just because of the increase of oxygen vacancy concentration but rather because of the increase of singly- or doubly-ionized oxygen vacancy concentration yielding mobile electrons. As-deposited films should have a greater number of neutral oxygen vacancies with the two electrons trapped at or near the vacancy and does not contribute to mobile carriers . In turn, this electron concentration increase brought the negative shift of the threshold voltage to be further explained below.