## 3 Results and Discussion

Table 1: Extracted electrical parameters of the a-IGZO TFTs with different P\({}_{\rm N/O}\). Figure 2: Evolution of the transfer curves against positive bias stress (PBS) time for the a-IGZO TFTs fabricated using P\({}_{\rm N/O}\) of (**a**) 0% at T = 70 \({}^{\circ}\)C, (**b**) 20% at T = 70 \({}^{\circ}\)C, and (**c**) 40% at T = 70 \({}^{\circ}\)C.  
the device interface region or insulator. To investigate the effect of N-doping on the carrier trapping process in the a-IGZO TFTs, the \(E_{\tau}\) is extracted by the Arrhenius plot of \(\tau\). As shown in Figure 3d, a good linear relationship in the ln\(\tau\)-1000/T plots is observed, which indicates that the carrier trapping process in the a-IGZO TFT is thermally activated [15]. Meanwhile, the \(E_{\tau}\) of the a-IGZO TFT with 20% P\({}_{\text{N/O}}\) (0.57 eV) is increased to that of the undoped a-IGZO TFT (0.37 eV). The increased \(E_{\tau}\) suggests that fewer channel carriers can be trapped into the a-IGZO/SiO\({}_{2}\) interface or insulator during the bias-stress process and the corresponding device exhibits better bias-stress stability. On the contrary, compared with the a-IGZO TFT with 20% P\({}_{\text{N/O}}\), the \(E_{\tau}\) of the a-IGZO TFT with 40% P\({}_{\text{N/O}}\) is decreased to 0.43 eV, which means that the interface quality is degraded when excess N is incorporated into the a-IGZO active layer. Therefore, the results indicate that the drift of \(V_{th}\) for the a-IGZO TFTs could be mitigated by the moderate N-doping. In addition, in real applications, switching TFTs are usually negatively biased for keeping off-state and exposed to light emitted from the backlight in active-matrix displays [17; 18]. Thus, the electrical reliability of the TFTs fabricated with different P\({}_{\text{N/O}}\) is also evaluated by negative- bias-stress illumination (NBSI). Figure 4a-c show the transfer curves of the a-IGZO TFTs fabricated with different P\({}_{\text{N/O}}\) against NBS time under white light illumination, in which the device is stressed at \(V_{GS}=-15\) V for 5000 s. The transfer curves of the TFTs exhibit a shift toward negative gate voltage direction with no apparent change in SS and \(\mu_{\text{FE}}\) after the NBSI condition, which indicates that the negative shift of \(V_{th}\) should be determined by photo-induced holes trapped into the a-IGZO/SiO\({}_{2}\) interface [19; 20]. Meanwhile, as shown in Figure 4a,b, it is clear that the negative shift of \(V_{th}\) is decreased from 3.0 V to 1.1 V for N-free a-IGZO TFT and 20% P\({}_{\text{N/O}}\) a-IGZO TFT after 5000 s NBSI, which means that the  
Figure 3: Time dependence of threshold voltage drift (\(\Delta V_{th}\)) for the a-IGZO TFTs fabricated using P\({}_{\text{N/O}}\) of (**a**) 0%, (**b**) 20%, (**c**) 40% at different stress temperatures, and (**d**) Stress time constant ln\(\tau\) as a function of the reciprocal temperature. a-IGZO/SiO\({}_{2}\) interface quality is improved by N-doping. However, as shown in Figure 4c, the a-IGZO TFT with 40% P\({}_{\text{N/O}}\) exhibits a large negative shift of \(V_{th}\) (2.65 V) compared with TFT with 20% P\({}_{\text{N/O}}\) after 5000 s NBIS, which indicates that additional defects are generated at the a-IGZO/SiO\({}_{2}\) interface by heavy N-doping. To reveal the mechanism of the effect of N-doping on the reliability of the a-IGZO TFTs, the chemical properties of the a-IGZO, a-IGZO: 20% P\({}_{\text{N/O}}\), and a-IGZO: 40% P\({}_{\text{N/O}}\) films are analyzed by the X-ray photoelectron spectroscopy (XPS) measurement. The deconvolution of XPS spectra of O 1s is shown in Figure 5a-c. The combined O 1s peak could be divided into three components by Gaussian fitting, which is located at 530.1 eV (O\({}_{\text{I}}\)), 531.3 eV (O\({}_{\text{II}}\)), and 532.4 eV (O\({}_{\text{III}}\)), respectively.