# Amorphous Oxide Thin Film Transistors with Nitrogen-Doped Hetero-Structure Channel Layers
## 3 Results
As shown in Figure 7a,b, moisture gas molecules could diffuse in and out of AOS thin films, donating extra electrons. This might largely influence the device performance (\(\mu_{\rm FE}\), SS, \(V_{\rm TH}\), etc.) [32]. As mentioned before, from the XPS measurements, the a-IZO:N film adsorbed much more ambient moisture gas than the a-IGZO:N films. Thus, there could be many more extra electron donations caused by moisture gas adsorption in Sample I than those in Sample II. This implied that many more extra electrons accumulated at the sub-channel in Sample I, participating in the current conduction when \(V_{\rm GS}\)>>\(V_{\rm TH}\), and thus leading to a larger \(\mu_{\rm FE}\) and a more negative \(V_{\rm TH}\) in Sample I relative to those in Sample II. According to Reference [32], moisture adsorption could bring about SS deterioration in AOS TFTs. Hence, as shown in Table 1, Sample I exhibited a worse SS value (0.8 V/dec) than Sample II, possibly due to the more moisture adsorptions in its channel layer. For the applications of TFTs in AMLCDs and AMOLEDs, it is important to keep \(V_{\rm TH}\) of 0 V (or a little larger). In our previous study [25], we designed a-IZO/a-IGZO:N TFTs, whose \(\mu_{\rm FE}\) was quite large (49.6 cm\({}^{2}\cdot\)V\({}^{-1}\cdot\)s\({}^{-1}\)), but \(V_{\rm TH}\) was too negative (\(-\)2.3 V). In this study, however, Sample II (a-IZO:N/a-IGZO:N FFT) had a positive \(V_{\rm TH}\) of 1.5 V, which might result from the fact that N-doping decreased the trap states in a-IZO and at the front channel interface [12; 13]. As shown in Figure 5a,b, the N-doped DSCL TFTs (Samples I and II) showed different PBS stability. Sample I had better PBS stability than Sample II. The stability of AOS TFTs could be influenced by the ambient O\({}_{2}\) gas during the PBS testing [10; 25; 32]. The reaction might be described as:  
\[{\rm O_{2(gas)}+e^{-}\to O_{2(solid)}}^{-}. \tag{4}\]  
The better PBS stability of Sample I could be attributed to the more moisture gas adsorptions on the film surface, which weakened the O\({}_{2}\) adsorption to the channel. Figure 7: The energy band diagrams for (**a**) a-IGZO:N/a-IZO:N TFT (Sample I) and (**b**) a-IZON/a-IGZO:N FFT (Sample II), when \(V_{\rm GS}\)>>\(V_{\rm TH}\). NBS tests might cause a moisture reaction at the back channels of AOS TFTs, which could be described as [25; 32; 33]:  
\[\mathrm{H_{2}O^{-}\to H_{2}O+e^{-}}. \tag{5}\]  
The XPS results indicated that the a-IZO:N films more easily adsorbed the ambient moisture gas than the a-IGZO:N films (Figure 2). Figure 5c,d indicate that Sample I showed very poor NBS stability (\(V_{\mathrm{TH}}\) shift = \(-\)9 V). This was likely because the sub-channel of Sample I (a-IGZO:N/a-IZO:N interface) was greatly influenced by the ambient moisture gas during the NBS testing. In contrast, Sample II showed a much smaller \(V_{\mathrm{TH}}\) shift (\(-\)0.5 V) under NBS tests. This might be due to the weak moisture adsorption effect of the a-IGZO:N layer, which was consistent with the XPS data (Figure 2). Thermal stability data in Figure 6 indicated that Sample I was unstable (\(V_{\mathrm{TH}}\) shift = \(-\)11.5 V) under thermal stressing; Sample II was much more thermally stable (\(V_{\mathrm{TH}}\) shift = \(-\)2.5 V). We reported that larger concentration variation of \(V_{\mathrm{O}}\) in AOS TFTs could lead to serious \(V_{\mathrm{TH}}\) shift during the thermal testing [29]. In addition, the literature [16; 34] indicated that N-doping suppressed \(V_{\mathrm{O}}\) formation in the bulk channels for the thermal tests of a-IGZO:N TFTs. Hence, the better thermal stability of Sample II might be attributed to the a-IGZO:N films, which acted as a passivation layer and suppressed the \(V_{\mathrm{O}}\) formation in the a-IZO:N layers.