# Amorphous Oxide Thin Film Transistors with Nitrogen-Doped Hetero-Structure Channel Layers
## 3 Results
In this study, we deliberately designed and prepared the N-doped DSCL TFTs (Samples I and II) using the hetero-structure channel layers composed of a 15-nm-thick a-IZO:N layer and a 15-nm-thick a-IGZO:N layer, and finally obtained good electrical performance (e.g., the large \(\mu_{\rm FE}\), the proper \(V_{\rm TH}\), the small SS, and the large \(I_{\rm ON}/I_{\rm OFF}\)), as shown in Table 1. On the other hand, although Sample I exhibited a larger field-effect mobility (\(u_{\rm FE}=31.9\) cm\({}^{2}\cdot\)V\({}^{-1}\cdot\)s\({}^{-1}\)), its \(V_{\rm TH}\) was rather negative (\(-5.0\) V), which was apparently not fit for driving FPDs. Sample II showed the optimum device performance (\(u_{\rm FE}=15.0\) cm\({}^{2}\cdot\)V\({}^{-1}\cdot\)s\({}^{-1}\), \(V_{\rm TH}=1.5\) V, SS \(=0.5\) V/dec) relative to Sample I. In addition, Sample I exhibited excellent PBS stability (\(V_{\rm TH}\)shift \(=+0.5\) V), whereas its NBS (\(V_{\rm TH}\) shift \(=-9.0\) V) and thermal stability (\(V_{\rm TH}\) shift \(=-11.5\) V) were poor. Compared to Sample I, more stable properties were observed for Sample II during NBS (\(V_{\rm TH}\) shift \(=-0.5\) V) and thermal stress (\(V_{\rm TH}\) shift \(=-2.5\) V) tests. Therefore, Sample II, rather than Sample I, was preferred because of its better electrical performance and stability. However, what accounted for these results? The related physical mechanisms should be ascertained by analyzing the microstructures and energy bands of the channel layers for these TFT devices. Figure 7 shows energy band diagrams of the two samples when \(V_{\rm GS}\)>>\(V_{\rm TH}\). As mentioned in the literature [15], \(V_{\rm O}\) was considered as the main source of free electrons in AOS films. According to the XPS result and the film resistivity (Figures 2 and 3), the a-IZO:N film contained more \(V_{\rm O}\) and exhibited a smaller resistivity with respect to the a-IGZO:N film, indicating that there could be even more electrons in the a-IZO:N film than those in the a-IGZO:N film. Due to the differences in carrier concentrations, the electrons injected from the a-IZO:N layer into the a-IGZO:N layer and formed the built-in electrical voltage (\(V_{\rm built-in}\)) near the interfacial region between the a-IZO:N and a-IGZO:N films. The \(V_{\rm built-in}\) might cause injection electrons to accumulate in the interface region between the a-IZO:N and a-IGZO:N layers. In addition, the polarity of \(V_{\rm built-in}\) was opposite for Samples I and II, as shown in Figure 7.  
Figure 6: Thermal stress of the N-doped DSCL TFTs: (**a**) a-IGZO:N/a-IZO:N TFT (Sample I); (**b**) a-IZO:N/a-IGZO:N TFT (Sample II). The transfer curves were obtained with \(V_{\rm DS}=10\) V.  
The N-doped DSCL AOS TFTs (Samples I and II) exhibited large \(\mu_{\rm FE}\), which resulted from the presence of double channels due to the hetero-structure channel layers in these samples. As shown in Figure 7a,b, some electrons might accumulate at the interfacial region between the a-IGZO:N layer and a-IZO:N layer due to the difference of the carrier concentration, as well as the formation of \(V_{\rm built\mbox{-}in}\). Apparently, these accumulated electrons could also participate in current conduction, forming a sub-channel at the a-IGZO:N/a-IZO:N or a-IZO:N/a-IGZO:N interfacial regions during the operation of the TFT devices. In other words, the current conduction operation of the DSCL AOS TFTs was dominated by not only the main-channel (the interface between channel layer and gate insulator), but also the sub-channel (the interface between a-IGZO:N and a-IZO:N films). The co-contribution of these two channels in current conduction led to the large \(\mu_{\rm FE}\) in the hetero-structure AOS TFTs. In particular, among the DSCL devices, Sample I exhibited larger \(\mu_{\rm FE}\) (31.9 cm\({}^{2}\cdot\)V\({}^{-1}\cdot\)s\({}^{-1}\)) than Sample II (15.0 cm\({}^{2}\cdot\)V\({}^{-1}\cdot\)s\({}^{-1}\)), which could be explained by the hetero structures in their energy bands when \(V_{\rm GS}\)>>\(V_{\rm TH}\).