# Amorphous Oxide Thin Film Transistors with Nitrogen-Doped Hetero-Structure Channel Layers
## 1 Introduction  
Amorphous oxide semiconductors (AOS), including amorphous InGaZnO (a-IGZO), amorphous InZnO (a-IZO), amorphous InGaO (a-IGO), etc., have been widely investigated as the channel layers of thin-film transistors (TFTs) since 2004 [1]. In fact, AOS TFTs are one of the major candidates to replace silicon TFTs for driving active-matrix liquid crystal displays (AMLCDs) and active-matrix organic light-emitting diodes (AMOLEDs) because of their high field-effect mobility (\(\mu_{\text{FE}}\)=10 cm\({}^{2}\cdot\)V\({}^{-1}\cdot\)s\({}^{-1}\)), good uniformity (substrate size could reach 2200 \(\times\) 2500 mm or even larger), simple processing methods, etc. [2; 3; 4; 5]. However, further improvements over the performance and stability of AOS TFTs are still needed for practical applications in flat panel displays (FPDs) and the other fields.  
[PERSON] et al. [6] used amorphous InSnZnO (a-ITZO) as the channel layers of TFTs and designed low RC-delay device structures to optimize the performance and reliability of pixel and driving circuits for AMOLEDs. [PERSON] et al. [7] improved the stability of a-IGZO TFTs by using the asymmetric device structure. [PERSON] et al. [8] co-sputtered IGZO and Al to form stable IGZAO TFTs [8]. [PERSON] et al. [9] prepared the rollable a-IGZO TFTs with good bias stability, good mechanical stability, and high operation frequency (\(-\)400 kHz) by using the solution-processed colorless-polyimide as the substrates [9].  
Recently, nitrogen-doping (N-doping) was shown to effectively improve the electrical properties (e.g., sub-threshold swing (SS), bias-stress stability, etc.) of AOS TFTs (e.g., a-IGZO:N TFTs [10], a-IZO:N TFTs [11], etc.). The N-doping decreased the channel/dielectric interface trap density, as well as the density of deep states, and the concentration of oxygen vacancies (\(V_{\text{O}}\)) in the channel layers ofa-IGZO TFTs [12]. Our previous work [13] indicated that N-doping improved the device performance and stability of a-IGZO TFTs by moderately adjusting the Vo concentration and largely suppressing the defects formation in the bulk channels, which was different from undoping and oxygen doping (O-doping). On the other side, N-doping unexpectedly degraded the mobility in a-IGZO:N [12], a-IZO:N [14], and ZnO:N [15], which might be attributed to the suppression of Vo, the main source of free electrons in AOS films [16].  
Another effective improving method is to form double-stacked channel layers (DSCL) for AOS TFTs, which has been studied by our group [17; 18], [PERSON] et al. [19], and [PERSON] et al. [20]. Among these studies, the DSCL structure was shown to be valid at improving the electrical characteristics (e.g., \(\mu_{\text{FE}}\), bias-stress stability, etc.) of TFTs. These improvements might result from the hetero-structure channel layers in DSCL devices [21; 22; 23]. In our recent reports, we obtained the DSCL structure by combining the a-IZO film with a-IGZO:N films, which made the corresponding TFTs exhibit quite a large \(\mu_{\text{FE}}\) (49.6 cm\({}^{2}\cdot\)V\({}^{-1}\cdot\)s\({}^{-1}\)) but too negative threshold voltage (\(V_{\text{TH}}\) \(\sim\)\(-\)2.3 V) [24; 25]. Therefore, further improvement is still necessary.  
In this study, we used a-IZO:N rather than a-IZO to combine with a-IGZO:N and formed two novel DSCL structures (a-IGZO:N/a-IZO:N and a-IZO:N/a-IGZO:N), achieving a good electrical performance and good stability of the corresponding TFT devices.