Rational Design of Different Ga Content Bilayer InGaZnO Thin-Film Transistors With Al₂O₃/HfO₂ Passivation Layer

This study investigated the effects of different Ga contents on the performance of single amorphous InGaZnO (a-IGZO) and bilayer InGa $_{\text{(\text{0}.\text{5}\% \text{)}}}$ ZnO/InGa $_{\text{(\text{1}\% \text{)}}}$ ZnO thin-film transistors (TFTs). Through rational design, a bilayer TFT exhibiting the best performance, including a $\textit{V}_{\text{th}}$ of 1.2 V, $\textit{I}_{\text{on}}$ / $\textit{I}_{\text{off}}$ of 1 $\times$ 10 $^{\text{8}}$ , SS of 0.28 V/decade, and $\mu _{\text{FE}}$ of 32.5 cm $^{\text{2}}$ /Vs, was obtained. This improved performance was attributed to the InGa $_{\text{(\text{0}.\text{5}\% \text{)}}}$ ZnO front layer enhancing the $\mu _{\text{FE}}$ with low surface defect and high $\textit{N}_{\text{e}}$ and the InGa $_{\text{(\text{1}\% \text{)}}}$ ZnO back layer controlling the $\textit{V}_{\text{th}}$ with low $\textit{V}_{\text{O}}$ and $\textit{N}_{\text{e}}$ in the bilayer device. Owing to the formation of energy band bending, the electrons transferred from the InGa $_{\text{(\text{0}.\text{5}\% \text{)}}}$ ZnO to InGa $_{\text{(\text{1}\% \text{)}}}$ ZnO layer. This resulted in the accumulation of free electrons near the interface, thereby enhancing the $\mu _{\text{FE}}$ of the bilayer device. Moreover, a minor shift in the $\textit{V}_{\text{th}}$ (0.4 and $-$ 0.5 V) of InGa $_{\text{(\text{0}.\text{5}\% \text{)}}}$ ZnO/InGa $_{\text{(\text{1}\% \text{)}}}$ ZnO TFTs with HfO $_{\text{2}}$ /Al $_{\text{2}}$ O $_{\text{3}}$ dual passivation layer (PVL) was observed under positive and negative gate bias light illumination stress with relative humidity 60% condition. This was attributed to the HfO $_{\text{2}}$ /Al $_{\text{2}}$ O $_{\text{3}}$ PVL protecting the channel from oxygen adsorption/desorption and environmental influences. Thus, the designed bilayer InGa $_{\text{(\text{0}.\text{5}\% \text{)}}}$ ZnO/InGa $_{\text{(\text{1}\% \text{)}}}$ ZnO TFTs with HfO $_{\text{2}}$ /Al $_{\text{2}}$ O $_{\text{3}}$ PVL have enabled new pathways for achieving high-performance and highly stable oxide TFTs.