Aluminum Capping-Induced Enhancement of Electrical Performance and Stability in Zinc Tin Oxide Thin-Film Transistors via a Low-Resistance Electron Pathway

We present a ZnSnO (ZTO) thin-film transistor (TFT) with enhanced mobility, achieved through the incorporation of a metal-capping layer. The fabrication of the ZTO active layer involves the deposition of a ZnO incubation layer, followed by super cycles of depositing ZnO and SnO2 layers through atomic layer deposition (ALD). An additional 60-nm-thick Al layer between the source and drain serves as a metal-capping layer and forms a conductive region. Rich in free electrons and located in the back channel away from the gate insulator, the conductive region leads to a primary ON-current path, thereby improving electrical characteristics and stability. Thus, the Al-capped ZTO TFT with a capping length of 200~\mu m exhibits decent performance with a saturation mobility of 16.89 cm2/V \cdot s, a threshold voltage of 0.81 V, a subthreshold swing (SS) of 0.59 V/dec, and an ON/OFF current ratio over 107. Moreover, it exhibits a minimal threshold voltage shift of 0.9 V in the positive bias test and −0.14 V in the negative bias stress test. On ALD-based ZTO TFTs, these results demonstrate the applicability of the Al-capping method, which successfully overcomes the tradeoff between mobility improvement and bias stability.