Triple-Stack ZnO/AlZnO/YZnO Heterojunction Oxide Thin-Film Transistors by Spray Pyrolysis for High Mobility and Excellent Stability.

We demonstrate a high mobility, triple-stack ZnO/AlZnO/YZnO heterojunction thin-film transistor (TFT) using the semiconductors deposited by spray pyrolysis at 350 °C on an Al2O3 gate insulator. A thin layer (5 nm) of AlZnO on the top of ZnO used as an active layer of an inverted coplanar-structured TFT increases the field-effect mobility (μFE) from 42.56 to 82.7 cm2 V-1 s-1. An additional 5 nm thick YZnO on the top of the ZnO/AlZnO TFT improves the electrical stability by reducing the defects in the bulk ZnO, AlZnO, and at the interface AlOx/ZnO. The ZnO-based materials show a nanocrystalline structure with the grain size less than 20 nm. The triple-stack oxide TFT shows a μFE of 71.3 cm2 V-1 s-1 with a threshold voltage (VTH) of 2.85 V. The hysteresis voltage for pristine ZnO, ZnO/AlZnO, and ZnO/AlZnO/YZnO TFTs is 0.52, 0.24, and 0.02 V, respectively. The ZnO/AlZnO/YZnO TFT shows a negligible VTH shift under temperature bias stress for 3600 s at 60 °C and excellent environmental stability over a few months, which is due to the presence of stronger Y-O and Al-O bonds in the back channel. The threshold voltage shift under positive bias temperature stress for pristine ZnO, ZnO/AlZnO, and ZnO/AlZnO/YZnO TFTs is 0.78, 0.40, and 0.15 V, respectively. Compared to the pristine ZnO TFT, the ZnO/AlZnO/YZnO TFT shows better environmental and bias stabilities with improved hysteresis. The experimental data of ZnO/AlZnO and ZnO/AlZnO/YZnO TFTs can be fitted by technology computer-aided design (TCAD) simulation using the density of states model of the oxide semiconductors. From the TCAD simulation, it is found that a 2D-like electron gas is formed at the narrow AlZnO layer between ZnO and YZnO.