Triple-Stack ZnO/AlZnO/YZnO Heterojunction Oxide Thin-Film Transistors by Spray Pyrolysis for High Mobility and Excellent Stability
Jewel Kumer Saha, Mohammad Masum Billah, Jin Jang
Abstract
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.