Impact of ambient moisture on gate controllability in ferroelectric-gate field-effect transistors with bottom-gate geometry

Ferroelectric-gate field-effect transistors (FeFETs) with a bottom-gate geometry consisting of a ferroelectric HfO2 gate and an oxide channel have been intensively studied in recent years. However, there has been no detailed investigation into the impact of atmospheric exposure on device performance, even though the channel is often exposed to ambient air for process simplification, especially at the research stage. In this study, the ambient stability of an indium tin oxide channel FeFET with a ferroelectric Ce-HfO2 bottom gate was investigated. We found that ambient degradation of the gate controllability was caused by an increase in physisorbed water in the device owing to the intrusion of moisture. Mobile ions, such as H+, which can easily move through a network of hydrogen bonds formed by adjacent physisorbed water, may compensate for ferroelectric polarization. Finally, we demonstrated that the observed degradation can be managed effectively without compromising the original device characteristics using Al2O3 passivation gently formed via plasma-free deposition.