Vacancy Defects in 2D Ferroelectric In2Se3 and the Conductivity Modulation by Polarization–Defect Coupling

α-In2Se3 is a promising two-dimensional (2D) ferroelectric semiconductor with unique phase transition behaviors and intrinsic n-type conductivity. However, the origin of this conductivity and the impact of defects on the phase transition remain unclear. In this study, we employed the WLZ method to calculate vacancies’ formation energy and ionization energy in monolayer α-In2Se3 and identified the defect-bound band edge states. Our results reveal a strong polarization–defect coupling effect, where the bottom-layer selenium vacancy drives intrinsic n-type conductivity in the sample with upward polarization while reversing the polarization-induced deep p-type defect. Furthermore, we demonstrate that a vacancy stabilizes the ferroelectric phase and reduces the phase transition rate to the paraelectric phase. Finally, we propose a defect-engineered ferroelectric field-effect transistor model that controls the resistance by leveraging the polarization–defect coupling effect. This work highlights the significant roles of vacancy defects in 2D α-In2Se3, offering strategies to design In2Se3 electronic devices at the nanoscale.