Subsurface State Bilayer in Tetragonal Ferroelectric BaTiO₃

Abstract The interface is an issue in the ferroelectric field effect transistor. In this work, the study presents a combined characterization of the subsurface structure and the electric polarization of (001)‐oriented and poled BaTiO 3 crystal sheets. The first layer is found from the last ≈1.22 Å of the crystal to its Ti─O surface, which holds the empty or partly‐filled pseudo excited states of high energy levels, to set up the potential well and trap compensation charges. The second layer from ≈2.45 to ≈1.75 Å beneath the surface, accommodates the distorted lattice, and particularly, the pairs of small polarons and O vacancies at the pseudo ground state and the pseudo excited states of low energy levels. As a concreteness of the depolarization field, the second layer generates the subsurface polaron‐type polarization of reverse ferroelectricity. Between these two layers, there is a gap of states with a thickness ≈0.53 Å. The state bilayer demonstrates a method to quantify the interface, proves the parasitic capacitance, validates the parallel‐plate capacitor configuration, and gives a telltale sign to the enhanced ferroelectric polarization, the surface proximity property, the flexoelectric effect, the insulating failure, and the photocatalytic phenomena.