Interface dipole effect on thin film ferroelectric stability: First-principles and phenomenological modeling

Utilization of the switchable spontaneous polarization of ferroelectric materials offers a promising avenue for the future of nanoelectronic memories and logic devices provided that nanoscale metal-ferroelectric-metal heterostructures can be engineered to maintain a bi-stable polarization switchable by an applied electric field. The most challenging aspect of this approach is to overcome the deleterious interface effects which tend to render ferroelectric polarization either unstable or unswitchable and which become ever more important as ferroelectric materials are produced thinner and thinner. Here we use first-principles density functional calculations and phenomenological modeling to demonstrate that a BaO/RuO₂ interface termination sequence in SrRuO₃/BaTiO₃/SrRuO₃ epitaxial heterostructures grown on SrTiO₃ can lead to a nonswitchable polarization state for thin BaTiO₃ films due to a fixed interface dipole. The unfavorable interface dipole at the BaO/RuO₂ interface leads to a strong preference for one polarization state and, in thin film structures, leads to instability of the second state below a certain critical thickness, thereby making the polarization unswitchable. We analyze the contribution of this interface dipole to the energetic stability of these heterostructures. Furthermore, we propose and demonstrate that this unfavorable interface dipole effect can be alleviated by deposition of a thin layer of SrTiO₃ at the BaO/RuO₂ terminated interface. Our first-principles and phenomenological modeling predict that the associated change of the interface termination sequence to SrO/TiO₂ on both sides of the heterostructure leads to a restoration of bi-stability with a smaller critical thickness, along with an enhancement of the barrier for polarization reversal. These results demonstrate that interface engineering is a viable approach to enhance ferroelectric properties at the nanoscale.