  Based on first-principles and model calculations we investigate the effect of
polar interfaces on the ferroelectric stability of thin-film ferroelectrics. As
a representative model, we consider a TiO2-terminated BaTiO3 film with LaO
monolayers at the two interfaces that serve as doping layers. We find that the
polar interfaces create an intrinsic electric field that is screened by the
electron charge leaking into the BaTiO3 layer. The amount of the leaking charge
is controlled by the boundary conditions which are different for three
heterostructures considered, namely Vacuum/LaO/BaTiO3/LaO, LaO/BaTiO3, and
SrRuO3/LaO/BaTiO3/LaO. The intrinsic electric field forces ionic displacements
in BaTiO3 to produce the electric polarization directed into the interior of
the BaTiO3 layer. This creates a ferroelectric dead layer near the interfaces
that is non-switchable and thus detrimental to ferroelectricity. Our
first-principles and model calculations demonstrate that the effect is stronger
for a larger effective ionic charge at the interface and longer screening
length due to a stronger intrinsic electric field that penetrates deeper into
the ferroelectric. The predicted mechanism for a ferroelectric dead layer at
the interface controls the critical thickness for ferroelectricity in systems
with polar interfaces.
