  We propose energy band engineering to enhance tunneling electroresistance
(TER) in ferroelectric tunnel junctions (FTJs). We predict that an ultrathin
dielectric layer with a smaller band gap, embedded into a ferroelectric barrier
layer, acts as a switch controlling high and low conductance states of an FTJ
depending on polarization orientation. Using first-principles modeling based on
density functional theory, we investigate this phenomenon for a prototypical
SrRuO3/BaTiO3/SrRuO3 FTJ with a BaSnO3 monolayer embedded in the BaTiO3
barrier. We show that in such a composite-barrier FTJ, ferroelectric
polarization of BaTiO3 shifts the conduction band minimum of the BaSnO3
monolayer above or below the Fermi energy depending on polarization
orientation. The resulting switching between direct and resonant tunneling
leads to a TER effect with a giant ON/OFF conductance ratio. The proposed
resonant band engineering of FTJs can serve as a viable tool to enhance their
performance useful for device application.
