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 $\mathrm{SrRu}{\mathrm{O}}_3\text{/}\mathrm{BaTi}{\mathrm{O}}_3\text{/}\mathrm{SrRu}{\mathrm{O}}_3$ FTJ with a $\mathrm{BaSn}{\mathrm{O}}_3$ monolayer embedded in the $\mathrm{BaTi}{\mathrm{O}}_3$ barrier. We show that in such a composite-barrier FTJ, ferroelectric polarization of $\mathrm{BaTi}{\mathrm{O}}_3$ shifts the conduction-band minimum of the $\mathrm{BaSn}{\mathrm{O}}_3$ 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 applications.

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• Electric polarization
• Ferroelectricity
• Local density of states
• Transport phenomena
• Tunnel junctions
• Density functional calculations