Resonant band engineering of ferroelectric tunnel junctions

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.