Enhanced Tunneling Electroresistance in Ferroelectric Tunnel Junctions due to the Reversible Metallization of the Barrier

Realizing a large tunneling electroresistance (TER) effect is crucial for device application of ferroelectric tunnel junctions (FTJs). FTJs are typically composed of a thin ferroelectric layer sandwiched by two metallic electrodes, where TER generally results from the dependence of the effective tunneling barrier height on the ferroelectric polarization. Since the resistance depends exponentially not only on barrier height but also on barrier width, TER is expected to be greatly enhanced when one of the electrodes is a semiconductor where the depletion region near the interface can be controlled via ferroelectric polarization. To explore this possibility, we perform studies of ${\mathrm{SrRuO}}_3/{\mathrm{BaTiO}}_3/n-{\mathrm{SrTiO}}_3$ FTJs, where $n\text{−}{\mathrm{SrTiO}}_3$ is an electron doped ${\mathrm{SrTiO}}_3$ electrode, using first-principles density functional theory. Our studies reveal that, in addition to modulation of the depletion region in $n\text{−}{\mathrm{SrTiO}}_3$, the ${\mathrm{BaTiO}}_3$ barrier layer becomes conducting near the interface for polarization pointing into $n\text{−}{\mathrm{SrTiO}}_3$, leading to dramatic enhancement of TER. The effect is controlled by the band alignment between the semiconductor and the ferroelectric insulator and opens the way for experimental realization of enhanced TER in FTJs through the choice of a semiconducting electrode and interface engineering.

Figure 1

Relative $z$ displacement between metal cation and anion (oxygen) in each atomic layer of the ${\mathrm{SrRuO}}_3/{\mathrm{BaTiO}}_3/n\text{−}{\mathrm{SrTiO}}_3$ supercell. Filled symbols correspond to $B{\mathrm{O}}_2$ layers ($B=\mathrm{Ru}$ or Ti) and open symbols correspond to $A\mathrm{O}$ layers ($A=\mathrm{Sr}$ or Ba). Red and blue curves correspond to polarization pointing into or away from the $n\text{−}{\mathrm{SrTiO}}_3$ electrode, respectively. Black curves correspond to the polarization extracted from the electrostatic model described in the text.

Figure 2

Local density of states on the ${\mathrm{TiO}}_2$ layers for ferroelectric polarization (a) into and (b) away from the ${\mathrm{BaTiO}}_3/n\text{−}{\mathrm{SrTiO}}_3$ interface. Circles indicate the position of the conduction band minimum, determined from the LDOS as described in the text.

Figure 3

Electrostatic model profile of the CMB in ${\mathrm{BaTiO}}_3$ and $n\text{−}{\mathrm{SrTiO}}_3$ for doping concentration (a) $n_0=0.09\mathrm{e}/\mathrm{f}.\mathrm{u}.$ (b) $0.03\mathrm{e}/\mathrm{f}.\mathrm{u}.$ and (c) $0.25\mathrm{e}/\mathrm{f}.\mathrm{u}.$ Red and blue curves correspond to polarization pointing into and away from $n\text{−}{\mathrm{SrTiO}}_3$, respectively, with the parameters provided in the text. The points in (a) are the CBM extracted from the LDOS as described in the text. Dashed curves in (c) include an additional intrinsic potential step $\mathrm{\Delta }V=0.5\mathrm{eV}$ across the ${\mathrm{BaTiO}}_3/n\text{−}{\mathrm{SrTiO}}_3$ interface.

Figure 4

(a) Fermi surface of $n\text{−}{\mathrm{SrTiO}}_3$ and (b) its projection along the $z$ direction. (c),(d) ${\mathbf{k}}_{||}$-resolved transmission through the tunnel junction with polarization pointing into (c) and away (d) from $n\text{−}{\mathrm{SrTiO}}_3$.