Giant Electroresistance in Ferroelectric Tunnel Junctions

The interplay between the electron transport in metal-ferroelectric-metal junctions with ultrathin ferroelectric barriers and the polarization state of a barrier is investigated. Using a model which takes into account screening of polarization charges in metallic electrodes and direct quantum tunneling across a ferroelectric barrier, we calculate the change in the tunneling conductance associated with the polarization switching. We find the conductance change of a few orders of magnitude for metallic electrodes with significantly different screening lengths. This giant electroresistance effect is the consequence of a different potential profile seen by transport electrons for the two opposite polarization orientations.

Figure 1

Electrostatics of a ${\mathrm{M}}_1\mathrm{\text{−}}\mathrm{FE}\mathrm{\text{−}}{\mathrm{M}}_2$ junction: (a) charge distribution and (b) the respective electrostatic potential profile (solid line). It is assumed that metal 1 (${\mathrm{M}}_1$) and metal 2 (${\mathrm{M}}_2$) electrodes have different screening lengths (${\delta }_1>{\delta }_2$) which lead to the asymmetry in the potential profile. The dashed line in (b) shows the potential when the polarization $\mathbf{P}$ in the ferroelectric is switched, resulting in the reversal of the depolarizing field $\mathbf{E}$.

Figure 2

Schematic representation of the potential profile $V(z)$ in a ${\mathrm{M}}_1\mathrm{\text{−}}\mathrm{FE}\mathrm{\text{−}}{\mathrm{M}}_2$ junction for polarization pointing to the left (a) and for polarization pointing to the right (b), assuming that ${\delta }_1>{\delta }_2$. The dashed lines show the average potential seen by transport electrons tunneling across the ferroelectric barrier. The horizontal solid line denotes the Fermi energy, $E_F$.

Figure 3

Calculated results as a function of screening length, ${\delta }_1$, in the metal 1 electrode for $P=20\mu \mathrm{C}/{\mathrm{cm}}^2$ and $d=2\mathrm{nm}$: (a) amplitudes of the potential at the ${\mathrm{M}}_1\mathrm{\text{−}}\mathrm{FE}$ (solid line) and ${\mathrm{M}}_2\mathrm{\text{−}}\mathrm{FE}$ (dashed line) interfaces; (b) conductance per unit area for polarization oriented to the right, $G_R/A$ (solid line) and for polarization oriented to the left, $G_L/A$ (dashed line); (c) conductance change, $G_R/G_L$, associated with the polarization switching in the ferroelectric barrier. The vertical dotted line indicates the value of ${\delta }_1={\delta }_2$ at which no asymmetry in the potential profile and, hence, no conductance difference is predicted.

Figure 4

GER ratio, $G_R/G_L$, as a function of ferroelectric barrier thickness $d$ for different values of polarization (a) and as a function of polarization $P$ for different barrier thicknesses (b). ${\delta }_1=6\AA$.