Millionfold Resistance Change in Ferroelectric Tunnel Junctions Based on Nickelate Electrodes

When electrons tunnel through an ultrathin ferroelectric, large variations of the tunnel transmission can result from the switching of the ferroelectric polarization. High-quality ultrathin films of BiFeO3 with atomically flat terraces and single unit-cell steps on epitaxial electrodes of LaNiO3 have been fabricated. The films crystallize in the pure polymorph of BiFeO3 with giant tetragonality; they show tunnel transport characteristics up to ten unit cells and a clear ferroelectric signal from scanning probe techniques. Sub-micrometer solid-state tunnel junctions defined from these heterostructures are switched by nanosecond voltage pulses. A complete reversal of the ferroelectric polarization results in large variations of the junction resistance, but partial switching of the polarization favors the reversibility of the resistance switching. Finally, the electrical transport characteristics of fully patterned tunnel junctions are investigated in the 300 K–80 K temperature range. Owing to the low resistivity of LaNiO3 electrodes, the junctions display record tunnel electroresistance values of more than 106 at 80 K combined with weak temperature dependences as expected for electron tunneling transport. These results will motivate further work to trigger metal–insulator transitions by electric-field effects in ferroelectric tunnel junctions combined with strongly correlated nickelates.