Strong Surface‐Termination Effect on Electroresistance in Ferroelectric Tunnel Junctions

Tunnel electroresistance in ferroelectric tunnel junctions (FTJs) has attracted considerable interest, because of a promising application to nonvolatile memories. Development of ferroelectric thin‐film devices requires atomic‐scale band‐structure engineering based on depolarization‐field effects at interfaces. By using FTJs consisting of ultrathin layers of the prototypical ferroelectric BaTiO 3 , it is demonstrated that the surface termination of the ferroelectric in contact with a simple‐metal electrode critically affects properties of electroresistance. BaTiO 3 barrier‐layers with TiO 2 or BaO terminations show opposing relationships between the polarization direction and the resistance state. The resistance‐switching ratio in the junctions can be remarkably enhanced up to 10 5 % at room temperature, by artificially controlling the fraction of BaO termination. These results are explained in terms of the termination dependence of the depolarization field that is generated by a dead layer and imperfect charge screening. The findings on the mechanism of tunnel electroresistance should lead to performance improvements in the devices based on nanoscale ferroelectrics.