Electroresistance Effect in Ferroelectric Tunnel Junctions with Symmetric Electrodes

Understanding the effects that govern electronic transport in ferroelectric tunnel junctions (FTJs) is of vital importance to improve the efficiency of devices such as ferroelectric memories with nondestructive readout. However, our current knowledge (typically based on simple semiempirical models or first-principles calculations restricted to the limit of zero bias) remains partial, which may hinder the development of more efficient systems. For example, nowadays it is commonly believed that the tunnel electroresistance (TER) effect exploited in such devices mandatorily requires, to be sizable, the use of two different electrodes, with related potential drawbacks concerning retention time, switching, and polarization imprint. In contrast, here we demonstrate at the first-principles level that large TER values of about 200% can be achieved under finite bias in a prototypical FTJ with symmetric electrodes. Our atomistic approach allows us to quantify the contribution of different microscopic mechanisms to the electroresistance, revealing the dominant role of the inverse piezoelectric response of the ferroelectric. On the basis of our analysis, we provide a critical discussion of the semiempirical models traditionally used to describe FTJs.