Finite-Bias Calculations of Perovskite Oxide Ferroelectric Tunnel Junctions using Multi-Space Density Functional Theory

Ferroelectric tunnel junctions (FTJs) have received considerable attention among all memory technologies as a solution to the statistical fluctuation in nanometer scales and the continued scaling-down requirement. In this work, based on the multi-space constrained-search density functional theory (MS-DFT) recently developed in our group [1-3], we study the finite-bias electronic structure and quantum transport characteristics of capacitor models composed of BaTiO₃ perovskite oxides sandwiched between the metallic SrRuO₃ electrodes. Based on the electric enthalpy concept, we calculate free energy profiles at different nonequilibrium bias conditions in a first-principles manner. Bias-dependent free energy plots then directly reveal the information regarding the competition between paraelectric and ferroelectric phases, polarization switching barrier height, and coercive voltage. The source of polarization switching will be analyzed by discriminating the electronic and ionic contributions and utilizing Hartree electrostatic potentials, Landauer residual dipoles, and depolarization fields. The results will be also compared with previous works obtained from equilibrium DFT calculations and their extensions. To our knowledge, our work represents the first study of nonequilibrium energetics in FTJs within the first-principles framework, demonstrating that MS-DFT is a promising tool for the future developments of energy and memory devices.