Oxygen-vacancy enhanced tunnel electroresistance in LaNiO₃/BaTiO₃/LaNiO₃ ferroelectric tunnel junctions

Oxygen vacancies (OVs) usually exist in perovskite oxides in ferroelectric tunnel junctions (FTJs), which significantly influence electron transport properties of FTJ. However, the role of OVs is currently not well understood since the OVs concentration is difficult to detect in experiments or to simulate using traditional first-principles methods. Here, using the density functional theory combined with nonequilibrium Green's function and coherent potential approximation (NECPA-DFT), we investigate electron transport properties of the LaNiO₃/BaTiO₃/LaNiO₃ FTJ, which has a low concentration OVs in the left LaNiO₃/BaTiO₃ interface. The tunnel barrier height monotonously decreases with the increase in the OVs concentration for the rightward polarization in BaTiO₃, leading to an increased electron tunneling coefficient. In contrast, for a leftward polarization, the barrier height only slightly decreases with the increasing OVs concentration, leading to a nearly invariant electron tunneling coefficient. The tunnel electroresistance ratio, therefore, increases monotonously with the OVs concentration and reaches to 5898% for a OVs concentration of 9%. Our results show that OVs play a critical role in determining electron transport properties of an FTJ as well as provide an alternative avenue to realize a natural asymmetric FTJ to improve its performance.