Performance manipulation of ferroelectric tunnel junctions via oxygen vacancies in barrier

Electronic and transport properties can be significantly influenced by oxygen vacancies (OVs) in ferroelectric tunnel junctions (FTJs). However, the effects of OVs with different types and locations in the ferroelectric barrier on the performance of FTJs remain unclear. Here, we reveal that the electronic structures and transport properties of the SrRuO3/BaTiO3/SrRuO3 FTJ are highly dependent on the types and locations of OVs in BaTiO3 barrier based on first-principles calculations (Vc type in BaO plane, with Ti-Vc-Ti chain along c axis, and Vab type in TiO2 a-b plane, located at the barrier/electrode interface or in the interior of the barrier). The Coulomb repulsion among the cations surrounding OV leads to the formation of local tail-to-tail domain walls (DWs). The tail-to-tail DWs can appear both near the interface and in the interior of BaTiO3 for Vc-type OVs, while they only form in the interior BaTiO3 for Vab-type OVs. Such tail-to-tail DWs alter the ferroelectricity of BaTiO3 and the local density of states in TiO2 layers. Consequently, the tunneling electroresistance (TER) effect is boosted in the presence of OVs and is sensitive to the types and locations of OVs, with the TER effect being larger when OVs are close to the interface than in the interior of the barrier. Our findings are significant for designing FTJs since OVs are generally unavoidable in ferroelectric oxides, and suggest the types and locations of OVs as a tuning knob to manipulate FTJs.