Increasing energy storage capabilities of space-charge dominated ferroelectric thin films using interlayer coupling

In our previous work (W. Zhang et al., Space-charge dominated epitaxial BaTiO3 heterostructures, Acta Mater. 85 (2015) 207–215), it was demonstrated that a space charge dominated BaTiO3 thin film can have much improved energy storage characteristics when compared with a regular insulating film of ferroelectric BaTiO3. However, the improved recoverable electric energy density (discharged energy per unit volume), Wre, is still constrained by the maximum/saturated polarization (Ps) of the BaTiO3 thin film. Here we propose a novel method to further improve Wre in ferroelectric thin film heterostructures by using interlayer coupling in an epitaxial ferroelectric bilayer. In our model structure, a ferroelectric BiFeO3 layer with a large remnant polarization (Pr ∼70 μC/cm2) was deposited in-situ on top of a BaTiO3/SrRuO3 heterostructure which shows a small remnant polarization (Pr ∼ 3 μC/cm2). The resulted bilayer structure showed a slim P-E hysteresis loop characterized by a small remnant polarization and a large saturated one (Ps ≫ Pr), which can be well explained by a competition between the effect of space charges (dominant at low field) and that of the interlayer charge coupling (dominant at high field). The large difference, i.e. Ps − Pr, increases the recoverable electric energy density by about 85% (from ∼28 J/cm3 to ∼51 J/cm3).