Designing functional ferroelectric interfaces from first-principles: dipoles and band bending at oxide heterojunctions

The fundamental phenomena at ferroelectric interfaces have been the subject of thorough theoretical and computational studies due to their usefulness in a large variety of emergent electronic devices, solar cells and catalysts. Ferroelectricity determines interface band-bending and shifts in electron energies, which can be beneficial or detrimental to device performance. However, the underlying mechanisms are still the subject of debate and investigation, as a deeper understanding of the electrochemistry is required to develop bona fide design principles for functional ferroelectric surfaces and interfaces. Here, using first principles calculations within the GGA + U formalism, we investigate the problem of band alignment in non-defective, asymmetric SrRuO 3 /PbTiO 3 /SrRuO 3 capacitors with ultra-thin ferroelectric layers. The effects of the dielectric size on the polar distortion stability and interface-specific properties are analyzed. It is shown that the critical size of the dielectric for polarization switching is ≈ 2 nm (5 PbTiO 3 u.c.). Below this limit there is no bulk-like region in the dielectric, the space charge accumulated at interfaces leads to the presence of gap states in the whole PbTiO 3 layer and ferroelectricity vanishes. We draw the band alignment diagrams as given by the band line-up and band structure terms, as well as by taking Ti 3 s semi-core states as reference. In the ferroelectric structures, both approaches predict a strong effect of band-bending on the type of contact, Schottky or Ohmic, at the asymmetric interfaces. The effect of interface states on the interface dipole amplitude and band alignment is discussed.