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

Abstract

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 3s 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.

Interface and surface stabilization of the polarization in ferroelectric thin films

“…For systems with fewer than 7 u.c. the paraelectric state is the most stable, showing the existence of a critical thickness for ferroelectricity for PTO on SRO, in good agreement with previous reports on the same thin film structure ( 34 , 35 ). Below this critical thickness, a metal SRO buffer at a single interface provides insufficient screening, and other interface- or surface-related effects are needed to stabilize ultrathin ferroelectricity.…”

Local polarization and valence distribution in LaNiO3/LaMnO3 heterostructures

“…Quantitatively, this estimation of the electric field intensity involves an uncertainty of ∼50 %, because the lowest energy B u modes range from 76 to 90 cm −1 , and the Born effective charge of the rare-earth ion R 3+ for RMnO 3 59-61 ranges from 3.3+ to 3.8+ instead of 3+. The obtained magnitude of the electric field is similar to the values reported for various perovskite heterostructures based on tunneling transport measurement 62 , crosssectional scanning tunneling microscopy 13 , and first-principles calculations 11,12 . The spatial distribution of Mn 4+ and that of the electric field are very similar to those of the metalsemiconductor junction except for the very small spatial scale.…”

“…Because of the high density of the electric charge in oxide materials, the spatial distribution of the potential gradient in oxide heterostructures is often limited to a few nanometers, which results in strong electric field. According to theoretical calculations 11,12 and cross-sectional measurements 13 , some transition metal oxide interfaces have a potential gradient of ∼1 GV/m. The spatial distribution and magnitude of the electric field differ by several orders of magnitude from those in classical semiconductors, in which a spatial distribution of 10 nm to 100 nm and local electric field of ∼ MV/m are observed.…”

Ferroelectricity Modulates Polaronic Coupling at Multiferroic Interfaces

“…This thickness ensures that the FE property remains stable, because the FE-polarization states tend to merge starting from a thickness of 5 u.c. 43 . The z-coordinates were allowed to relax until the Hellman-Feynman forces on each atom were less than 2.5 meV/Å.…”