Interface and surface stabilization of the polarization in ferroelectric thin films

Abstract

Ferroelectric perovskites present a switchable spontaneous polarization and are promising energy-efficient device components for digital information storage. Full control of the ferroelectric polarization in ultrathin films of ferroelectric perovskites needs to be achieved in order to apply this class of materials in modern devices. However, ferroelectricity itself is not well understood in this nanoscale form, where interface and surface effects become particularly relevant and where loss of net polarization is often observed. In this work, we show that the precise control of the structure of the top surface and bottom interface of the thin film is crucial toward this aim. We explore the properties of thin films of the prototypical ferroelectric lead titanate (PbTiO3) on a metallic strontium ruthenate (SrRuO3) buffer using a combination of computational (density functional theory) and experimental (optical second harmonic generation) methods. We find that the polarization direction and strength are influenced by chemical and electronic processes occurring at the epitaxial interface and at the surface. The polarization is particularly sensitive to adsorbates and to surface and interface defects. These results point to the possibility of controlling the polarization direction and magnitude by engineering specific interface and surface chemistries.

In-situ monitoring of interface proximity effects in ultrathin ferroelectrics

“…We next pinpoint the mechanism promoting pervasive downwards polarization at the top interface. We rule out the intrinsic chemistry of the PTO termination as a possible explanation because, according to ab initio calculations 22 , both the PbO and the TiO 2 top termination favor an upwards polarization. Our tests with other ferroelectric perovskites rather point to non-stoichiometry as the likely origin because the striking polarization evolution (ii-iii) was only observed for materials with A-site volatility, i.e., PTO, Pb [Zr x Ti 1−x ]O 3 (PZT), and BiFeO 3 (BFO), in contrast to BaTiO 3 (BTO) 4 (see Supplementary Note 3).…”

Piezoelectricity in Nominally Centrosymmetric Phases

“…Here, both flexoelectricity [49] and, theoretically, surface piezoelectricity [50][51][52][53] can generate strain that can potentially be picked in our measurements. Both are very weak effects in comparison to the piezoelectric effect on bulk crystals and ceramics [51,54,55].…”

“…Surface piezoelectricity: Surface piezoelectricity is theoretically predicted in all materials [50][51][52][53]. By symmetry, surface piezoelectricity can only manifest itself in experiments where the equivalence between opposite surfaces is broken, i.e.…”

Self-consistently derived sample permittivity in stabilization of ferroelectricity due to charge accumulated at interfaces

“…charge accumulation in outer electrodes sandwiching the material, or charge accumulated in contaminating layers; 2 (iii) an origin related to surface chemical reconstructions or to other quantum chemical phenomena related to the translational symmetry breaking at surfaces. 4,5 Evidence for this charge accumulation is brought mainly by photoelectron spectroscopy. 6,7 The usual approach was to consider that these charges are accumulated such as to compensate the depolarization field produced by fixed charges, which would elsewhere destroy the single domain state.…”