Polarization In Ultrathin Films Research Articles

Interface and surface stabilization of the polarization in ferroelectric thin films

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.

Nanomechanics of flexoelectric switching

We examine the phenomenon of flexoelectric switching of polarization in ultrathin films of barium titanate induced by a tip of an atomic force microscope (AFM). The spatial distribution of the tip-induced flexoelectricity is computationally modeled both for perpendicular mechanical load (point measurements) and for sliding load (scanning measurements), and compared with experiments. We find that (i) perpendicular load does not lead to stable ferroelectric switching in contrast to the load applied in the sliding contact load regime, due to nontrivial differences between the strain distributions in both regimes: ferroelectric switching for the perpendicular load mode is impaired by a strain gradient inversion layer immediately underneath the AFM tip; while for the sliding load regime, domain inversion is unimpaired within a greater material volume subjected to larger values of the mechanically induced electric field that includes the region behind the sliding tip; (ii) beyond a relatively small value of an applied force, increasing mechanical pressure does not increase the flexoelectric field inside the film, but results instead in a growing volume of the region subjected to such field that aids domain nucleation processes; and (iii) the flexoelectric coefficients of the films are of the order of few nC/m, which is much smaller than for bulk $\mathrm{BaTi}{\mathrm{O}}_{3}$ ceramics, indicating that there is a ``flexoelectric size effect'' that mirrors the ferroelectric one.

Phase field simulations of the poling behavior of BaTiO3 nano-scale thin films with SrRuO3 and Au electrodes

Abstract A continuum phase field model is used to investigate the poling behavior of BaTiO 3 (BTO) thin films with thicknesses ranging from 1 nm–80 nm. The finite element simulations focus on the influence of the electrode material on the electrical hysteresis curves. The 15 nm thick electrodes consist of either SrRuO 3 (SRO) or Au and are explicitly included in the simulations. Experimental evidence suggests that there is a depolarization field in SRO-electroded BTO thin films. This depolarization field, which is due to an imperfect charge compensation by the electrodes, causes a suppression of ferroelectricity for film thicknesses of a few unit cells. The depolarization field and its effect on the electrical behavior is captured by a Robin type boundary condition for the order parameter of the phase field model. This boundary condition is applied at the interface between the thin film and the top and bottom electrode. The simulation results show that the overall poling behavior strongly depends on the lattice parameter of the electrode material. Films with SRO electrodes, which have a lattice parameter smaller than the a -parameter of tetragonal BTO, observe a switchable polarization larger than the spontaneous polarization. If Au is used (which has lattice parameter larger than the a -parameter of tetragonal BTO) then switching is inhibited for films which are less than 20 nm thick. It is also shown that the Robin boundary condition induces a depolarization field which has the effect of dramatically reducing the switchable polarization in ultrathin films.

Space charge effect in ultrathin ferroelectric films

Space charge influence on the depolarization field becomes pronounced in nano-scale ferroelectric films in static state. We have studied theoretically the ferroelectric, dielectric, and piezoelectric properties of ultrathin ferroelectric films with the consideration of influence from space charges, surface, incomplete screening, and misfit strain. It is found that space charges accumulated near the surface of the film can modify the depolarization field, which results an enhancement of the polarization in ultrathin films and a persisting polarization even below the critical thickness. The singularities of the dielectric constant and piezoelectric coefficient were all being rounded near the critical thickness.