Piezoresponse of ferroelectric films in ferroionic states: Time and voltage dynamics

Abstract

The interplay between electrochemical surface charges and bulk ferroelectricity in thin films gives rise to a continuum of coupled ferro-ionic states. These states are exquisitely sensitive to chemical and electric conditions at the surfaces, applied voltage, and oxygen pressure. Using the analytical approach combining the Ginzburg-Landau-Devonshire description of the ferroelectricity with Langmuir adsorption isotherm for the ions at the film surface, we have studied the temperature-, time-and field-dependent polarization changes and electromechanical response of the ferro-ionic states. The responses are found to be inseparable in thermodynamic equilibrium and at low frequencies of applied voltage. The states become separable in high frequency dynamic mode due to the several orders of magnitude difference in the relaxation times of ferroelectric polarization and surface ions charge density. These studies provide an insight into dynamic behavior of nanoscale ferroelectrics with open surface exposed to different kinds of electrochemically active gaseous surrounding.* Corresponding author. E-mail: sergei2@ornl.gov (S.V.K.) 1 Within 70 years, the development of perovskite ferroelectric applications has evolved from capacitor radio-ceramics bulk units [1,2], multifunctional piezoelectric thin sheet transducers [3,4] and pyroelectric thin layer detectors [5] to thin film ferroelectric memories [6,7]. From early 1970-ies to present there are growing demands to ferroelectric structures in chemotronics [ 8 ] and electrochemical sensorics [9] for design the new classes of sensing and converting devices. The first experiments [10,11, 12,13] have proved the interaction of the ferroelectric barium titanate, BaTiO 3 , surface with oxygen, hydrogen and methyl-alcohol molecules resulted in the significant influence on polarization reversal, dielectric permittivity and pyroelectric currents. Electric coupling between electrochemical surface charges and bulk ferroelectricity gives rise to a chemical switching [14,15,16] and continuum of coupled ferro-ionic states [17,18] in ferroelectric thin films. These states are exquisitely sensitive to chemical and electric conditions at the film surfaces [14][15][16][17][18]. The analytical description of coupled ferro-ionic states was developed via the combination of Ginzburg-LandauDevonshire description of the film ferroelectric properties [19,20,21] Here, we explore the dynamic behavior of the ferro-ionic states in thin films, representing the polarization and electromechanical response as measured by the piezoresponse force microscopy (PFM) [23,24]. We demonstrate that the piezoelectric responses in ferroelectric, ferro-ionic and nonferroelectric electret-like ionic states are inseparable at low frequencies of applied voltage. The states become separable in high frequency dynamic regime due to the several orders of magnitude difference in the relaxation times of ferroelectric polarization and surface ions. This effect calls up with pyroelectric response under slow enough and fa...

Control of polarization reversal temperature behavior by surface screening in thin ferroelectric films

“…shifts along ϕ-axis [40,41,42]. In particular the dependence ( ) ϕ σ 0 is shifted towards positive potentials ϕ for the case (see e.g.…”

“…Mathematical statement of the problem is listed in Refs. [40,41,42], as well as in Appendix A. The polarization components of uniaxial ferroelectric film depend on the inner electric field E as…”

“…Note that the abrupt step-like dependence is left-shifted with respect to 0 = ψ for , symmetric for and right-shifted for (see Refs. [40,41,42]).…”

“…and screening charge density can be described via the coupled nonlinear algebraic equations derived in Refs. [40,41].…”

“…are related with the ( ) asymmetry of the screening conditions realized in the case , polarization reversal in the ferroelectric film is facilitated for one polarity and is difficult for another polarity of applied voltage U, which leads to the shift of and their shape deformation[40,41,42]. The ferroelectric state is not realized in ultrathin films because of its suppression by finite size effect at K due to the ionic electric field at , at that the hysteresis region corresponds to the negative polarity of applied voltage.…”