Ji Young Jo

Domain Switching Kinetics in Disordered Ferroelectric Thin Films

We investigated domain kinetics by measuring the polarization switching behaviors of (111)-preferred polycrystalline Pb(Zr,Ti)O3 films, which are widely used in ferroelectric memories. Their switching behaviors at various electric fields and temperatures could be explained by assuming the Lorentzian distribution of logarithmic domain-switching times. We suggested that the local field variation due to dipole defects at domain pinning sites could explain the Lorentzian distribution.

Nonlinear Dynamics of Domain-Wall Propagation in Epitaxial Ferroelectric Thin Films

We investigated the ferroelectric domain-wall propagation in epitaxial Pb(Zr,Ti)O3 thin film over a wide temperature range (3-300 K). We measured the domain-wall velocity under various electric fields and found that the velocity data is strongly nonlinear with electric fields, especially at low temperature. We found that, as one of surface growth issues, our domain-wall velocity data from ferroelectric epitaxial film could be classified into the creep, depinning, and flow regimes due to competition between disorder and elasticity. The measured values of velocity and dynamical exponents indicate that the ferroelectric domain walls in the epitaxial films are fractal and pinned by a disorder-induced local field.

ac Dynamics of Ferroelectric Domains from an Investigation of the Frequency Dependence of Hysteresis Loops

We investigated nonequilibrium domain wall dynamics under an ac field by measuring the hystere- sis loops of epitaxial ferroelectric capacitors at various frequencies and temperatures. Polarization switching is induced mostly by thermally activated creep motion at lower frequencies, and by vis- cous ow motion at higher frequencies. The dynamic crossover between the creep and ow regimes unveils two frequency-dependent scaling regions of hysteresis loops. Based on these findings, we constructed a dynamic phase diagram for hysteretic ferroelectric domain dynamics in the presence of ac fields.

Domain wall motion in epitaxial Pb(Zr,Ti)O3 capacitors investigated by modified piezoresponse force microscopy

We investigated the time-dependent domain wall motion of epitaxial PbZr0.2Ti0.8O3 capacitors 100nm thick using modified piezoresponse force microscopy (PFM). We obtained successive domain evolution images reliably by combining the PFM with switching current measurements. We observed that domain wall speed (v) decreases with increases in domain size. We also observed that the average value of v, obtained under applied electric field (Eapp), showed creep behavior, i.e., ⟨v⟩∼exp[−(E0∕Eapp)μ] with an exponent μ of 0.9±0.1 and an activation field E0 of about 700kV∕cm.

Enhanced thermoelectric performance of PEDOT:PSS/PANI–CSA polymer multilayer structures

A layer-by-layer deposition of two conducting polymers, each layer of which is a few tenths of nanometer thick, has been successfully performed to enhance the thermoelectric power factor of organic thin films, which are critical components of flexible thermoelectric energy harvesting devices. The multilayer films were deposited via multiple solution processes, which exhibit enhanced electrical conductivity without any significant degradation of the Seebeck coefficient, in contrast to a coupling behavior between the electrical conductivity and the Seebeck coefficient in bulk materials. The electrical conductivity and power factor—proportional to the electrical conductivity—of 5(PEDOT:PSS/PANI–CSA) multilayer films are 1.3 and 2 times higher than those of a single PEDOT:PSS layer. Transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) reveal distinct interfaces through which an enhanced electrical conductivity and power factor have been achieved in our multilayer films. From the TEM, EELS, and Raman analyses, a model for the enhancement of the electrical conductivity has been proposed. The enhancement of electrical conductivity occurs via stretching of PEDOT and PANI chains and hole diffusion from the PANI–CSA layer to the PEDOT:PSS layer. The band alignment in the multilayer structure not only enhances electrical conductivity but also maintains the Seebeck coefficient at an optimum value. Our study suggests that the layer-by-layer deposition of polymer thin films is a promising technique for manipulating the thermoelectric properties of each polymer component to enhance thermoelectric performance.

FERROELECTRIC SWITCHING DYNAMICS AND PULSE-SWITCHING POLARIZATION MEASUREMENTS

ABSTRACT Pulse switching properties were studied in polycrystalline Pb(Zr,Ti)O3 capacitors to check a recently proposed nucleation limited switching (NLS) model, where very wide switching was observed. The backing-switching responses (P b) at the end of writing pulses were studied to understand the role of writing pulse together with switching polarization (Δ P). The P b+Δ P, with high enough write voltage, is almost independent of writing pulse width (t w), even though Δ P is observed to increase with increasing t w. From these results, it is concluded that the NLS dynamics are attributed to the switching and poling processes with external fields around coercive fields.

Structural consequences of ferroelectric nanolithography.

Domains of remnant polarization can be written into ferroelectrics with nanoscale precision using scanning probe nanolithography techniques such as piezoresponse force microscopy (PFM). Understanding the structural effects accompanying this process has been challenging due to the lack of appropriate structural characterization tools. Synchrotron X-ray nanodiffraction provides images of the domain structure written by PFM into an epitaxial Pb(Zr,Ti)O(3) thin film and simultaneously reveals structural effects arising from the writing process. A coherent scattering simulation including the superposition of the beams simultaneously diffracted by multiple mosaic blocks provides an excellent fit to the observed diffraction patterns. Domains in which the polarization is reversed from the as-grown state have a strain of up to 0.1% representing the piezoelectric response to unscreened surface charges. An additional X-ray microdiffraction study of the photon-energy dependence of the difference in diffracted intensity between opposite polarization states shows that this contrast has a crystallographic origin. The sign and magnitude of the intensity contrast between domains of opposite polarization are consistent with the polarization expected from PFM images and with the writing of domains through the entire thickness of the ferroelectric layer. The strain induced by writing provides a significant additional contribution to the increased free energy of the written domain state with respect to a uniformly polarized state.

Piezoelectricity in the Dielectric Component of Nanoscale Dielectric-Ferroelectric Superlattices

The origin of the functional properties of complex oxide superlattices can be resolved using time-resolved synchrotron x-ray diffraction into contributions from the component layers making up the repeating unit. The CaTiO3 layers of a CaTiO3/BaTiO3 superlattice have a piezoelectric response to an applied electric field, consistent with a large continuous polarization throughout the superlattice. The overall piezoelectric coefficient at large strains, 54  pm/V, agrees with first-principles predictions in which a tetragonal symmetry is imposed on the superlattice by the SrTiO3 substrate.

Composition-dependent polarization switching behaviors of (111)-preferred polycrystalline Pb(ZrxTi1−x)O3 thin films

We investigated the time-dependent polarization switching behaviors of (111)-preferred polycrystalline Pb(ZrxTi1−x)O3 thin films with various Zr concentrations. We could explain all the polarization switching behaviors well by assuming Lorentzian distributions in the logarithmic polarization switching time [refer to J. Y. Jo et al., Phys. Rev. Lett. 99, 267602 (2007)]. Based on this analysis, we found that the Zr ion substitution for Ti ions would induce broad distributions in the local field due to defect dipoles, which makes the ferroelectric domain switching occur more easily.

Nonlinearity in the High-Electric-Field Piezoelectricity of Epitaxial BiFeO3 on SrTiO3

The piezoelectricity of a multiferroic BiFeO3 thin film deviates from its low-field linear response in electric fields higher than 150 MV/m. Time-resolved synchrotron x-ray microdiffraction reveals a low-field piezoelectric coefficient of 55 pm/V and a steeper increase in strain at higher fields, with an effective piezoelectric coefficient of 86 pm/V. The strain reaches 2% at 281 MV/m, a factor of 1.3 higher than expected based on an extrapolation from low fields. The peak intensity of the BiFeO3 (002) Bragg reflection decreases throughout the high-electric-field regime, accompanied by increased diffuse scattering, consistent with lattice softening lattice near a field-induced phase transition.