Role of ferroelectric polarization during growth of highly strained ferroelectric materials.

In ferroelectric thin films and superlattices, the polarization is intricately linked to crystal structure. Here we show that it can also play an important role in the growth process, influencing growth rates, relaxation mechanisms, electrical properties and domain structures. This is studied by focusing on the properties of BaTiO thin films grown on very thin layers of PbTiO using x-ray diffraction, piezoforce microscopy, electrical characterization and rapid in-situ x-ray diffraction reciprocal space maps during the growth using synchrotron radiation.

Domain alignment within ferroelectric/dielectric PbTiO/SrTiO superlattice nanostructures.

The ferroelectric domain pattern within lithographically defined PbTiO/SrTiO ferroelectric/dielectric heteroepitaxial superlattice nanostructures is strongly influenced by the edges of the structures. Synchrotron X-ray nanobeam diffraction reveals that the spontaneously formed 180° ferroelectric stripe domains exhibited by such superlattices adopt a configuration in rectangular nanostructures in which domain walls are aligned with long patterned edges. The angular distribution of X-ray diffuse scattering intensity from nanodomains indicates that domains are aligned within an angular range of approximately 20° with respect to the edges.

Photoinduced Domain Pattern Transformation in Ferroelectric-Dielectric Superlattices.

The nanodomain pattern in ferroelectric-dielectric superlattices transforms to a uniform polarization state under above-band-gap optical excitation. X-ray scattering reveals a disappearance of domain diffuse scattering and an expansion of the lattice. The reappearance of the domain pattern occurs over a period of seconds at room temperature, suggesting a transformation mechanism in which charge carriers in long-lived trap states screen the depolarization field.

Thermal Fluctuations of Ferroelectric Nanodomains in a Ferroelectric-Dielectric PbTiO_{3}/SrTiO_{3} Superlattice.

Ferroelectric-dielectric superlattices consisting of alternating layers of ferroelectric PbTiO_{3} and dielectric SrTiO_{3} exhibit a disordered striped nanodomain pattern, with characteristic length scales of 6 nm for the domain periodicity and 30 nm for the in-plane coherence of the domain pattern. Spatial disorder in the domain pattern gives rise to coherent hard x-ray scattering patterns exhibiting intensity speckles. We show here using variable-temperature Bragg-geometry x-ray photon correlation spectroscopy that x-ray scattering patterns from the disordered domains exhibit a continuous temporal decorrelation due to spontaneous domain fluctuations.

In situ X-ray diffraction and the evolution of polarization during the growth of ferroelectric superlattices.

In epitaxially strained ferroelectric thin films and superlattices, the ferroelectric transition temperature can lie above the growth temperature. Ferroelectric polarization and domains should then evolve during the growth of a sample, and electrostatic boundary conditions may play an important role. In this work, ferroelectric domains, surface termination, average lattice parameter and bilayer thickness are simultaneously monitored using in situ synchrotron X-ray diffraction during the growth of BaTiO3/SrTiO3 superlattices on SrTiO3 substrates by off-axis radio frequency magnetron sputtering.

Field-dependent domain distortion and interlayer polarization distribution in PbTiO3/SrTiO3 superlattices.

The remnant polarization of weakly coupled ferroelectric-dielectric superlattices is distributed unequally between the component layers, and as a result the components respond differently to applied electric fields. The difference is apparent in both the nanometer-scale structure of striped polarization domains and in the development of piezoelectric strain and field-induced polarization. Both effects are probed with in situ time-resolved synchrotron x-ray diffraction in a PbTiO(3)/SrTiO(3) superlattice in fields up to 2.

Nanosecond dynamics of ferroelectric/dielectric superlattices.

The nanosecond response of a PbTiO(3)/SrTiO(3) ferroelectric/dielectric superlattice to applied electric fields is closely linked to the dynamics of striped domains of the remnant polarization. The intensity of domain satellite reflections observed with time-resolved x-ray microdiffraction decays in 5-100 ns depending on the magnitude of the electric field. The piezoelectric response of the superlattice within stripe domains is strongly suppressed due to electromechanical clamping between adjacent regions of opposite polarization.

Improper ferroelectricity in perovskite oxide artificial superlattices.

Ferroelectric thin films and superlattices are currently the subject of intensive research because of the interest they raise for technological applications and also because their properties are of fundamental scientific importance. Ferroelectric superlattices allow the tuning of the ferroelectric properties while maintaining perfect crystal structure and a coherent strain, even throughout relatively thick samples. This tuning is achieved in practice by adjusting both the strain, to enhance the polarization, and the composition, to interpolate between the properties of the combined compounds.

Advanced fabrication and characterization of epitaxial ferroelectric thin films and multilayers.

Understanding the behavior of ferroelectrics on the nanoscale level requires the production of materials of the highest quality and advanced characterization techniques for probing the fascinating properties of these systems with reduced dimensions. Here we give an overview of our recent achievements in this area, which includes the detailed study of the suppression of ferroelectricity in PbTiO3 thin films, the fabrication of PbTiO3/SrTiO3 superlattices in which ferroelectricity shows some surprising behavior, and finally the manipulation of nanoscale ferroelectric domains using the atomic force microscope which leads to the precise analysis of domain wall creep and roughness in Pb(Zr,Ti)O3 thin films.