Nanosecond Structural Dynamics of Ferroelectric Oxide Thin Films
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The functionalities of ferroelectric materials are closely related to the switchable spontaneous polarization and the polarization-structure coupling. A persistent effort has been made since the discovery of ferroelectricity to understand and manipulate the electronic and structural properties of ferroelectrics. Recently, studies of ferroelectrics have been focused on novel materials, including ferroelectric/dielectric superlattices and multiferroics that provide additional means to modify the functional properties. This thesis provides an in-depth study of the coupling of the polarization with other degrees of freedom, based on experimental measurements of the structural changes induced by electric fields and optical excitation. We have studied the mechanism associated with the transformation from a nanoscale polarization domain state to uniform polarization in ferroelectric/dielectric PbTiO3/SrTiO3 superlattices. The switching process was probed using time-resolved X-ray microdiffraction, which allowed us to follow the domain dynamics at their characteristic nanosecond timescale. PbTiO3/SrTiO3 superlattices exhibit a weak coupling between the polarizations of the component layers, and, as a result, the competition between the energy associated with the depolarization field and the energy of domain walls leads to the formation of stripe domains. The dielectric layers have a smaller polarization than the ferroelectric layers. The formation of stripe domains and the unequal distribution of the polarization have important consequences in the response of the superlattice to applied electric fields. We found that the switching of the stripe domains occurs heterogeneously at the submicron scale, with a timescale for switching that depends on the magnitude of the applied electric field. Each component layer responds differently to applied electric fields. The dielectric SrTiO3 layers are initially less polarized and thus exhibit a large distortion of domains before the transformation is complete. A significant piezoelectric expansion of the SrTiO3 layers is found after the transformation to the uniform polarization state, which is consistent with the change in polarization due to the elimination of stripe domains. The second component of this thesis focuses on the structural response of epitaxial multiferroic BiFeO3 thin films to high electric fields and femtosecond laser excitation. We have found that the piezoelectricity of a BiFeO3 thin film deviates from its low-field linear response in electric fields higher than 150 MV/m. The increase of the piezoelectricity as well as a simultaneously observed increase in the diffuse scattering is consistent with the softening of the lattice in the proximity of an electric-field-induced phase transition. The sub nanosecond structural dynamics of BiFeO3 thin films were also probed with time-resolved x-ray scattering following above-bandgap femtosecond laser excitation. A photoinduced strain on the order of 0.5% develops within 100 ps after a laser pulse with a 3.1 eV photon energy and a transmitted fluence of 6 mJ/cm2. Two potential mechanisms are discussed for this expansion: a piezoelectric response to the screening of the depolarization field in the presence of photoinduced carriers, and a mechanical response to the large induced population of excited carriers. The relaxation of the strain can be interpreted as a carrier recombination process, which is on the order of one nanosecond depending on the film thickness. The widths of Bragg reflections increase under large laser fluence, an effect that can be attributed to strain inhomogeneity.