High spatial and temporal resolution studies of ferroelectric thin films.

摘要

The subject of this thesis is the investigation of the polar structure and dynamics of ferroelectric thin films using newly developed high resolution optical, scanning-force microscopy and time-resolved methods. A technique based on confocal scanning optical microscopy (CSOM) is used to image the ferroelectric polarization of Ba_xSr_1−xTiO ₃ (BST) thin films at room temperature with sub-micron spatial resolution. Films of both paraelectric (x = 0.5) and ferroelectric ( x = 0.8) compositions show a coexistence of both paraelectric and ferroelectric phases on the smallest scale resolvable with this technique. These results suggest that non-uniform stress is responsible for the strong inhomogeneous thermal broadening of the ferroelectric phase transition, and that dielectric loss in thin films may be dominated by a relatively small fraction of nanometer-sized regions.; Apertureless near-field scanning optical microscopy (ANSOM) is used to map the inhomogeneous ferroelectric polarization in Ba_xSr _1−xTiO₃ thin films. Images of nanometer-scale ferroelectric domains in Ba_xSr_1−xTiO₃ thin films are obtained with 30 Å spatial resolution using ANSOM. The images exhibit inhomogeneities in the ferroelectric polarization over the smallest scales that can be observed, and are largely uncorrelated with topographic features. The application of an in-plane static electric field causes domain reorientation and domain-wall motion over distances as small as 40 Å. These results demonstrate the promise of ANSOM for imaging near-atomic-scale polarization fluctuations in ferroelectric materials. Interferometric ANSOM is described in detail, including a practical description of how ANSOM images are acquired. A discussion of the various contrast mechanisms in ANSOM is followed by a prescription for eliminating a certain class of topographic artifacts. For the imaging of polarization in ferroelectric thin films, the linear electro-optic effect provides the central contrast mechanism. High-resolution ANSOM images show the existence of polar nanodomains in Ba_xSr_1−x TiO₃ films, providing strong direct evidence of its relaxor character.; To study the temporal behavior of these thin-films, the microwave dielectric response of a ferroelectric thin film is measured locally using time-resolved confocal scanning optical microscopy (TRCSOM). Measurements performed on an ensemble of nanometer-scale regions show a well-defined phase shift between the paraelectric and ferroelectric response at 2–4 GHz. Application of a static electric field produces large local variations in the phase of the ferroelectric response. These variations are attributed to the growth of in-plane ferroelectric nanodomains whose size-dependent relaxation frequencies lead to strong dielectric dispersion at mesoscopic scales.