High-resolution scanning optical microscopy of ferroelectric thin films

Abstract

Two optical techniques are used to map the electric polarization in BaxSr1-xTiO3 (BST) thin films at room temperature. The first, confocal scanning optical microscopy (CSOM), is a diffraction-limited technique with wide applicability and ease of use. CSOM images taken at different static electric fields show reorientation of ferroelectric nanodomains. Local hysteresis loops, obtained by cycling the applied electric field at one position on the sample, suggest a coexistence of both paraelectric and ferroelectric phases at compositions x which exhibit paraelectric behavior in the bulk. Non-uniform strain is postulated to account for the apparent spatial variations of the Curie temperature. The second technique, apertureless near-field scanning optical microscopy (ANSOM), is used to image polarization fluctuations with a spatial resolution of 30 Å. ANSOM images of the same samples show inhomogeneities in the ferroelectric polarization over the smallest scales that can be observed, changes which 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 utility of both CSOM and ANSOM in characterizing domain structure and dynamics in ferroelectric thin films.