Direct structural determination in ultrathin ferroelectric films by analysis of synchrotron x-ray scattering measurements

In order to better understand ferroelectricity in thin films, it is important to explore the atomic-scale structure and the spatial distribution of polarization near the interfaces. We present sub-Angstrom-resolution electron density maps of three ultrathin PbTiO3 films grown epitaxially on SrTiO3 (001) substrates. The maps were obtained by analysis of synchrotron x-ray scattering measurements of Bragg rod intensities using the recently developed coherent Bragg rod analysis method. A four- and a nine-unit-cell-thick film were studied at room temperature, and a nine-unit-cell-thick film was studied at 181 degrees C. The results show that at room temperature, the PbTiO3 films are polar, monodomain, and have their polarization oriented away from the substrate. The four-unit-cell film may be the thinnest monodomain perovskite film found to be in the polar phase. At 181 degrees C, the electron density map of the nine-unit-cell film is consistent with the presence of 180 degrees stripe domains. In the monodomain samples, details of the atomic-scale structure of the PbTiO3/SrTiO3 interface are observed, which may provide evidence for the nature of the positive charge layer required to stabilize polarization in monodomain films.