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-Ångstrom-resolution electron density maps of three ultrathin ${\mathrm{PbTiO}}_3$ films grown epitaxially on ${\mathrm{SrTiO}}_3$ (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 °C. The results show that at room temperature, the ${\mathrm{PbTiO}}_3$ 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 °C, the electron density map of the nine-unit-cell film is consistent with the presence of 180° stripe domains. In the monodomain samples, details of the atomic-scale structure of the ${\mathrm{PbTiO}}_3∕{\mathrm{SrTiO}}_3$ interface are observed, which may provide evidence for the nature of the positive charge layer required to stabilize polarization in monodomain films.