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-{}ngstrom-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 ^∘{}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 ^∘{}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 PbTiO₃∕SrTiO₃ interface are observed, which may provide evidence for the nature of the positive charge layer required to stabilize polarization in monodomain films.