X-ray Studies of Ferroelectricity in Ultrathin Films

G. B. Stephenson

Materials Science Division, Argonne National Laboratory

For decades, researchers have found that ferroelectric behavior is typically suppressed in films that are sufficiently thin. Various explanations have been put forward: intrinsic suppression of polarization at surfaces, the effect of depolarizing electric fields, or extrinsic effects of composition or strain. As a result, despite decades of effort, the size dependence of the paraelectric-to-ferroelectric phase transition remains an unresolved issue, in particular for the technologically-important perovskites. We have been using in situ synchrotron x-ray scattering to investigate the ferroelectric properties of ultra-thin, coherently strained epitaxial films of PbTiO3 as a function of film thickness, temperature, vapor ambient, and electrical boundary conditions. The ability to perform x-ray scattering in the film growth chamber allows us to determine optimum growth conditions, to control the thickness of the films to sub-unit-cell accuracy, and to control surface and film stoichiometry during high temperature study. When films are grown on insulating SrTiO3, we find that the ferroelectric phase forms as nanoscale 180° stripe domains. When films are grown on conducting SrRuO3 layers on SrTiO3, the polar phase forms in a single domain. Although we observe a thickness-dependent TC, in both cases the polar phase is stable at room temperature in films with thicknesses as small as three unit cells. Recently we have found that the polarity of the monodomain films can be switched by changing the oxygen pressure in the ambient. The results are in good agreement with Landau theory for stripe domains, and with density-functional calculations of the effects of ionic adsorbates at neutralizing the depolarizing field.

*Collaborators: R.-V. Wang, F. Jiang, D. D. Fong, S. K. Streiffer, C. Thompson, J. A.Eastman, P. H. Fuoss, A. M. Kolpak, A. M. Rappe, and K. R. Elder.