Nanostructured Complex Oxides
Reduced Dimensionality and Strain in Ferroelectrics
Ferroelectrics exhibit spontaneous polarization of electric dipoles that is fundamentally interesting and technologically useful. An interesting question is: what happens to dipole ordering when the dimensions of the ferroelectric become extremely small. Prof. Norton’s group has been investigating size effects in KTaO3/KNbO3 superlattices grown by a film-growth technique known as pulsed-laser deposition. KTaO3 is a cubic perovskite oxide that is paraelectric at all temperatures. KNbO3 is an oxide with similar crystalline structure that undergoes a paraelectric to ferroelectric phase transition at 701 K.
When KTaO3/KNbO3 multilayered thin-film structures are synthesized in which the KNbO3 layer thicknesses approach a single unit cell, the properties of the ferroelectric change dramatically. First, strain reduces the ferroelectric transition temperature, known as the Curie temperature, for layer thickness down to 5 nm. For periodicities less than 5 nm, structures behave as a random alloy indicating long-range ferroelectric coupling occurs across the paraelectric KTaO3 layers. Finally, when the KNbO3 layer thickness is reduced to one or two unit cells, the electric dipole order completely changes from parallel (ferroelectric) to anti-parallel (known as antiferroelectric). This type of dipole ordering is never seen in K(Ta,Nb)O3 bulk materials and represents a new phenomenon completely due to the nanoscale dimensionality of the material. For practical applications, this result suggests that ferroelectric devices, such as ferroelectric random access memory, could be reduced in size down to a few unit cells, but that structures approaching two unit cells in size would no longer be effective ferroelectric materials.
Conceptually, one can envision the engineering of these and other oxide materials material properties at the atomic level so as to tailor to the specific functions relevant to material performance. This requires an understanding of the material properties at the nanoscale, as well as development of advanced synthesis and measurement techniques. Several growth processes have demonstrated atomic-level control of oxide growth, including pulsed-laser deposition, molecular beam epitaxy, and chemical vapor deposition. These advancements enable the study and engineering of oxide materials at the nanoscale.
Related Papers and Review Articles:
"Dielectric response of asymmetric KNbO3/KTaO3 superlattices," J. Sigman, H. J. Bae, D. P. Norton, J. Budai, and L. A. Boatner , J. Vac. Sci. Technol. A 22, 2010 (2004)
"Antiferroelectric behavior in symmetric KNbO3/KTaO3 superlattices," Sigman, J., Norton, D. P., Christen, H. M., Fleming, P. H. and Boatner, L. A., Physical Review Letters, Volume 88, Number 9, p. 097601-1, 2002.
"Long-range ferroelectric interactions in KTaO3/KNbO3 superlattice structures," Christen, H.-M. ; Specht, E.D.; Norton, D.P.; Chisholm, M.F.; Boatner, L.A., Applied Physics Letters, Volume 72, Issue 20, 1998, Pages 2535-2537.
Additional information on this and related research can be seen at http://norton.mse.ufl.edu.