Ferroelectricity at the Nanoscale
Chun-Gang Duan, R. Sabirianov,1 W.-N. Mei,1 S. S. Jaswal, and E. Y. Tsymbal
1 Department of Physics, University of Nabraska, Omaha, Nebraska
A continuing demand to further miniaturize electronic devices brings up a problem of the existence of ferroelectricity at the nanometer scale. Recent experimental and theoretical results demonstrate that in thin films ferroelectricity persists down to film thickness of a few unit cell which opens a new direction for novel electronic devices such as ferroelectric tunnel junctions. In particular, it was discovered that, in organic ferroelectrics, ferroelectricity can be sustained in thin films of a few monolayer thickness. In perovskite ferroelectric oxides, ferroelectricity was observed down to a nanometer scale. These findings raise questions about factors controlling ferroelectricity and the nature of the ferroelectric state at the nanoscale. Here we report a first-principles study of KNbO3 ferroelectric thin films placed between two metal electrodes, either SrRuO3 or Pt. We show that bonding at the ferroelectric-metal interfaces imposes severe constraints on the displacement of atoms, destroying the bulk tetragonal soft mode. If the interface bonding is sufficiently strong the ground state represents a ferroelectric domain with an interface domain wall, driven by the intrinsic oppositely-oriented dipole moments at the two interfaces. The critical thickness for the net polarization of the KNbO3 film is predicted to be about 1 nm for Pt and 1.8 nm for SrRuO3 electrodes. Funded by NSF and NRI.
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