Finite Size Effects In Ferroelectric Nanosystems: absence of mode softening
D. Wolpert, W. Cox, J. Cerne, A. Markelz, T. Zhao, R. Ramesh
University at Buffalo, US
Keywords: ferroelectric, FRAM, phonon, Terahertz
Ferroelectrics are currently used for DRAM capacitors, nonvolatile memory elements, (FRAM), and switching and routing elements in optical communications systems. Further miniaturization and large scale application of these systems is limited by apparent finite size effects diminishing the dielectric response. It has been suggested that the observed decrease in the dielectric constant and coercive field for nanoscale ferroelectric systems is due to a fundamental shift in lattice dynamics from a second order displacive transition to a first order transition and disappearance or weakening of the phonon mode softening.1 Here we present terahertz time domain spectroscopy (TTDS)2 results showing the near absence of mode softening in a 500 nm PbZr1-xTixO3 (PZT) thin film. At room temperature two modes are observed at 1.1 THz (37 cm-1) and at 2.3 THz (77 cm-1). As the temperature in increased toward Tc we do not see a dramatic shifting of modes, but rather a spectral weight transfer from the high frequency mode to the low frequency mode. This absence of mode softening is somewhat more dramatic than that reported by other investigators. These results suggest a dramatic change in lattice dynamics for nanoscale ferroelectric films that may be highly dependent on sample preparation techniques.
NSTI Nanotech 2003 Conference Technical Program Abstract