Intrinsic Dielectric Response in Ferroelectric Nanocrystals
Measurements on 'free-standing' single crystal barium titanate capacitors with thicknesses down to 75nm show a dielectric response typical of large single crystals rather than conventional thin films. There is a notable absence of any broadening or temperature shift of the dielectric peak or loss tangent. Peak dielectric constants of ca. 25,000 are obtained, and Curie-Weiss analysis demonstrates 1st order transformation behavior. This is in surprising contrast to results on conventionally deposited thin film heterostructures, which show large dielectric peak broadening and temperature shifts [e.g. C. B. Parker, J-P. Maria and A. I. Kingon, Appl. Phys. Lett., 81, 340 (2002)], as well as an apparent change in the nature of the paraelectric-ferroelectric transition from 1st to 2nd order. Our data are compatible with the recent model by Bratkovsky and Levanyuk, which attributes dielectric peak broadening to gradient terms that will exist in any thin film capacitor heterostructure, either through defect profiles introduced during growth, or through subtle asymmetry between top and bottom electrodes. The observed recovery of 1st order transformation behavior is consistent with the absence of significant substrate clamping in our experiment, as modeled by Pertsev et al. Phys. Rev. Lett., 80, 1988 (1998), and illustrates that the 2nd order behaviour seen in conventionally deposited thin films cannot be attributed to the effects of reduced dimensionality in the system, nor to the influence of an intrinsic universal interfacial capacitance associated with the electrode-ferroelectric interface.