Nonlinear local piezoelectric deformation in ferroelectric thin films studied by scanning force microscopy

Local piezoelectric deformation is investigated in (Pb,La)TiO3 (PLT) and Pb(Zr,Ti)O-3 (PZT) thin films via scanning force microscopy (SFM) as a function of the ac voltage V-ac applied between the conducting tip and the bottom electrode. Thus obtained voltage dependence of the effective piezoelectric coefficient (local piezoelectric nonlinearity) is compared with the corresponding macroscopic piezoelectric behavior determined by laser interferometry. As expected, the local piezoresponse of PLT films measured inside uniformly polarized areas (ferroelectric domains) remains almost linear with increasing V-ac until the driving voltage becomes comparable with the coercive one. The corresponding macroscopic response is substantially nonlinear, suggesting significant contribution from the motion of 90 degrees domain walls. On the contrary, in PZT films the local piezoelectric behavior is strongly nonlinear, whereas the macroscopic piezoelectric coefficient is almost field independent. Moreover, depending on the polarity of probed as-grown domains, the local piezocoefficient of PZT films is found to display either enhancement or reduction with increasing ac voltage. The positive domains (i.e., domains having polarization vector pointing to the film-free surface) are often unstable with increasing V-ac and switch into the opposite polarization state under an ac voltage several times smaller than that required for global polarization reversal. This effect is explained by the presence of charged domain boundaries below the surface and their local depinning induced by external field. It is envisaged that SFM can be used not only for polarization mapping of ferroelectric surfaces but also as a probe for studying local polarization profiles beneath the SFM tip. (c) 2005 American Institute of Physics.