Combined effect of grain size and tensile stresses on the ferroelectric properties of sol-gel (Pb,La)TiO 3 thin films
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Combined effect of grain size and tensile stresses on the ferroelectric properties of sol-gel (Pb,La)TiO3 thin films M. Alguero´, M.L. Calzada, and L. Pardo Instituto de Ciencia de Materiales, Consejo Superior de Investigaciones Cientı´ficas, Cantoblanco 28049, Madrid, Spain
E. Snoeck Centre d’Elaboration des Materiaux et de´s Etudes Strucuturales, Centre National de la Recherche Scientifique, BP4347, F-31055 Toulouse, France (Received 18 September 1998; accepted 28 September 1999)
Transmission electron microscopy has shown that the grain size of sol-gel-prepared lanthanum-modified lead titanate films increases from ∼100 to ∼1 m when the excess of PbO in the precursor solution is reduced from 20 to 10 mol%. Switchable polarization is higher in the films with a smaller grain size. Profilometry and the temperature dependence of the dielectric permittivity indicate that films are tensile stressed by the substrate. The grain-size effect on polarization switching is explained by taking into account this tensile stress, which is thought to induce some a-domain orientation and 90° domain wall clamping in the grains attached to the substrate.
I. INTRODUCTION
Lanthanum-modified lead titanate (PTL) ferroelectric thin films have been studied1–3 for their interest in infrared sensing, electromechanical sensing, and actuating applications. The incorporation of small amounts of La3+ into the perovskite structure substituting for Pb2+ in lead titanate allows the reduction of the tetragonal distortion, which facilitates the poling of the films, maintaining a high value of the spontaneous polarization.4 This report is focused on films of PTL with 8% La, prepared by a diol-based sol-gel method and deposited on Si-based substrates.5 For this and other modified compositions, the sol-gel method has been shown to provide perovskite phase films with good electrical properties.6,7 We have reported in a previous paper8 on the ferroelectric properties of the PTL films prepared from precursor solutions with an excess of PbO that were crystallized at a heating rate of 10 °C min−1. In Ref. 8, it was found the development of a Pb-deficient second phase, coexisting with the perovskite in the films, occurred without an excess of PbO. This phase has been shown to be a metastable intermediate phase adopting the pyrochlore structure, as that of Pb2Ti2O6 (JCPDD-ICDP File No. 26-142), which can be fully converted into perovskite phase by prolonged heat treatment at 650 °C.9 The second phase damages the ferroelectric response of the films, although it completely disappears when an excess of PbO equal or higher than 10 mol% is used. The switchable polarization increases when the excess of PbO is raised from 10 to 20 mol%, which cannot be related to the detrimental second phase. 4570
J. Mater. Res., Vol. 14, No. 12, Dec 1999
In the same work,8 the effect of using a high heating rate (>500 °C min−1) in the crystallization thermal treatment was also studied. This topic has technological relevance as rapid
