A layer of reduced switchable polarisation in (Pb,La)TiO3 films leading to a thickness dependence of the ferroelectric parameters

Two sets of ferroelectric (Pb,La)TiO₃ thin films have been prepared by a diol-based sol-gel technique by varying the heating rate to the crystallisation temperature of 650°C. Films of increasing thickness were obtained by repeating the solution deposition from 1 to 5 times. The switchable polarisation was evaluated from hysteresis loops and from the integration of the switching current transients during pulse tests, and was found to significantly depend on film thickness. Measurements of the thickness dependence of the reciprocal capacitance and Rutherford backscattering spectrometry results indicate that a layer with different dielectric permittivity, and composition for one set of films, existed next to the bottom electrode. In the one set of films it originated because of interdiffusion between the film and the substrate, while in the other set, it was linked to the tensile stress at the film/substrate interface. These layers had a reduced switchable polarisation, which caused the observed dependence of their properties on thickness.