Monodomain strained ferroelectric PbTiO₃ thin films: Phase transition and critical thickness study

This work demonstrates that instead of paraelectric PbTiO3, completely c -oriented ferroelectric PbTiO3 thin films were directly grown on (001) -SrTiO3 substrates by pulsed-laser deposition with thickness up to 340 nm at a temperature well above the Curie temperature of bulk PbTiO3. The influence of laser-pulse frequency, substrate-surface termination on growth, and functional properties were studied using x-ray diffraction, transmission electron microscopy, and piezoresponse force microscopy. At low growth rates (frequency <5 Hz) the films were always monodomain. However, at higher growth rates (frequency >8 Hz) a domains were formed for film thickness above 20-100 nm. Due to coherency strains the Curie temperature (Tc) of the monodomain films was increased approximately by 350°C with respect to the Tc of bulk PbTiO3 even for 280-nm-thick films. Nonetheless, up to now this type of growth mode has been considered unlikely to occur since the Matthews-Blakeslee (MB) model already predicts strain relaxation for films having a thickness of only ∼10 nm. However, the present work disputes the applicability of the MB model. It clarifies the physical reasons for the large increase in Tc for thick films, and it is shown that the experimental results are in good agreement with the predictions based on the monodomain model of Pertsev [Phys. Rev. Lett. 80, 1988 (1998)].