Built-in voltage in thin ferroelectric PbTiO3 films: the effect of electrostatic boundary conditions

We have studied the built-in voltage in heterostructures composed of ferroelectric PbTiO₃ and dielectric spacers of SrTiO₃, grown on SrTiO₃ substrates with SrRuO₃ bottom electrodes. We used piezoresponse force microscopy to determine the domain structure of the samples and perform local switching experiments. By increasing the thickness of the SrTiO₃ spacer layers, we were able to tune the polarization configuration in the PbTiO₃ films by gradually going from a monodomain configuration to a polydomain one. The built-in voltage was found to be linked to the choice of electrodes and to the thickness of the spacer layer. The stability of artificially poled regions was also affected, with a faster back-switching for the samples with the thicker SrTiO₃ spacers. We have additionally compared samples with and without a top SrRuO₃ electrode, showing that a top electrode layer results in a stronger built-in voltage, responsible for a preferential monodomain configuration and a very fast back-switching of artificially poled regions.