Phase transitions and domain structure in heteroepitaxial ferroelectric (Ba1-xSrx)TiO3//(100)MgO and PbTiO3//(100)MgO thin films

Heteroepitaxial PbTiO3 and (Ba1-xSrx)TiO3 thin films were obtained by the r.f. sputtering method. The (100) surface cuts of MgO single crystals served as substrates. The films obtained exhibited ferroelectric properties closely similar to those of the ceramic targets. Depending on substrate temperature (Ts) with other deposition parameters constant, heteroepitaxial films of various degrees of perfection of the crystal structure were obtained. Films deposited at a temperature slightly higher than critical temperature (Tcr) for the heteroepitaxy process exhibited an insular growth mechanism while for Ts ≥ Tcr layer-by-layer growth was found. At room temperature the crystal structure of the PbTiO3//(100)MgO films and of the (Ba1-xSrx)TiO3//(100)MgO films for × ≤ 0,3 exhibit symmetry of the tetragonal system P4mm. Phase transitions in the (Ba1-xSrx)TiO3//(100)MgO films unlike those in the PbTiO3//(100)MgO films exhibited marked diffusion.The domain structure of the heteroepitaxial films, in comparison with that of free ferroelectric single crystals shows certain singular features. The PbTiO3//(100)MgO films exhibit characteristically a-c-domain structure while the (Ba1-xSrx)TiO3//(100)MgO films have c-c-domain structure. A simple phenomenological model is presented here in order to explain the causes of the appearance of various domain configurations in the studied films.