Epitaxial Growth Of High-mobility Ferroelectric Metal Single Crystal Thin Film Ba_0.5La_0.5TiO₃

Ferroelectric metal is a material that coexists with ferroelectric polarization and metallicity,which is very rare.Classical theory once thought that this kind of material could not exist,because the free electrons in the conductor would completely shield the electric dipole moment and could not form long-range polarity.Making this material is a challenge.At present,the reported ferroelectric(or polar)metal materials have two problems:(1)the mobility of itinerant electrons is low,and the conductivity is poor;(2)the ferroelectric polarization is difficult to reverse.In response to the above challenges,this article is based on the classic ferroelectric material Ba TiO₃,through the doping of La element,a series of high-quality ferroelectric metal thin films Ba_0.5La_0.5TiO₃ have been successfully prepared.and carefully studied the ferroelectric polarization and metallicity of the film,and the effect of substrate strain on the metallicity of the films was also investigated.The main research contents of this article include:1.Epitaxial preparation of single crystal thin films.Using lanthanum oxide,barium carbonate and titanium dioxide powder as raw materials,a high-quality Ba_0.5La_0.5TiO₃target was prepared by a solid-phase sintering method.The distribution of each element is relatively uniform,and the atomic ratio of La to Ba is 1:1.The Ba_0.5La_0.5TiO₃ thin film was prepared on the Sr TiO₃ substrate by using a self-developed atomic-level precision high-pressure magnetron sputtering system.The optimum growth conditions for this thin film are 800 ^oC and argon pressure 0.01 Torr.The film is a co-epitaxial single crystal film in a fully strained state with a thickness of about 71.2 nm.The in-plane lattice constant and the out-of-plane lattice constant are 3.905 and 4.003(?),respectively,and it has a tetragonal phase structure.2.Research on the metallicity and ferroelectric polarization of single crystal thin films.The Ba_0.5La_0.5TiO₃ film exhibits good metallic properties.The type of carrier is electron,from 300 K to 20 K,and the carrier concentration increases about 50 times,and the mobility is as high as 110 cm²·V^-1·s^-1 at low temperature.Using a piezoelectric force microscope,the flip of the ferroelectric polarization was successfully achieved in the film.The characterization of high-resolution scanning transmission electron microscopy shows that the offset of Ti ion from the center position is about 0.1(?),and the second harmonic generation measurement shows that the change of carrier concentration with temperature is consistent with the change of polarization with temperature.3.The effect of strain on the metallicity of thin films.By selecting substrates with different lattice constants and changing the in-plane lattice constants,the Ba_0.5La_0.5TiO₃film gradually transitions from a fully strained state to a relaxed state,and the film changes from a metallic state to an insulating state.