Electronic Structure And Optical Properties Of Doped Barium Titanate Thin Films

Recently,the research on the optical properties of ferroelectric materials is very hot due to the potential applications of ferroelectric materials in photoelectric functional devices.Ferroelectric thin films have many advantages,such as regulating photovoltaic characteristics by electric field,higher photoelectric conversion efficiency and higher output photovoltage.Ferroelectricity has been considered to be unable to coexist with metallicity because free carriers can screen the coulomb interaction between dipoles.However,recent experiments have demonstrated that ferroelectricity and metallicity can coexist.For example,with the increase of the electron doping concentration of BaTiO₃,the resistance of the system exhibits the conductive behavior of the metal material,and the tetragonal phase to cubic phase transformation will be happen at a critical doping concentration.Therefore,research on electronic doping ferroelectric materials and ferroelectric thin films is very significant,especially the metal-insulating phase transitions,structural phase transitions and optical properties.In this paper,the first-principles calculation method was used to study La-doped BaTiO₃ films and electron-doped BaTiO₃bulk materials.The effects of doping on the electronic structure and optical properties of ferroelectric materials were studied in detail.Our research can be summarized in the following two aspects:1.Doping effect of ferroelectric thin filmBy first-principles calculations,Ba atoms in the BaO atomic layer near the surface of the BaTiO₃ film were replaced by La atoms to study the effect of doping on the polarization of ferroelectric thin films.It was found that when the spontaneous polarization points to the inside of the film,the surface state will change from a metallic state to an insulating state under a suitable concentration of doped electrons,and a gradient structure of energy gap is formed from the surface layer to the inside of the film.In the case of polarization pointing up,the surface of the La-doped system always shows a metallic state.Therefore,the metal-insulator phase transition of the BaTiO₃ film surface can occurs under an applied electric field.2.Effect of electron doping on optical properties of BaTiO₃ bulk materialThe uniform doped Ba TiO₃ bulk material occurs,as the electron doping concentration reaching a critical value 0.11e/u.c,structural phase transition from the tetragonal phase to the cubic phase and the ferroelectricity disappears.Moreover,due to the doped electrons at the bottom of the conduction band transition to the high energy level,a new absorption peak appears in the low energy region,and the blue shift phenomenon occurs in the original absorption edge of the BaTiO₃.In addition,we dope a La atom in a 2*2*2 Ba TiO₃supercell（doping concentration is 0.125 e/u.c by a La atom replacing a Ba atom）.Under this doping concentration,the homogeneous electron doping system shows a cubic phase,and the ferroelectricity disappears.However,in the case of La-doping system,due to the doped electrons localized around the La atom,the interaction between the dipoles cannot be completely screened,and ferroelectricity and metallicity can coexist in this system.We hope that our theoretical research can provide guidance for the application and design of ferroelectric materials in optoelectronic functional devices.