Preparation Of Novel Perovskite Ferroelectric Thin Films

Ferroelectric materials are a class of materials with spontaneous polarisation that can be reversed with the application of an electric field.By utilising the strong coupling between spontaneous polarisation and light,electric field,magnetic field,ferroelectric thin films can be applied in memory,transducer,and sensor,etc.The performances of known ferroelectric thin film materials have met bottlenecks,including a low polarisation value at the nanoscale,weak thermal stability,and a high dielectric constant,etc.This thesis concentrates on the preparation and structure of novel ferroelectric thin film materials so as to extend the variety of ferroelectric materials and candidate materials in various application fields.Magnetron sputtering method have been incorporated to prepare these perovskite films.Introducing oxygen vacancies and epitaxial strains in CaTiO3,SrZrO3,and BaZrO3 thin films,these non-ferroelectric materials are converted into ferroelectric materials in this thesis.The crystal structure,electronic structure,and relationship between them and ferroelectricity in these thin films are investigated in an effort to generate new strategy for the study of novel ferroelectric thin film materials.The deposition of low dielectric constant SrZrO3 thin films onto the SrTiO3 substrates can induce ferroelectricity while maintaining the low dielectric constant properties.Combining first-principles calculations and spherical electron microscopy analysis confirmed that high compressive strain induced the phase transition from orthorhombic(Pbnm)to monoclinic(Pm).The ferroelectricity of SrZrO3 films were derived from the off-center displacement of Zr ions caused by high compressive strain.The in-plane strain geometric phase analysis(GPA)indicates that there is strain relaxation in the SrZrO3 film,so as the thickness increases,the ferroelectricity will gradually decrease.This study confirms that utilising strain engineering to induce paraelectric materials with a low dielectric constant into ferroelectric materials is an effective method for preparing ferroelectric materials with a low dielectric constant.Magnetron sputtering method was used to deposite BaZrO3 films with significant lattice mismatch on SrTiO3 substrates,and spherical electron microscopy revealed the existence of out-of-phase boundaries in the films.The outof-phase boundary is originated from the large lattice mismatch between the film and the substrate,and its construction and extension require a suitable amount of Ba element to sustain.According to X-ray absorption spectroscopy(XAS),the ferroelectricity of BaZrO3 films is caused by the displacement of Zr ions induced by compression strain.The in-plane strain GPA demonstrates that the complete outof-phase boundary can maintain the uniform distribution of the in-plane strain in the thickness direction,reduce the critical thickness limit of epitaxial strain,and then sustain the ferroelectricity in the thickness direction.By introducing oxygen vacancies into the CaTiO3 film,strong roomtemperature ferroelectricity was achieved.The type of bottom electrode has minimal impact on the ferroelectric properties of CaTiO3 film.CaTiO3 films grown on the bottom electrode of La0.7Sr0.3MnO3 have good surface quality;its maximum polarization value of 60 μC/cm2 is 5 times that of the largest reported in the CaTiO3 film.The origin of the ferroelectricity of thin films is the oxygen vacancy-induced displacement of Ti ions along the c-axis,as shown by synchrotron radiation and XAS experiments.The higher the oxygen vacancy content,the stronger the ferroelectricity of the CaTiO3 film,and that the ferroelectricity could be regulated by varying the annealing atmosphere.This study demonstrated that oxygen vacancy content is a crucial factor in the paraelectric-ferroelectric phase transition of CaTiO3 films.PbTiO3:ε-Fe2O3 composite multiferrous films were prepared by magnetron sputtering,which had good ferroelectricity(Pr of 19 μC/cm2)and ferromagnetism(25 emu/cc)at room temperature.By two-phase recombination,the ferroelectric properties of(111)-oriented PbTiO3 can be stabilized and the magnetic coercive field of ε-Fe2O3 is reduced.The surface morphology and ferroelectric properties of the composite film can be effectively improved by inserting a PbTiO3 buffer layer.