| Bismuth layered perovskite ferroelectric(BLPF)materials have been investigated extensively due to their promising applications in non-volatile ferroelectric random access memories,pyroelectric and piezoelectric devices.It is crucial to control the structure of the films for their future applications due to their strong anisotropic structure.And this is on the point of prevalent issue.Most recently,four-Bi-layered perovskite such as SBTi and CBTi have attracted much attention owing to their high Curie temperature and having potential to be used in high-temperature piezoelectric devices.Similar to SBT and BLT,the major polarization vector of SBTi,CBTi and CSBTi is also along the a axis,thus,fabrication of (100)-oriented SBTi,CBTi and CSBTi films is effective to increase the piezoelectric coefficient.In this paper,the structure control and decreasing the coercive field of four-Bi-layered perovskite ferroelectric thin films,systematic research works have been carried out as follows:(1)The study of LNO electrode.It has been proved that LNO is a promising candidate as the electrode material for ferroelectric random access memories.The structure of SBTi or CBTi or CSBTi films maybe controlled by varying the orientation of LNO films.For controlling the orientation of LNO films,the effects of preheating temperature and time, annealing temperature,and the amount of acetic acid in the precursor solution on the structure and resistivity of LNO films were investigated systematically.(100)- and(110)-oriented LNO films were obtained on Si(100)substrates by further optimizing these processing conditions.(2)The study of SBTi films.SBTi films were fabricated on SiO2/Si(100)and LNO-coated SiO2/Si substrates.The influence of the orientation of LNO films on the structure and ferroelectric properties of SBTi films were studied.A epitaxial relationship between the c-axis-oriented SBTi film and(100)-oriented LNO was proposed.The(100)-predominant SBTi film were fabricated on(110)-oriented LNO electrode by using the layer-by-layer method.On the other hand,(100)-predominant SBTi films were obtained directly on Si substrates.The formation of(100)-predominant SBTi films can be attributed to the higher growth rate of the(100)-oriented grains compared to(001)-oriented grains.Thus the c-axis-oriented grains will be covered by(100)-oriented grains under the layer-by-layer conditions.Since high activation energy is required for the growth of(100)-oriented grains, (100)-oriented SBTi films can not be obtained at temperatures lower than 700℃.The (100)-predominant SBTi film exhibits good insulating and dielectric properties.(3)The study of CBTi films.The influence of excess bismuth content and processing conditions on the structure and ferroelectric properties of CBTi thin films were investigated. Good ferroelectric properties can be obtained in the CBTi film with 7.5mol%excess bismuth. Annealing temperature of 700℃with soak time of 300 second can favor the growth of a-axis-oriented grains.The remanent polarization(Pr),coercive field(Ec)and rectangular ratio for CaBi4.3Ti4O15film are 6.1μC/cm2,80kV/cm and 0.45,respectively.(4)The study of CSBTi films.The dependence of the structure and ferroelectric properties of(Ca,Sr)Bi4Ti4O15(CSBTi)films on the molar ratio of Sr to Ca,processing conditions,La doping,and film thickness were investigated systematically.It was found that,①(CaxSr1-x)Bi4Ti4O15film with x=0.4 in the air or oxygen atmosphere show (100)-predominant and excellent ferroelectric proportion.②The volume ratio of the (100)-oriented grain increased with the increase of doping content of La,i.e.,doping of La can favor the growth of(100)-oriented grains and inhabitation of oxygen vacancies..Meanwhile, the remanent polarization also increased with the increase of La content.The CSLBTi0.24 (the doping content of La is 0.24 mol)show the lower coercive field of 90 kV/cm and higher remanent polarization of 22μC/cm2,and rectangular ratio of 0.67,much better than that of the undoped CSBTi film.(5)The optimized processing conditions for the fabrication of(100)-predominant SBTi and CBTi films on Pt/Ti/SiO2/Si(100)substrates were adopted for the preparation of CSBTi firm at fist time.It can be found that the relative intensity of(200)peak increased with the film thickness and became predominant for the thickness larger than 720 nm.The thickness required for the formation of(100)-predominant CSBTi film is larger than that for the (100)-predominant SBTi or CBTi films on SiO2/Si(100)substrates.This can be ascribed to the Pt bottom electrode rather than the doping of Ca.Contrary to the trend of the relative intensity of(100)diffraction peak with the film thickness,the remanant polarization of SCBTi films decrease with the increase of film thickness.This should be due to that part of the thick SCBTi film is consisted of the grains with different orientations along the direction perpendicular to the film surface.By optimizing the processing conditions and controlling the stoichiometry of the film, high quality four-layer BLSF thin films that can meet the requirements of the ferroelectric memories were obtained.Our works on the structure control and improve ferroelectric properties for SBTi and CBTi and CSBTi films will provide some useful information for the study of whole family of bismuth-layered-perovskite oxides.
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