Structure Design And Performance Investigation Of (K_xNa_1-xNbO₃)-（BiFeO₃） Multilayer Films

As the use of lead zirconate titanate（PZT）is gradually limited,K_0.5Na_0.5Nb O₃（KNN）has become one of the most promising perovskite oxide materials to replace PZT.At the same time,with the rapid development of the electronics industry,the development trend of miniaturization and miniaturization of electronic devices makes perovskite thin film materials gradually become a research hotspot.KNN films have been widely studied because of their high Curie temperature and better electrical properties.The preparation methods of perovskite thin film materials can be divided into chemical methods and physical methods.The Sol-Gel method in the chemical preparation method has the advantages of low cost,controllable components,and convenient large-scale production,so it is gradually applied in the preparation of thin film materials.In this paper,KNN thin films will be prepared by Sol-Gel method.However,KNN thin films prepared by Sol-Gel method still have many shortcomings:the stoichiometric ratio error caused by the water absorption of alkali metal ions during the solution configuration process;the volatilization of alkali metal ions during the annealing process..These defects lead to poor electrical properties of KNN films.In this paper,we tried to use alkali metal element control and excessive doping to prepare KNN films with chemical compositions close to K:Na:Nb=1:1:2.At the same time,the problem of poor crystalline quality of KNN thin films prepared by Sol-Gel method can be improved by other means.Interface regulation is one of the ways to improve the properties of KNN thin films.By changing the substrate and using an appropriate buffer layer,the crystalline quality and electrical properties of KNN thin films can be improved to a certain extent.Because KNN has the same perovskite structure and similar lattice constants as BiFeO₃（BFO）and BaTiO₃（BTO）,BFO film and barium titanate BTO film are selected as buffer layers to promote high-quality crystallization of KNN.The third chapter introduces the BFO thin film and BTO thin film prepared on the bottom electrode based on LaNiO₃（LNO）in detail.The experimental comparison shows that the BFO thin film prepared by magnetron sputtering has better ferroelectric properties.Therefore,BFO was finally selected to promote the high-quality crystallization of KNN.This paper introduces epitaxial interface and lattice strain between KNN and BFO through the KNN/BFO heterostructure,which is helpful for improving the crystalline quality of KNN.To this end,this paper explores the two-layer films of Au/KNN/BFO/LNO and Au/BFO/KNN/LNO.It is found that the BFO film prepared based on the LNO bottom electrode has high-quality crystallization along the（001）orientation,and BFO promotes the high-quality crystallization of the upper KNN film through interface regulation.The KNN crystals in the KNN/BFO/LNO bilayer films are denser and have a higher degree of orientation,and the polarization strength P_r of the hysteresis loop is improved.In the BFO/KNN/LNO bilayer film,KNN will be annealed again during the preparation of BFO.At this time,the huge kinetic energy impact of the plasma makes the recrystallization of KNN more dense,and the defects in the film are improved.Therefore,a multi-layer film with a BFO/KNN/BFO/LNO three-layer structure was designed by combining the two principles,and its dielectric loss was lower than that of KNN/BFO/LNO and BFO/KNN/LNO.The change of film thickness will affect the crystal morphology and size,which in turn affects the electrical properties of the film.This paper further explores the effect of BFO thickness in KNN/BFO/LNO bilayer films on the film properties.The study found that when the thickness of KNN is 100 nm and the thickness of BFO varies in the range of 200-400 nm,the grain size of BFO increases with the increase of the thickness of BFO film.This reduces the grain boundaries in the film and reduces the leakage current,and at the same time increases the grain size of KNN based on interface regulation.In order to avoid the influence of the different KNN:BFO thickness ratio on its dielectric properties caused by the different dielectric constants of KNN and BFO,a control experiment was designed in which the KNN:BFO thickness ratio was unchanged and the total thickness was changed.The study found that the increase of the thickness of BFO can promote the improvement of the crystalline quality of the film to a certain extent,the dielectric constant is improved,and the dielectric loss is reduced.The effects of different bottom electrodes on the electrical and optical properties of KNN-BFO multilayer films were also discussed.It is found that the KNN/BFO/ITO（222）bilayer film has a larger leakage current than the KNN/BFO/LNO bilayer film,which is caused by the excessive lattice mismatch between the bottom electrode and the film grain boundary.Then,the optical properties of the film were studied,and it was found that the transmittance of the KNN/BFO double-layer film was lower than that of the BFO single-layer film due to the diffraction effect in the film.At the same time,the optical band gap of the film was also studied,and it was finally found that the band gap of the double-layer film was lower than that of the single-layer film,which was helpful for the application of solar cells and optoelectronic devices.