Fabrication And Electrical Properties Of Na_0.5Bi_0.5TiO₃-Based Epitaxial Ferroelectric Thin Films

To meet the development demands of highly integrated electronic devices,the development of low-power,high-storage capacity,high-precision,multifunctional,and environmentally friendly devices has become crucial.Na0.5Bi0.5TiO3(NBT),a lead-free A-site complex perovskite structure(ABO3)ferroelectric material,possesses significant advantages such as large polarization,high ferroelectric Curie temperature(Tc≈320 ℃),high dielectric constant,relaxation ferroelectric characteristics at certain temperatures,and ease of forming solid solutions with other materials.As a result,NBT has been extensively studied in various fields,including dielectric energy storage,ferroelectric memory,resistive switching memory,optoelectronics,photocatalysis,and bone repair biology.Furthermore,with the trend towards miniaturization and integration of devices,research on thin film devices has become increasingly important.Compared to polycrystalline thin films,epitaxial thin films exhibit more stable performance.In this study,we employed the chemical solution deposition(CSD)method,which offers advantages such as large-scale preparation,low cost,easy control of chemical composition,and simple industrial operation,to prepare NBT-based epitaxial thin films.The performance of these films was investigated,and the main research findings are as follows:(1)Preparation and energy storage performance study of NBT films with different epitaxial orientations.NBT films with(100),(110),and(111)epitaxial orientations were prepared on single-crystal LaAlO3 substrates using the CSD method,with La0.7Sr0.3MnO3(LSMO)thin films as the bottom electrode.The results show that all the prepared films are epitaxial films with a rhombohedral structure,and their spontaneous polarization orientation is(111).Polarization current curves and hysteresis loops demonstrated that the(111)oriented films exhibit the highest maximum and remnant polarization values under the same electric field.However,these films also have significant hysteresis losses,resulting in the lowest breakdown field strength of only 1500 kV/cm.On the other hand,the(100)oriented films exhibit the lowest hysteresis losses and leakage current,along with the highest breakdown field strength of 2200 kV/cm.Consequently,the(100)oriented films achieve the optimal recoverable energy storage density(28.1 J/cm3)and energy storage efficiency(45.4%).Additionally,they exhibit good fatigue stability with up to 106 cycles and excellent stability from room temperature to 80℃.(2)Preparation and energy storage performance study of epitaxial(100)oriented BiFeO3(BFO)/NBT/LSMO multilayer films with different BFO thicknesses.By constructing multilayer films with BFO,which possesses a large ferroelectric polarization value(～100μC/cm2),it is expected to enhance the overall polarization value of the thin films.Furthermore,the introduction of the NBT/BFO interface has the potential to impede the development of electronic trees within the films,thereby significantly increasing the breakdown field strength.The results show that the leakage current decreases and the breakdown field strength increases from 1929 kV/cm(NBT/LSMO)to 2751 kV/cm(2L BFO/NBT/LSMO)when two layers of BFO are incorporated into the multilayer films.The 2L BFO/NBT/LSMO films achieve the optimal recoverable energy storage density(37.0 J/cm3)and exhibit superior temperature stability from room temperature to 140℃ due to their reduced leakage current.(3)Preparation and energy storage performance study of epitaxial(100)oriented high-entropy Na0.5Bi0.5Ti0.7Hf0.1Zr0.1Sn0.1O3(NBTHZS)films.The results show that,due to element doping and the high-entropy effect,the high-entropy NBTHZS films exhibit a denser microstructure,reduced leakage current,and decreased remnant polarization compared to NBT films.These characteristics result in a high breakdown field strength(4.58 MV/cm)and significantly reduced hysteresis losses.Consequently,the high-entropy films achieve superior energy storage performance,with a recoverable energy storage density of 81.0 J/cm3 and an energy storage efficiency of 74.1%(approximately 16 times higher than the energy density of 5.1 J/cm3 achieved by NBT films in this study).The high-entropy films also have good fatigue stability and temperature stability.(4)Preparation of epitaxial NBT/LSMO films and study of resistive switching effects.By adjusting the process parameters of NBT films,the leakage current is increased to achieve stable resistive switching effects.Hysteresis loops and large dielectric losses indicated a higher leakage current.After 100 current-voltage cycles,the on/off ratio under positive bias decreases from an initial level of 103(1770)to 102(617).Fitting the current mechanism reveals that the resistive switching effect is mainly controlled by the film’s bulk-limited conduction mechanism,with oxygen vacancies playing a major role.This study optimized the dielectric energy storage performance and resistive switching effects of NBT-based thin films through the control of epitaxial orientation,multilayer film construction,high-entropy engineering,and adjustment of process parameters.The results demonstrate the enormous potential applications of epitaxial NBT-based thin films in the fields of energy storage and resistive switching.