Effect of zr and la based co doping on electrical properties of lead free barium titanate batio3 thin films

HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY MASTER’S THESIS Effect of Zr and La based co-doping on electrical properties of lead-free barium titanate BaTiO3 thin films TRAN THI DOAN Doan.TT202748M @sis.hust.edu.vn Major: Materials Science Supervisor: Prof Vu Ngoc Hung Department: Institute: Micro-Electro-Mechanical-System Laboratory International Training Institute for Materials Science HANOI, 05/2022 HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY MASTER’S THESIS Effect of Zr and La based co-doping on electrical properties of lead-free barium titanate BaTiO3 thin films TRAN THI DOAN Doan.TT202748M @sis.hust.edu.vn Major: Material Science Supervisor (Sign and write full name) HANOI - 05/2022 Acknowledgment Firstly, I would like to express my deepest appreciation to my supervisor, Prof Vu Ngoc Hung, who directly instructed me throughout this research project I am also grateful to Dr Nguyen Duc Minh of MESA+ Institute for Nanotechnology at University of Twente, Netherlands I am extremely thankful and indebted to him for sharing his expertise as well as the valuable guidance and encouragement extended to me I would like to extend my sincere thanks to MSc Dang Thi Ha, Dr Vu Thu Hien, Dr Ngo Duc Quan, and all of members in MEMS laboratory for their enthusiasm to help me through the process of research I take this opportunity to express gratitude to all professors, lecturers, and employees at ITIMS for their kindness to support me during a period I have already studied and worked there Finally, I want to give special thanks to my family for providing me with their unfailing support and encouragement during my years of study and research Abstract BaTiO3-based materials with a perovskite structure have attracted interest because some of them are potentially valuable materials due to their environment-friendly properties In this study, lead-free Ba(Zr0.25Ti0.75)O3 (BZT) and La-doped Ba(Zr0.25Ti0.75)O3 (BLZT) thin films were grown on Pt/Ti/SiO2/Si substrates via a sol-gel spin-coating method The effects of various annealing temperatures (450–700 oC) for BZT thin films on microstructure, dielectric, and energy storage performances were systematically investigated As XRD result, it was found that the degree of crystallization of the films increased with the increasing annealing temperature (Ta) This result indicated a pure polycrystalline perovskite phase of BZT thin films achieved at 700 oC At the same time, the dielectric constant of the BZT film also increases as the annealing temperature increases In particular, the optimal energy-storage density of 30.9 J/cm3 and a large energy-storage efficiency of 67.8% could be obtained in the film annealed at 500 oC, which not only achieved a large breakdown strength (up to 7000 kV/cm) but also exhibited a great temperature-dependent energy storage performance stability in a wide temperature (from 30 oC to 200 oC) a good frequency-dependent energy storage performance stability in ranging from 100 to 10000 Hz and an excellent chargedischarge cycling life with fatigue-free performance up to 109 cycles Moreover, the effects of La doping of BZT thin films (from – mol.%) on microstructure, dielectric, and energy storage performances were also investigated As XRD result, it was shown that La doping enhanced the ability of crystallization of the films with the perovskite BZT phase achieved at 650 oC La-doped BZT thin films indicated prominently increasing relaxor behavior with increasing La-doping concentration In particular, the films with mol.% Ladoping simultaneously exhibit a quite high recoverable energy-storage density (~7.0 J/cm3) and a large energy-storage efficiency (~ 60.7%) under an EBD of 1650 kV/cm Moreover, dielectric constant of La-doped BZT thin films was found to be significantly improved, reaching the maximum value of 164 for mol.% La-doped BZT thin film These results indicated that lead-free Ba(Zr0.25Ti0.75)O3 and La-doped Ba(Zr0.25Ti0.75)O3 thin films were expected to become a candidate materials for energy-storage capacitors Master student (Sign and write full name) TABLES OF CONTENTS LIST OF ABBREVIATIONS i LIST OF FIGURES ii LIST OF TABLES v INTRODUCTION vi CHAPTER LITERATURE REVIEWS 1.1 1.2 Overview of ferroelectric and relaxor ferroelectric properties 1.1.1 Perovskite structure 1.1.2 Ferroelectrics 1.1.3 Relaxor ferroelectric Principles for High Energy-Storage in Dielectric Capacitors 10 1.2.1 Basic knowledge on dielectric capacitor 10 1.2.2 Measuring methods of energy-storage density for dielectric capacitor… 11 1.2.3 1.3 Potential dielectrics for high energy-storage application 14 Overview of barium titanate-based materials 15 1.3.1 Barium titanate (BaTiO3) structure 15 1.3.2 Effects of doping on BaTiO3 properties 17 CHAPTER EXPERIMENTS AND METHODS 20 2.1 Fabrication of BZT and BLZT thin films by sol-gel spin coating method… 20 2.1.1 Overview of sol-gel spin coating method 20 2.1.2 Fabrication of BZT and BLZT Sols 23 2.1.3 Fabrication of BZT and BLZT thin films 25 2.2 Methods to investigate the structure and properties of BZT and BLZT films… 27 2.2.1 Structural characteristics 27 2.2.2 Ferroelectric behaviors 29 CHAPTER RESULTS AND DISCUSSION 32 3.1 Effect of annealing temperature (Ta) on properties of BZT thin films 32 3.1.1 Structural characteristics 32 3.1.2 Ferroelectric properties and breakdown strength (EBD) 34 3.1.3 Energy - storage properties 36 3.1.4 Dielectric properties 38 3.2 3.3 Effects of La-doping on properties of BZT thin films 39 3.2.1 Structural characteristics 39 3.2.2 Ferroelectric properties and breakdown strength (EBD) 41 3.2.3 Energy-storage properties 42 3.2.4 Dielectric properties 44 Thermal stability, frequency stability and fatigue endurance 45 3.3.1 Thermal stability 45 3.3.2 Frequency stability 46 3.3.3 Fatigue endurance 48 CONCLUSIONS 52 LIST OF PUBLICATIONS 54 REFERENCES 55 LIST OF ABBREVIATIONS Acronyms Description AFE Anti-ferroelectrics CVD Chemical Vapor Deposition DSC Differential scanning calorimetry BTO Barium titanate, BaTiO3 BZT Barium zirconate titanate, Ba(Zr,Ti)O3 BLZT Barium lanthanum zirconate titanate, (Ba,La)(Zr,Ti)O3 FE Ferroelectrics HR-SEM High resolution scanning electron microscopy LD MEMS MPB P-E PLD PZO PZT PLZT Linear dielectric Micro electro mechanical systems Morphotropic phase boundary Polarization hysteresis loop Pulsed laser deposition Lead zirconate, PbZrO3 Lead zirconate titanate, Pb(Zr,Ti)O3 Lead lanthanum zirconate titanate, (Pb,La)(Zr,Ti)O3 PNRs Polar nanoregions PMN Lead magnesium niobate, Pb(Mg1/3Nb2/3)O3 PZT RFE TGA XRD Lead zirconate titanate, Pb(Zr,Ti)O3 Relaxor ferroelectric Thermogravimetric analysis X-Ray diffraction i LIST OF FIGURES Figure 1.1 (a) Cubic perovskite unit cell ABO3, (b) Perovskite lattice structure BO6 [12] Figure 1.2 Polarization as a function of temperature in (a) first and (b) second order phase transition [18] Figure 1.3 Frequency dependence of polarization [19] Figure 1.4 Ferroelectric hysteresis loop [19] Figure 1.5 Orientation of dipoles in the ferroelectric materials (a) absence of electric field (b) under electric field and (c) after removal of electric field [23] Figure 1.6 Temperature evolution of dielectric constant showing the characteristic temperatures in RFE Representative hysteresis loops for each temperature interval are showed below [24] Figure 1.7 Hysteresis behavior in (a) ferroelectric and (b) relaxor materials [27] Figure 1.8 Polarization and temperature in (a) ferroelectric and (b) relaxor materials [28] Figure 1.9 (a) Sharp transition in BT and (b) diffuse transition in PMN single crystals [28] Figure 1.10 The diagram of charge separation in parallel-plate capacitor under the function of electric field [30] 11 Figure 1.11 The diagram of measurement circuit for the energy-storage density [30] 12 Figure 1.12 (Color online) The typical dependence of (a) polarization and (b) permittivity on electric field of ferroelectrics in the first quarter [30] 13 Figure 1.13 Diagram of hysteresis and energy storage density for (a) linear dielectrics, (b) ferroelectrics, (c) relaxor ferroelectrics, and (d) anti-ferroelectrics The green area in the first quadrant is the recoverable energy density Ureco, and the red area is the energy loss Uloss [32] 14 Figure 1.14 Schematic of the perovskite structure of BaTiO3 (a) Cubic lattice (above Curie temperature, 120oC), (b) Tetragonal lattice (below Curie temperature, 120oC) [34] 15 Figure 1.15 Reversal in the direction of spontaneous polarization in BaTiO3 by reversal of the direction of the applied field [35] 16 Figure 2.1 An overview of the various stages of the sol-gel process 21 Figure 2.2 Schematic representation of: (a) dip coating; (b) spin coating; and (c) spray coating [45] 22 Figure 2.3 Example of processing routes to obtain sol-gel spin coatings [46] 23 Figure 2.4 Flow diagram for producing BZT and BLZT sols 24 Figure 2.5 TGA-DSC plots of BZT material 25 ii Figure 2.6 Schematic of spin-coating and heat treatment process for producing BZT thin films 26 Figure 2.7 Spin-coating machine at International Training Institute for Materials Science (ITIMS) 27 Figure 2.8 Schematic representation of the Bragg‟s law for diffraction [48] 28 Figure 2.9 The working principle diagram of X-ray diffractometer and the PANalytical X‟Pert PRO system [48] 28 Figure 2.10 The schematic drawing of a Sawyer-Tower circuit used for hysteresis measurement of ferroelectric thin film [49] 30 Figure 2.11 Equipment for measuring ferroelectric properties of materials BLZT (aixACCT- TF2000) 31 Figure 3.1 Cross-sectional SEM images of BZT thin films, grown on Pt/Ti/SiO2/Si, at various annealing temperatures (a) 450 oC, (b) 500 oC, (c) 600 o C, (d) 650 oC, (e) 675 oC, (f) 700 oC 32 Figure 3.2 (a) XRD patterns of BZT thin films, grown on Pt/Ti/SiO2/Si, at various annealing temperatures, Ta, (b) Schematic of the dependence of phase transition on annealing temperature, Ta for BZT thin films 33 Figure 3.3 Polarization-electric field (P-E) hysteresis loops and (b) values of Pmax, Pr and Pmax - Pr for BZT thin films at various annealing temperatures The measurements were performed at 1000 kV/cm and kHz 34 Figure 3.4 Electric field dependence of Pmax and Pr values for BZT thin films at various annealing temperatures, measured until their corresponding electric breakdown strength (EBD) The data were calculated from the corresponding P-E loops 35 Figure 3.5 Dependence of volumetric energy-storage density Ustore, recoverable energy-storage density Ureco, and energy-storage efficiency (η) on applied electric field for BZT thin films at various annealing temperatures The data were calculated from the corresponding P-E loops 37 Figure 3.6 (a) Energy-storage densities, recoverable energy-storage and (b) energy-storage efficiency were measured at the corresponding EBD values, for BZT thin films at various annealing temperatures 38 Figure 3.7 (a) Dielectric constant – electric field (-E) curves and (b) dielectric loss curves of BZT thin films at various annealing temperatures, measurement at room temperature frequency kHz 38 Figure 3.8 SEM images of (a) BZT and (b) BL5ZT thin films 39 Figure 3.9 XRD patterns of BLZT thin films grown on Pt/Ti/SiO2/Si with various La-doping contents 40 Figure 3.10 Polarization-electric field (P-E) hysteresis loops and (b) values of Pmax, Pr and Pmax - Pr for BZT thin films with various La doping contents (0-8 mol.%) The measurements were performed at 1000 kV/cm and kHz 41 iii Figure 3.11 Electric field dependence of Pmax and Pr, values for La-doped BZT thin films at various La doping contents, measured until their corresponding electric breakdown strength (EBD) The data were calculated from the corresponding P-E loops 42 Figure 3.12 Dependence of volumetric energy-storage density (Ustore), recoverable energy-storage density (Ureco), and energy-storage efficiency (η) on applied electric field for BZT thin films with various La doping contents (a) 0%, (b) 3%, (c) 5%, (d) 8% The data were calculated from the corresponding P-E loops 43 Figure 3.13 (a) Energy-storage densities, recoverable energy-storage and (b) energy-storage efficiency were measured at the corresponding EBD values, for BZT thin films with various La doping contents 44 Figure 3.14 (a) Dielectric constant – electric field (-E) curves and (b) dielectric loss curves of BZT thin films with various doping content, measurement at room temperature frequency 1000 Hz 44 Figure 3.15 The operating-temperature dependence of (a) P-E loops, (b) Pmax, Pr and Pmax – Pr values for BZT thin film at an annealing temperature of 500 oC The measurements were performed at 4000 kV/cm and 1000 Hz 45 Figure 3.16 The operating-temperature dependence of (a) energy storage density (U) and (b) energy-storage efficiency (η) for BZT thin film at annealing temperature of 500 oC The measurements were performed at 4000 kV/cm and 1000 Hz 46 Figure 3.17 The frequencies-temperature dependence of P-E loops for BZT thin film at annealing temperature of 500 oC The measurements were performed at 4000 kV/cm and room temperature 47 Figure 3.18 The operating-frequencies dependence of (a) Pmax, Pr and Pmax - Pr values, (b) Ec values, (c) energy storage density (U) and (d) energy-storage efficiency (η) for BZT thin film BZT thin film at annealing temperature of 500 o C The measurements were performed at 4000 kV/cm and room temperature 47 Figure 3.19 a) Comparison of P-E hysteresis loops measured at different charge-discharge cycles, (b) Pmax and Pr values as a function of number chargedischarge cycles under an applied electric field of 4000 kV/cm and kHz, for the BZT thin film at annealing temperature of 500 oC The fatigue testing was performed by applying a bipolar electric field of pulse height 200 kV/cm and at pulse width 100 kHz (or μs) 49 Figure 3.20 Dependence of (a) energy storage density and (b) energy storage efficiency (η) on cycling for BZT thin film at an annealing temperature of 500 oC The data were calculated from the corresponding P-E hysteresis loops performed at 4000 kV/cm, kHz and room temperature 49 iv % due to the slimmer P-E loop, and then decreases with a further increase in dopant content From the above analysis, it is shown that the mol.% La-doped BZT film has a quite high recoverable energy-storage density (~7.0 J/cm3) and a large energy-storage efficiency (60.7%) under an EBD of 1650 kV/cm, which leads to the conclusion that the La-doped BZT thin films are promising lead-free candidate materials for environmentally friendly energy-storage devices Figure 3.13 (a) Energy-storage densities, recoverable energy-storage and (b) energystorage efficiency measured at the corresponding EBD values, for BZT thin films with various La doping contents 3.2.4 Dielectric properties In this section, to investigate the La-doping dependence of the dielectric constant, the ε–E curves of BZT films with various La doping contents from 8%, measured at 120 kV/cm and 1000 Hz, are shown in Figure 3.14(a) Figure 3.14 (a) Dielectric constant – electric field (-E) curves and (b) dielectric loss curves of BZT thin films with various doping content, measurement at room temperature frequency 1000 Hz The dielectric loss measurements as a function of bias voltage show a curvature similar to the tuning curves in Figure 3.14(b) It can be seen that both dielectric constant and dielectric loss of the films at zero bias increase for Ladoped BZT thin films (Table 3.4), reaching a maximum value of 164 for 3%.mol 44 La-doped BZT thin film This is due to the improvement of the crystallinity of BZT films can be achieved by La-doping (as shown in the XRD analysis) Table 3.4 The measured dielectric constant, dielectric loss for BZT thin films with various La doping contents (0 - 8%) La doping (%) Dielectric constant Dielectric loss 22 164 157 130 0.008 0.020 0.019 0.009 3.3 Thermal stability, frequency stability, and fatigue endurance 3.3.1 Thermal stability It is well known that industrial implementation not only demands high energy storage performance at room temperature but also requires good thermal stability The recent urgent need for the high-temperature capability of energy storage device-based dielectric capacitors has increased for new applications, such as automotive industries (as hybrid electric vehicles with a working temperature of ~140 ) [50] and underground oil and gas explorations (working temperature of ~200 ) [51] In this section, the temperature-dependent polarization of BZT thin films at an annealing temperature of 500 oC was measured at a wide range of operating temperatures (30–200 oC) Figure 3.15 (a) indicates that the P-E loops, measured at 4000 kV/cm and 1000 Hz frequency, gradually slightly broader with increasing operating temperature Figure 3.15 The operating-temperature dependence of (a) P-E loops, (b) Pmax, Pr and Pmax – Pr values for BZT thin film at annealing temperature of 500 oC The measurements were performed at 4000 kV/cm and 1000 Hz 45 As shown in Figure 3.15 (b), it can be seen that both maximum polarization (Pmax) and remanent polarization (Pr) increase slightly, however, the value of (Pmax – Pr) is almost constant as the operating temperature increases Based on the data from P-E loops, the values of Ustore and Ureco were calculated, as shown in Figure 3.16 (a) As the operating temperature increases, the Ustore values exhibit a slight increase, and the Ureco values almost insignificantly changed, resulting in a mild decrease in energy storage efficiency () values, as shown in Figure 3.16 (b) The minor fluctuation in Ureco is less than 2.4 % and the change of  is less than 5.9 %, suggesting good temperature stability throughout a wide range of operating temperatures Figure 3.16 The operating-temperature dependence of (a) energy storage density (U) and (b) energy-storage efficiency (η) for BZT thin film at annealing temperature of 500 o C The measurements were performed at 4000 kV/cm and 1000 Hz 3.3.2 Frequency stability In this section, the frequency-dependent polarization of BZT thin films at an annealing temperature of 500 oC was measured at a wide range of operating frequencies (100–10000 Hz) The P-E hysteresis loops of BZT thin films measured at 4000 kV/cm, room temperature, and a frequency (f) range from 100 to 10000 Hz are shown in Figure 3.17 The results indicated that the frequencydependence is not very significant, although a tiny change in the shape of the P-E loops (Figure 3.17 (a) and (b)) would have a visible impact on maximum polarization (Pmax), remanent polarization (Pr) and coercive field (Ec) This change in polarization with frequency is mostly due to the effect of the mobile defect (such as oxygen vacancies) are easily accumulated near the film/electrode interfaces, because of their high mobility vacancies, they form the interfacial layers under an external electric field As shown in Figures 3.18 (a) and (b), it is observed that Pmax, Pr, Pmax - Pr, and Ec values are almost no significant change in the low-frequency region (100– 1000 Hz) Such properties can be related to the nucleation rate Under such a low-frequency region, most nuclei involved in the transformation might originate in the defects at the film/electrode interfaces and/or inside the film, and so arise 46 as soon as the electric field is applied Thus, the nucleation rate occurs slowly in the low-frequency region Figure 3.17 The frequencies-temperature dependence of P-E loops for BZT thin film at annealing temperature of 500 oC The measurements were performed at 4000 kV/cm and room temperature Figure 3.18 The operating-frequencies dependence of (a) Pmax, Pr and Pmax - Pr values, (b) Ec values, (c) energy storage density (U) and (d) energy-storage efficiency (η) for BZT thin film at annealing temperature of 500 oC The measurements were performed at 4000 kV/cm and room temperature However, in the higher field region (1000-10000 Hz), both Pr and Ec increase rapidly with increasing frequency due to the increase in the number of 47 nucleation sites for opposite domains during the transformation The shape of the P-E loops becomes broadened with decreasing frequency in the high-frequency region (1000–10000 Hz), as shown in Figure 3.17 (b) Thus, larger energy loss is observed, leading to lower energy efficiency, as presented in the next section From the P-E loops of BZT thin films measured at 4000 kV/cm and under different operating frequencies, the energy-storage density and energy-storage efficiency are calculated and presented in Figures 3.18 (c) and (d) As the operating frequency rises from 100 to 1000 Hz, both Ustore and Ureco values exhibit an insignificant change, which is mainly due to the constant of the Pmax – Pr value Thus, the value of  with the rise in frequency is also almost unchanged, and a value of 87.2 % is achieved at 1000 Hz However, when the operating frequency exceeds 1000 Hz, the Ureco values exhibit a decrease from 10.2 to 8.4 J/cm3, and the Ustore values decrease slightly, resulting in a reduction of  values from 87.8% to 74.7% It can be clearly seen that a high Pmax –Pr and small Ec are achieved at low the frequency region (100-1000 Hz), leading to a high Ureco a large  value under an applied electric field of 4000 kV/cm These results indicate that high energy storage density and efficiency can be obtained simultaneously by tuning the operating frequencies 3.3.3 Fatigue endurance Long-term stability during the charge-discharge cycling process is considered to be an important factor for the practical application of energystorage capacitors in pulse-power electronic systems The superior chargedischarge endurance guarantees the operation of energy storage devices over long-duration cycling Thus, fatigue in polarization and piezoelectric properties has been one of the most significant topics of academic research in the past several decades In this section, fatigue in polarization and electrical properties of BZT thin films at an annealing temperature of 500 oC will be presented Figure 3.19 shows the fatigue behavior of the BZT thin film at an annealing temperature of 500 oC as a function of the charge-discharge cycles up to 109 cycles, and Figure 3.19 (a) gives the P-E loops after 0.1, 104, 106, and 109 cycles, (b) Pmax, Pr, and Pmax – Pr values as a function of number charge-discharge cycles, which were performed at 4000 kV/cm, kHz and room temperature The fatigue testing was performed by applying a bipolar electric field of pulse height 200 kV/cm and at pulse width 100 kHz (or μs) Clearly, no significant difference was found in the P-E loops, as well as Pmax, Pr, and Pmax – Pr values, with fatigue-free behavior observed for this sample Both Pmax and Pr values remain steady at 5.7 and 0.4 µC/cm2, respectively, and Pmax – Pr is 5.3 µC/cm2, resulting in relative stability for Ustore and Ureco and  values with a small ~1 % fluctuation, as shown in Figures 3.20 (a) and (b) 48 Figure 3.19 a) Comparison of P-E hysteresis loops measured at different chargedischarge cycles, (b) Pmax and Pr values as a function of number charge-discharge cycles under an applied electric field of 4000 kV/cm and kHz, for the BZT thin film at an annealing temperature of 500 oC The fatigue testing was performed by applying a bipolar electric field of pulse height 200 kV/cm and at pulse width 100 kHz (or μs) Figure 3.20 Dependence of (a) energy storage density and (b) energy storage efficiency (η) on cycling for BZT thin film at an annealing temperature of 500 oC The data were calculated from the corresponding P-E hysteresis loops performed at 4000 kV/cm, kHz, and room temperature Clearly, an excellent charge-discharge cycling life with fatigue-free performance up to 109 cycles was observed for amorphous BZT thin films with metallic Pt layers as the top and bottom electrodes For the ferroelectric perovskite thin films, strong polarization fatigue is always observed [52, 53] This difference is attributed to the fact that for PZT thin-film capacitors, in each fatigue cycle, the polarization switching occurs at coercive fields less than the maximum applied field According to the Feng Yang study [54], during domain switching in ferroelectric materials, the mobile charged defects (such as oxygen vacancies) are easily accumulated near the film/electrode interfaces, because of their high mobility, they form the interfacial layers under an external electric field [55] In this study, the amorphous BZT films remain in linear-like behavior Hence, the formation of interfacial layers does not occur during switching in this film Alternatively, no degradation in polarization, or the energy storage performance, of BZT films is observed under the fatigue conditions 49 Conclusions In this chapter, the effects of annealing temperatures (from 450 to 700 ) and La doping (0 – mol.%) of BaZr0.25Ti0.75 (BZT) thin films on microstructure, dielectric, and energy storage performances were systematically investigated Moreover, the operating temperature and frequency dependence of the energy storage performance and fatigue behavior of BZT thin films have been investigated in the frequency range from 100 to 10000 Hz and the operating temperature ranging from 30 °C to 200 °C It was found that the degree of crystallization and surface roughness of the films increased with the increasing temperature The result indicated the formation of crystallization of the perovskite BZT phase achieved at 700 °C The results show that the BZT film annealed at 500 oC has the highest recoverable energy-storage density, reaches 30.9 J/cm3 value, and a large energystorage efficiency with 67.8% under a high 7000 kV/cm breakdown strength An increase in the dielectric constant with increasing annealing temperature can be attributed to the effect of crystal structure formation The results indicated that the BZT films annealed at 700 °C achieved the highest dielectric constant due to the formation of a pure perovskite crystal structure The BZT films exhibit a great temperature-dependent energy storage performance stability in wide temperatures (from 30-200 °C) The results show that as the operating temperature increases, the Ustore values exhibit a slight increase, and the Ureco values almost insignificantly changed, resulting in a mild decrease in energy storage efficiency () values The minor fluctuation in Ureco is less than 2.4 % and the change of  is less than 5.9 % The frequency dependence of the energy storage performance of BZT thin films has been investigated in the frequency range from 100 to 10000 Hz The results indicated that both Pr and Ec values are proportional to frequency A high Pmax - Pr and small Ec are achieved at low the frequency region (100-1000 Hz), leading to a high Ureco a large  value under an applied electric field of 4000 kV/cm when the operating frequency exceeds 1000 Hz, the Ureco values exhibit a decrease from 10.2 to 8.4 J/cm3, and the Ustore values decrease slightly, resulting in a reduction of  values from 87.8% to 74.7% Moreover, an excellent charge-discharge cycling life with fatigue-free performance up to 109 cycles was also realized in the BZT thin films The results indicated that both Pmax and Pr values remain steady, resulting in relative stability for Ustore, Ustore, and  values with a small ~1 % fluctuation La-doped BZT thin films indicated prominently increasing relaxor behavior with increasing La-doping concentration BLZT thin films with mol.% La-doping simultaneously exhibit a quite high recoverable energy-storage density (~7.0 J/cm3) and a large energy-storage efficiency (60.7%) under an EBD of 1650 kV/cm Furthermore, the dielectric constant of the La-doped BZT thin 50 films was found to be significantly improved, reaching the maximum value of 164 for 3%.mol La-doped BZT thin film 51 CONCLUSIONS In this study, lead-free Ba(Zr0.25Ti0.0.75)O3 (BZT) and La-doped BZT (BLZT) thin films, grown on Pt/Ti/SiO2/Si substrates, were fabricated successfully by a sol-gel spin-coating method The main research results have been achieved as follows:  BZT thin film The effect of annealing temperatures (450 °C - 700 °C) of BZT thin films on the microstructure was systematically investigated The results indicated the degree of crystallization of the films increased with the increasing annealing temperature of films As XRD result, the formation of crystallization of the perovskite BZT phase was achieved at 700 °C The film annealing temperature dependence of the energy-storage properties indicated the BZT film annealed at 500 °C, amorphous phase, has the highest recoverable energy-storage density, reaches 30.9 J/cm3 value, and a large energy-storage efficiency with 67.8% under a high 7000 kV/cm breakdown strength Dielectric constant of the BZT films rises with increasing annealing temperatures This is due to the effect of the formation of a pure perovskite crystal structure The BZT films exhibit a great temperature-dependent energy storage performance stability in a wide temperatures (from 30 - 200 °C) The results show that as the operating temperature increases, the Ustore values exhibit a slight increase, and the Ureco values almost insignificantly changed, resulting in a mild decrease in energy storage efficiency () values The minor fluctuation in Ureco is less than 2.4 % and the change of  is less than 5.9 % The frequency dependence of the energy storage performance of BZT thin films has been investigated in the frequency range from 100 to 10000 Hz The results indicated that both Pr and Ec values are proportional to frequency A high Pmax – Pr and small Ec are achieved at low the frequency region (100-1000 Hz), leading to a high Ureco and a large  value under an applied electric field of 4000 kV/cm when the operating frequency exceeds 1000 Hz, the Ureco values exhibit a decrease from 10.2 to 8.4 J/cm3, the Ustore values decrease slightly, resulting in a reduction of  values from 87.8% to 74.7% Moreover, an excellent charge-discharge cycling life with fatigue-free performance up to 109 cycles was also realized in the BZT thin films The results indicated that both Pmax and Pr values remain steady, resulting in relative stability for Ustore, Ureco, and  values with a small ~1 % fluctuation  BLZT thin film The effect of La doping of BZT thin films (BLZT) on microstructure was systematically investigated The results indicated that La doping enhanced the 52 ability of crystallization of the films As XRD result, the formation of crystallization of the perovskite BZT phase was achieved at 650 °C La-doped BZT thin films indicated prominently increasing relaxor behavior with increasing La-doping concentration BLZT thin films with mol.% La-doping simultaneously exhibit a quite high recoverable energy-storage density (~7.0 J/cm3) and a large energy-storage efficiency (60.7%) under an EBD of 1650 kV/cm Compared with undoped-BZT thin film, the dielectric constant of the Ladoped BZT thin films were found to be significantly improved, reaching the maximum value of 164 for 3%.mol La-doped BZT thin film Future research directions are expected: Investigate the effect of thickness on dielectric, and energy storage properties for BZT thin films Investigate the effect of various annealing temperatures on microstructure, dielectric, and energy storage performances for La-doped BZT thin films (BLZT) Investigate the thermal stability, frequency stability and fatigue endurance of capacitors using La-doped BZT (BZLT) materials 53 LIST OF PUBLICATIONS Minh D Nguyen, Doan T Tran, Ha T Dang, Chi T Q Nguyen, Guus Rijnders and Hung N Vu (2021), “Relaxor-ferroelectric films for dielectric tunable applications”: effect of film thickness and applied electric field, Materials 2021, 14, 6448 Tran Thi Doan, Nguyen Duc Minh, Vu Ngoc Hung, “ Impact of Ladoping on ferroelectric and energy storage properties of sol-gel BZT thin films”, SPMS 2021 - The 12th National 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