Effects Of Annealing Temperature And Doping On Electric And Mechanics Properties For Titanate Ferroelectric Thin Films

With prominent properties such as fast reorientation at low operating voltage, low power consumption, high sensitivity for a wide dynamic range, low hysteresis and high energy density, ferroelectric thin film is proper to apply at memory unit, sensor, MEMS, etc. The titanate ferroelectric thin films can be a candidate for the device applications, due to its lead-free composition, excellent ferroelectric and piezoelectric properties. More and more researchers are focused on the preparation and properties of the titanate ferroelectric thin films. However, almostly researchers focused on the electric properties for the titanate ferroelectric thin films, but rarely studied on the electric and mechanics properties such as piezoelectric performance, residual stress, relaxor property and domain evolution. The piezoelectric coefficient is key parameter for ferroeletric thin films applied to the MEMS, the residual stress is critical aspect for performance of ferroelectric thin film and reliability of ferroelectric advice, and relaxor properties are associated with safe working temperature, thermal stability and working efficiency for piezoelectric transducers. Meanwhile, for ferroelectric thin films, the domain state is produced by its own uniformity and mechanical constraint. The structure, type, number and evolution of domain decide physical properties and the applied direction of ferroelectric thin films. Therefore, it’s worth a thorough research on the piezoelectric properties, residual stress and relaxation behavior for titanate ferroelectric thin films.The Bi4Ti3O12(BIT)-based and Na0.5Bi0.5TiO3(NBT)-based ferroelectric thin flims were prepared by metal organic deposition (MOD) method at different annealing temperatures, and effects of annealing temperature and doping on the electric and mechanics properties were studied in details for titanate thin films, in this paper. The the results of this paper are listed as follows:1. The Bi3.25Eu0.75Ti3O12thin films were deposited by a MOD method at annealing temperature600-800°C. Using field-emission scanning electron microscope (FE-SEM) and XRD the microstructure of BET0.75thin films were identified. The piezoelectric properties were obtained by scanning force microscopy (SPM). Furthermore, the mechanisms concerning the dependence of the enhancement d33are discussed according to the phenomenological equation. The results show:(a) There are a few pinholes and voids on the BET0.75film surfaces but crack-free.(b) Thin films are of the Bi-layer pervoskite structure.(c) The largest remnant polarization2Pr67.3μC/cm2and piezoelectric coefficient65.7pm/V of BET thin film were obtained with annealing at700°C.(d) The mechanisms concerning the dependence of the enhancement d33are discussed according to the phenomenological equation.2. The Bi3.15Eu0.85Ti3O12thin films were deposited by a MOD method at annealing temperature600-750°C. Using FE-SEM and XRD the microstructure of BET0.85thin films were identified. The piezoelectric properties were obtained by SPM, and chemical composition was investigated by EDS. Furthermore, the mechanisms concerning the dependence of the enhancement d33are discussed according to the Bi3+ions volatile. The results show:(a) Thin films are of the Bi-layer pervoskite structure.(b) The optimized annealing temperature700°C is determined, and Bi3+ions volatile quantity increases with the annealing temperature, and it enhances sharply for BET thin film annealed at750°C.(c) The largest remnant polarization2Pr81.7μC/cm2and piezoelectric coefficient46.7pm/V of BET0.85thin film were obtained with annealing at700°C.(d) The degeneration for electric and piezoelectric properties at high annealing temperature is discussed by non-stoichiometric defects and oxygen vacancy induced by Bi3+ions volatile.3. The trilayered Bi3.15Eu0.85Ti3O12/Bi3.15Nd0.85Ti3O12/Bi3.15Eu0.85Ti3O12(BET/BNT/BET) thin films were deposited by a MOD method at annealing temperature600-750°C. The microstructure and electric properties were studied, the mechanical properties were investigated by nanoindenter, and the residual stress was calculated by traditional sin2ψ method. Meanwhile, the enhanced polarization property can be interpreted by M-W capacitor model. The results show:(a) The BET/BNT/BET ferroelectric thin films are polycrystalline without preferred orientation, and there are the typical Bi-layered perovskite polycrystalline phase without any pyrochlore or other phase related to Eu and Nd, indicating that the substitution of Nd3+and Eu3+to Bi3+occurs.(b) All the surfaces are crack-free, and they are of dense surface.(c) For the trilayered thin film annealed at700°C, remnant polarization2Pr (100.1μC/cm2), and dielectric constant εr (1391) are largest among all thin films. And they are larger than others of pure BET and BNT thin films.(d) The enhanced mechanism of εr is interpreted by M-W capacitor model, in which the discontinuous interfaces of dielectric layers with different conductivities, and overall polarization effects are dominated by charge accumulation at discontinuous interfaces within the dielectric. 4. The A-and A-sites cosubstituted Bi3.15(Eu0.7Nd0.15)Ti3O12(BENT) thin films were prepared by MOD at different annealing temperatures from600to750°C. The surface and interface profiles, crystalline structure and chemical composition were investigated with FE-SEM, XRD and EDS. The ferroelectric, dielectric and leakage current properties were measured by ferroelectric tester, impedance analyzer and semiconductor characterization system. The mechanical properties were investigated by nanoindenter, and the residual stress were calculated by traditional sin2ψ method. And the enhanced remnant polarization is explained by cosubstitution and deformation effect of Ti-O octahedron. The results show:(a) Bi-layered perovskite polycrystalline phase has no any pyrochlore phase in BENT thin films.(b) A uniform, compact and dense surface is observed in BENT thin films.(b) The optimized annealing temperature650°C is determined, it is lower than those of A-site substituted BIT thin films (700°C).(c) The remnant polarization2Pr and dielectric constant εr are103μC/cm2and1086for BENT thin film annealed at650°C, and it is of the largest εr and2Pr among the thin films.(d) The residual stress values are calculated as-743,-622,-547and-530MPa by traditional sin2ψ method for BENT thin films annealed at600,650,700and750°C.(e) The enhanced remnant polarization is explained by cosubstitution and deformation effect of Ti-O octahedron for the A-and A-sites.5. The0.94(Na0.5Bi0.5)TiO3-0.06BaTiO3(NBT-BT6) relaxor thin films have been fabricated by MOD. The effects of annealing temperature on microstructure, and on piezoelectric, ferroelectric, and dielectric properties were investigated by AFM, FE-SEM, XRD, SPM, and use of a ferroelectric tester and an impedance analyzer. The relaxation properties of NBT-BT6thin films were characterized by use of εr-T plots acquired by means of an impedance analyzer and a high-temperature sintering furnace. The in-plane residual stress in polycrystalline NBT-BT6ferroelectric thin films were evaluated by the orientation average method. Meanwhile, the mechanisms concerning the effect of residual stress on the porlazation response was discussed. The results show:(a) These are indicative of a polycrystalline structure without any preferred orientation, and of the perovskite and pyrochlore phases.(b) Among the thin films, NBT-BT6thin film annealed at750°C has the largest effective piezoelectric coefficient95.1pm/V.(c) For the NBT-BT6thin film annealed at750°C, a wide temperature range,183-210°C, around the phase transition temperature was observed in the dielectric temperature plots, and the diffusion coefficients were quantitatively assessed as1.6,1.78, and1.6.(d) The effect of residual stress on the polarization properties is discussed by Landau-Devonshire theory. The simulation result shows the P3decreases with the tensile residual stress, while increases with the compressive stress. The simulation result is in agreement with our experimental result.6. According to the optimized annealing temperature determined by the study for the NBT-BT6relaxor ferroelectric thin films, the (1-0.01x)(Na0.5Bi0.5)TiO3-0.01xBaTiO3(NBT-BTx)(x=4,5,6,7and8) relaxor thin film have been fabricated by MOD annealed at750°C. The microstructure of NBT-BTx thin film was investigated by FE-SEM and XRD. The initial domain structures of NBT-BT5thin film are obtained by PFM. Meanwhile, the domain evolution and piezoelectric properties of ferroelectric thin films is observed by PFM under different external electric fields and forces. The main results are as follows.(a) A irregularly grain size surfaces with unclear grain boundaries and pores were obtained.(b) This is indicative of a polycrystalline structure with (100) orientation, and of the perovskite and pyrochlore phases.(c) We have observed90°domain switching due to the external mechanical forces exerted by the SFM tip on the surface of NBT-BT5thin film. Meantime, the movement of90°domain wall driven by the mechanical force is observed.(d) The domain was written by opposite DC voltages for NBT-BT5thin films. The written area was then keeping in environment for different duration to detect the retention of the thin film and the retention loss is low. The result shows the NBT-BT5thin film is a potential material for memory unit application.(e) The piezoelectric coefficients46-110pm/V were obtained for NBT-BT5thin film.We expect that the research can offer useful guidelines to the design of titanate ferroelectric thin film devices.