Growth And Properties Of Perovskite Ferroelectric Device Materials

Due to the process of industrialization and population growth, global energy consumption increased significantly in the past few decades, the whole community is faced with the depletion of fossil energy and environmental issues. These challenges can be resolved through renewable energy. Producing nano-generator is particularly attractive, because it can take advantage of the human biomechanics energy such as heart beat, blood circulation, muscle stretch, and put it into electricity power to support the implanted biological devices. Piezoelectric material is multi-functional, which has a wide range of applications from medical diagnosis and treatment, industrial process control, environmental monitoring, automotive and robotics industry to multimedia and communication. The piezoelectric material can convert mechanical energy to electrical energy, or convert electrical energy into mechanical energy. This nature is particularly prominent in the solid solution of the perovskite type ferroelectric. However, for the low-power electronic devices driven indoor or closed environment and implantable biological devices, there is still need innovative approaches to achieve. Fabricating high voltage output ferroelectric generator by using P(VDF-TrFE) can get higher output than other devices with similar structure. Relaxor perovskite PMN-PT/PIN-PMN-PT single crystal is a very promising compound as the candidate of ferroelectric material for electromechanic transducers due to its excellent piezoelectric coefficients d33, ultrahigh strain levels with low hysteresis, high dielectric constant, large electro-optic coefficients, and large electromechanical coupling factor k33. Physical properties of PMN-PT are sensitive to Ti content, poling strength, crystallographic orientation. The structure of PMN-PT contains the coexistence of neighboring phases in a broad composition range near the morphotropic phase boundary (MPB). Although the dielectric permittivity, domain structure, hysteresis loop, and piezoelectric have been widely investigated, the reports on electronic transitions of PMN-PT are still scarce. On the other hand, the investigations on PMN-PT crystals at low temperature have been mainly focused on the phase transition behavior and domain structures. Up to date, less attention has been paid to the optical properties, especially the temperature dependence of electronic structures for PMN-PT materials. As we know, transmittance spectra can provide optical band gap (OBG), optical constants, absorption characteristics, band tail state behavior, and optical phonon modes. In this thesis, we analysis the optical properties of ferroelectrics and provide a electronic band model. And we use the perovskite ferroelectrics to make thin film nano-generator which has a high-voltage performance and large current output. The following is the main works and results of this dissertation.1. The transmittance spectra of Pb(Mg1/3Nb2/3)O3-PbTiO3single crystals have been studied and we found that the temperature dependence of direct band gap Egd, indirect band gap Egi, and phonon energy Ep for different PMN-PT crystals. A phenomenological model of band structure vs. temperature is proposed to explain the negative and positive band narrowing coefficient dEgdldT in ferroelectric PMN-PT/PIN-PMN-PT crystals around the MPB where multiple phases coexist.The transmittance spectra of (1-x)Pb(Mg1/3Nb2/3)03-xPbTi03(x=0.24and0.31) single crystals have been studied in the temperature range of5.3-300K. It was found that the direct band gap Egd is3.150±0.016eV, indirect band gap Egi is2.939±0.014eV, and the phonon energy Ep is0.098±0.014eV for the PMN-0.24PT crystal at300K. With increasing the temperature, the Egd of the PMN-0.24PT crystal decreases from3.263±0.017to3.150±0.016eV while the Egd of the PMN-0.31PT crystal increases from3.050±0.015to3.101±0.016eV. The peculiar characteristic can be ascribed to the monoclinic and rhombohedral multiphase coexistence in the PMN-0.31PT crystal. The optical properties of ferroelectric Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3(PIN-PMN-PT) single crystals around the morphotropic phase boundary (MPB) have been investigated using ultraviolet-infrared transmittance spectra in the temperature range of8-300K. Based on the temperature-dependent spectral measurement of the band gap, we propose a phenomenological model of band structure vs. temperature to explain both the negative and positive band narrowing coefficient dEgdldT in ferroelectric PIN-PMN-PT crystals around the MPB where multiple phases coexist. The peculiar positive coefficient only exists in the fragile multiphase region of the MPB, while the negative coefficient, caused by thermal expansion of the lattice and renormalization of the band structure by electron-phonon interaction, exists in the rhombohedral or tetragonal single-phase region as well as in the stationary multiphase region of the MPB. The origin of the positive coefficient is a long-range increasing fraction of coexistence from the monoclinic phase with small band gap to rhombohedral phase with large band gap at elevated temperature. In agreement with optical transmittance results of PMN-PT/PIN-PMN-PT, the model predicts that these unusual positive band narrowing coefficients may exist for all ferroelectrics around the MPB where the coexistence of phases lacks thermodynamic stability.2. Anomalous structure deformations involving octahedral rotations and tilting angle of polarization can be found around200K in by Raman spectra and XRD. The reflectance spectra of LaNiO3film on silicon under different applied voltage are studied. The energy of1.96eV is corresponding to the O2p to Ni3d electronic transition, which increases with increasing applied voltage.We report direct observation for the structural transformations of relaxor ferroelectric Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3single crystals with the aid of temperature-dependent polarized Raman scattering. In addition to the two well-known phase transitions above room temperature, anomalous structure deformations involving octahedral rotations and tilting angle of polarization can be found around200K. A comparison of experimental results with the Devonshire expansion of the free energy allows us to elucidate the peculiar characteristic as the variation of volume fractions among coexistence of three phases, including a first-order phase transition between the orthorhombic and rhombohedral phases and a second-order phase transition between the monoclinic and orthorhombic ones at low temperature. We provides direct observation of the phase/structural transitions of ferroelectric materials. The reflectance spectra of LaNiO3film on silicon have been investigated in the wavelength range of190-2650nm (0.47-6.5eV) under different external direct-current voltage. The Drude-Lorentz dispersion model is used to extract the optical function. The O2p to Ni3d electronic transition can be uniquely assigned to the energy of about1.96eV and decreases with decreasing applied voltage. The discrepancy from the real part of dielectric function with the applied voltage has a strong spectral dependence. The optical conductivity variation under different external voltage indicates that the electrical field can induce the modification of the carrier transport.3. The temperature dependences of the optical properties of Bi3.25La0.7sTi3O12films have been studied. The tetragonal phase of the BLT film transforms to orthorhombic phase at300K and then to monoclinic phase at160K. A P(VDF-TrFE) film device is made, its size is5*5mm,with the thickness of film3um. The voltage output is0.6V while the current output is3nA.The optical properties of perovskite (ABO3)-type Bi3.25La0.75Ti3O12(BLT) FE nanocrystalline films grown by using the sol-gel method have been investigated from the near-infrared to the ultraviolet photon energy region. The temperature dependences of the electronic properties in BLT nanocrystalline films have been determined by using the spectral transmittance at temperatures from80to480K. The maximum transmittance is strongly related to phase/structural transitions. The dielectric functions in the photon energy range of1.1-6.5eV have been extracted by fitting the experimental data with Adachi’s model. The dielectric function and the fundamental optical transition energy show an obvious dip pattern at160K and discontinuous variation at300K. Moreover, the long-wavelength refractive index n(0), which is related to the total effective number of valence electrons per atom in the materials, presents a similar trend. The result indicates that the tetragonal structure of the BLT nanocrystal gradually transforms to an orthorhombic structure and then to a monoclinic structure with decreasing temperature. The phase transition temperatures are located at about300and160K, respectively.Young’s Modulus are closely relative to microstructure of P(VDF-TrFE) films annealing at different temperature. The maximum Young’s modulus are achieved at the highest percentage of crystallinity. Young’modulus increases from0.47Gpa at the room annealing temperature to5.3GPa at130℃annealing temperature. Further increasing the annealing temperature leads to the decrease of Young’s modulus due to the decrease of crystallinity above melting temperature Tm. P(VDF-TrFE) generator is made, the size is5*5mm, the thickness of film is3um, the voltage output is0.6V, the current output is3nA.