Synthesis And Optical Properties Of Low Diamention Bi-based Oxide Materials

Bismuth titanate (Bi4Ti3O12, BIT) ferroelectric materials have received much attention because of its low conductivity, higher fatigue resistivity, high dielectric constant, etc. It can be used in the applications of pyroelectric detectors, ultraviolet detectors, uncooled infrared detectors, infrared focal plane arrays and ferroelectric memories due to the unique optical and electrical properties. As we know, bismuth titanate belongs to the family of Aurivillius compounds consists of three (Bi2Ti3O10)2-blocks sandwiched between two (Bi2O2)2+sheets along the tetragonal c axis. More importantly, BIT films can be deposited at675℃, which is significantly lower than the synthesis temperature of other layer-type ferroelectric materials such as SrBi2Ta2O9. In addition, compared with undoped BIT, lanthanum (La)-substituted bismuth titanate (Bi4-xLaxTi3O12, BLT) has the significant advantages in electrical properties such as larger remnant polarization (Pr), smaller coercive field (Ec), lower dielectric loss and higher fatigue resistivity. The electrical properties of the BLT bulk and film materials have been the most extensively studied, while the detailed microstructure and optical properties, which are directly related to electronic band structures, have not been fully clarified. In addition, among multiferroic materials such as BiFeO3(BFO), BiMnO3, YMnO3, TbMnO3and HoMnO3, BFO is known to be the only one with simultaneous ferroelectric (Tc≈1103K) and G-type antiferromagnetic (TN≈643K) orderings at room temperature. BFO film has a large remanent polarization of about100μC/cm2along the [111]c or [001]h directions of pseudo-cubic or hexagonal structure in theory and experiments, as compared to its bulk counterpart. Therefore, it can be used for nonvolatile ferroelectric random access memories (NVFRAMs), which has large capacity, ultra-fast operating speed (10-9s), low-power consumption, and high ratio of resistance in forward and reverse directions. However, there are some obstacles to be overcome in BFO films for NVFRAMs:high leakage current, ferroelectric reliability, high coercive field, the formation of the schottky contact between metallic electrode and Si, etc. Chemical modification of the Bi site for BFO is expected to improve the ferroelectric behaviors. Among the doping elements, the rare earth elements such as La are adopted to enhance ferroelectric properties and decrease leakage current because of the reduced oxygen vacancies by stabilizing oxygen octahedron. Moreover, it has been reported that the Mn3+(d5configuration) substitution of Fe3+site (Fe3+, d4configuration) can increase the resistivity in the high electric field region because of the formation of more distorted (Fe, Mn)O6octahedra. Thus, ferroelectric behaviors of (La, Mn)-substitutions BFO films on n+-Si substrates should be investigated systematically for the applications of spintronics, multiple-state memories, information storage process, and uncooled infrared sensors, etc.In this work, iron-doped titanium dioxide, lanthanum (La)-substituted bismuth titanate and (La and Mn)-substitutions BFO films and nanotubes have been prepared by the Sol-Gel technology due to some superiorities:a fast fabrication process, large-area deposition, composition control, and low cost. The studies of these optical and electrical properties are helpful for the applications of optoelectronic and memories devices. The main works and innovations of this dissertation are listed in details as following:Ⅰ. Bi3.25Lao.75Ti3O12nanotube arrays with the different outer diameters of about50,100, and200nm have been prepared by the template-assisted Sol-Gel method.(a) The Raman scattering enhancement has been observed from highly ordered ferroelectric Bi3.25La0.75Ti3O12nanotube arrays. The enhancement factor are evaluated to be about92,257, and623corresponding to the outer diameters of50,100, and200nm, respectively. The phenomena can be attributed to the unique surface, microstructure, grain size, and tensile stress in the curved nanotube walls.(b) The photoluminescence emissions are enhanced signifcantly and the ferroelectricity of BLT-NT arrays have been well remained.(c) The optical and electrical behaviors of the BLT nanostructure demonstrate that it is suitable for fabricating multifunctional devices.Ⅱ. Based on the investigations of the optical and electrical properties of the BLT nanotube arrayes, which is suitable for fabricating multifunctional devices, we expectate to combine as many as physicochemical properties of BiFeO3material for fabricating multifunctional devices. As we know, BFO is the only one with simultaneous ferroelectric (Tc≈1103K) and G-type antiferromagnetic (TN≈643K) orderings at room temperature. However, there are some obstacles to be overcome in BFO film:high leakage current, ferroelectric reliability, high coercive field, the formation of the schottky contact between metallic electrode and Si, etc. As the first step, the optical, electrical and magnetic properties of the bismuth ferrite films should be investigated in detail,(a) Based on the hypothesis of Spaldin, La-substituted Bi1-xLaxFe0.92Mn0.08O3(BLFMx,0≤x≤0.2) films were directly deposited on heavily As-doped Si(100) by the Sol-Gel technology.(b) The XRD analysis shows that the films exhibit pure perovskite phase structure. The rhombohedral structure is distorted to a tetragonal one by doping Mn and La elements.(c) Moreover, the substitutions of Mn and La impress the leakage current density and enhance the ferroelectric behavior by reducing oxygen vacancies, formation of homogeneous microstructure, stabilizing perovskite structure, inducing lattice distortion and so on. These results could be crucial for the silicon-based technology, non-volatile ferroelectric random access memory applications.(d) It is important to further investigate the optical, ferroelectric, and magnetic properties of the bismuth ferrite nanotubes, which is suitable for fabricating multifunctional devices.III. Rutial and anatase iron-doped titanium dioxide nanocrystalline (TiO2：Fe) film with differente compositions have been deposited on Si(100) substrates by a facile nonhydrolytic sol-gel route.(a) For the rutile TiO2:Fe films, as the Fe concentration increasing, the surface becomes more dense, Raman-active phonon modes are shifted toward a lower frequency side, and the broadening of the infrared active phonon modes are changed. A four-phase structure model and the Adachi dispersion function as well as ellipsometric data fitting were used to extract the optical constants (band gap and absorption coefficients) in the near infrared-visible-near ultraviolet range. It suggests that the band gap of titanium dioxide is reduced by the introduction of a new level (Fe t2g) within the band gap. In addition, iron induces the phase transform from rutile to anatase.(b) For the iron doped anatase TiO2polycrystalline films, as the Fe concentration increasing, the intensity of Raman active phonon mode B1g (515cm-1) increases, while that of the Aig (519cm-1) phonon mold decreases. The second active Raman modes may be are originated from surface morphology, oxygen vacancies, crystal defects and so on. The intensity of the photoluminescence peaks at room temperature decreases rapidly with increasing iron concentration since the Fe incorporation could prolong the radiative life time and/or shorten the non-radiative life time. Finally, we study the low temperature photoluminescence spectrum of the anatase Fe:TiO2films, the emissions comes from oxygen vacancy (1.9eV), localized exciton (2.0eV and2.2eV), self-bound exciton (2.4eV), and the X1a and F1b indirect transition (2.6eV).