Electronic Band Transitions Of Several Typical Functional Oxide Film Materials

For several decades, electronic products in the areas of microelectronics, optoelectronics, and solar cell based on informational and functional oxide materials have been widely used in our daily life. In particular, wide-band gap semiconducting oxides, transition-metal oxides, and perovskite structure oxides are the hottest research materials. This is because these oxide materials are the most promising oxide materials for applications in light emitting diodes, information storage memories, uncooled infrared focal plane arrays, spintronic quantum control and photovoltaic devices as transparent windows, electrode layers, and dielectric layers. In order to exploit its potential applications, it is significant to further investigate the optical and electronic properties of the oxide materials. Moreover, the optoelectronic transitions, which could be directly correlated with the electronic band structures, play an important role in the optoelectronic device design. By optical spectroscopy analysis, one can further clarify the electronic bands, phase transitions, and free carrier behavior. In order to make sure optoelectronic device works well at variable temperature, understanding and describing the evolutions of optical constants, electronic excitations, and absorption coefficient at different temperature are pre-requisite. The experimental results such as temperature dependence of the optical band gap (OBG) energy can provide the information about the electron-phonon interactions and optical excitation process. On the other hand, it is well-known that films are expected to yield better sensitivity and faster response than the bulk materials. Therefore, it is necessary to carry out a delicate study on the optoelectronic transitions of oxide film materials.Optical spectroscopy is a nondestructive probe technique, which is a powerful tool for optical characterization of functional oxides. By spectral measurements, one can determine the optical constants, lattice dynamics, OBG, Photoluminescence (PL) properties, and electronic transitions of the materials. These important informations correlate with the carrier mobility, physical transition, chemical composition, crystalline quality, energy level of impurities, and the presence of defects in the materials. In this dissertation, the optoelectronic transitions of several informational and functional oxide film materials have been investigated using optical spectroscopy. The main works and innovations of this dissertation are listed as follows:1. The influences from As doping concentration and growth laser energy on the phonon frequencies, PL properties, OBG, and optical constants of ZnO films have been obtained. The optoelectronic properties of magnetic elements doped ZnO and SnO2films have been studied. The magnetic element dependence of the optoelectronic and magnetic properties has been analyzed. It can be found that the electronic band structures change with the temperature and composition. The relationship among dielectric functions, phonon modes, OBG, and composition has been established.The optoelectronic transitions of As doped ZnO (ZnO:As) nanocrystalline films have been systematical studied. The influences from As doping concentration and growth laser energy on the the frequency of A1longitudinal optical (LO) phonon mode, PL properties, OBG, and optical constants in ZnO films have been obtained. The inner relationship between the microstructure and optical constants of the ZnO:As films has been established. The effects from growth laser energy on the electronic band structure of rutile TiO2films have been investigated. The experimental results reveal that the OBG is strongly influenced by the crystallinity and packing density of the films, which decreases with increasing the laser power energy. The effects from different transition metal (Cr, Mn, and Ni) on the RT ferromagnetic properties of ZnO films and its intrinsic origin have been studied. By magnetic, PL, and absorption spectral measurements, one can further find that the RT ferromagnetic properties strongly depend on the oxygen vacancies, oxygen defects, and doping elements. In particular, the competition between the above mechanisms can change the magnetic properties. The relationship among magnetic elements, OBG, and PL characteristics has been obtained. The doping magnetic composition dependence of the electronic band structure and optoelectronic transitions in Mn doped SnO2films have been systematical investigated. The variations of the electronic band structure in the films with the temperature and composition have been observed. A series of the physics law between composition and some physical parameters (dielectric functions, OBG, etc.) have been established.2. The evolutions of optoelectronic transitions, dispersion constants, and optical conductivity in VO2material near the metal-insulator transition (MIT) region have been discovered. The OBG decreases from about0.5eV (insulator state) to zero (metal state) when the MIT occurs. The energies of electronic transitions show the hysteresis behavior near the MIT region. The variations of phase transition temperature with the external electric field and its physical micro-origins in relaxation behavior have been discovered. A prototype of field effect transistor based on the VO2material has been fabricated.The optoelectronic transitions and evolutions of the phase transition under external electric field in VO2nanocrystalline film have been investigated. By electrical transport measurements and temperature-dependent Raman scattering technology, the variations of MIT with the external electric field and its relaxation behavior have been revealed. Thus, an efficient method has been found for manipulating the physics properties by adding external field. Moreover, above phenomena have been explained using the band structure theory. It indicates that the shortening of the V-V distance with the electric field and the change of carriers between electrons and the mixing of electrons and holes could be the major cause for the MIT. It further confirms the inner relationship between the MIT of bulk and the structural phase transition (SPT). Furthermore, the delayed temperature of SPT was increased by0.4℃/V with increasing the positive voltage because the electric field can pull the electrons and push the holes. Finally, we fabricated a prototype of field effect transistor based on the VO2material.The evolutions of the optoelectronic transitions in high quality VO2film material near the metal-insulator transition region have been investigated. The OBG decreases from about0.5eV (insulator state) to zero (metal state) when the MIT occurs, which can be confirmed directly by spectral results. This is because the a1g and egπ bands are moved close and finally overlap with the temperature. Moreover, the energies of interband electronic transitions show the hysteresis behavior near the MIT region, which offers the foundation for explaining the origins of the MIT of bulk and the SPT. On the other hand, the OBG of VO2film at low-temperature decreases with increasing the temperature. The three higher-order interband electronic transitions can be observed and uniquely distinguished. The temperature effects on the higher-order transition energy become much weaker.3. The temperature dependence of optical constants and OBG in BiFeO3film has been discovered. Four interband optoelectronic transitions related to the charge transfer excitation from Fe to O can be uniquely assigned. Two magnetic transitions located at about150and200K in the BiFeO3material have been observed. With increasing substrate temperature, the optical conductivity of La0.5Sr0.5CoO3films changes from insulator-like behavior to metal-like behavior. The discrepancies of the optical conductivity and electronic band structures in LaNiO3and La0.5Sr0.5CoO3conductive metallic oxide films have been discovered.The temperature dependence of optoelectronic transitions and OBG in BiFeO3film has been studied by transmittance spectra. Four interband electronic transitions related to the charge transfer excitation from Fe to O can be uniquely assigned. Moreover, two magnetic transitions located at about150and200K in the material have been observed. The variation law among the direct OBG, electronic band structure, and temperature has been established. With increasing the temperature, the OBG of BiFeO3films decreases from2.69to2.65eV. The band gap narrowing coefficient of the film is estimated to be about-1.65×10-4eV/K at RT. The preparation craft of the La0.5Sr0.5CoO3(LSCO) films have been investigated using pulsed laser deposition. The preparation parameters dependence of the crystal quality has been researched by changing the substance temperature. With increasing substrate temperature, the LSCO films become better crystalline and show the highly (110) preferred orientation. The optical conductivity shows a remarkably different variation trend and changes from insulator-like behavior to metal-like behavior with increasing the growth temperatures, which is strongly dependent on the crystal quality of the films. The discrepancies of the electronic band structures in nanostructured LaNiO3(LNO) and LSCO conductive metallic oxide films have been investigated. Four electronic characteristics related to the interband electronic transitions from O2p to Ni/Co3d energy states can be uniquely assigned. The influences from grain size on the electronic state characteristics have been discovered. Moreover, the crystal structure and nano-scale dependences of the physical parameters (electronic transitions, dispersion constants, and optical conductivity, etc) for different nanostructured LNO and LSCO conductive metallic oxides have been obtained.