Synthesis And Properties Of Doped TiO₂Thin Films

In recent years, Titanium dioxide (TiO2) has been intensively studied because of its actual and potential application in many fields. Many studies show that the mixed crystalline TiO2with anatase and rutile phase (M-TiO2) has higher activity than anatase phase, and study on optical properties of mixed media is beneficial to the preparation of the graded-index films, so a study on the M-TiO2is very meanful. The M-TiO2may be fabricated by controlling anatase to rutile phase transformation (ART). Many factors influence the ART, such as atmosphere, temperature, grain size, time, and dopant. The influence of dopant on the ART has been widely investigated; however, the mechanism of dopant catalyzing the ART is disputed. In addition, many studies have focused on the ART of powders TiO2, whereas very limited research has been reported regarding the ART in Tio2films and optical properties of M-TiO2films. This is detrimental to the research and application of TiO2. In this study, metal ion doped TiO2(MI-TiO2) films were fabricated by sol-gel method, and influence of dopant on the ART and optical properties of M-TiO2films were investigated and discussed. Additionally, BaTiO3film and BaTiO3/(rutile MI-TiO2) multilayer films were fabricated and studied. The main work and results are as follows:1. Optical properties of rutile Ti1-xCoxO2films (0≤x≤0.1) on quartz substrates. The OBG first increase and then decreases with higher Co content, reaching its maximum value when x is about0.03. The competition between increasing and decreasing mechanisms of OBG leads to the phenomena.X-ray diffraction (XRD) pattern indicate all films are rutile phase. Atom force microscope (AFM) results show that with increasing Co content, surface of the films becomes smooth and compact. Raman spectra indicate Raman bands slightly shift low frequency with increasing Co content. It can be seen from transmittance spectra that the fundamental absorption edge first shows the blue shift and then the red shift. According to Tauc's law, the optical band gap (OBG) was got from transmittance spectra. The OBG varies between3.10and3.26eV. The OBG increase and then decreases with higher Co content, reaching its maximum value when x is about0.03, which was not reported by other groups. Refractive index and thickness of Ti1-xCoxO2films were obtained by fitting transmittance spectra with Swanepoel method. Refractive index value increases with increasing Co content. Decrease of grain size, compressive stress, and reduction of rutile TiO2increase OBG; and defect and impurity decrease the OBG. The competition between increasing and decreasing mechanisms of OBG leads to the strange change of OBG.2. Fabrication and the ART of MI-TiO2films on Si substrates. Co, Ni and Fe doping catalyze the ART. The evolution of rutile content with dopant content is an "S" shape. At the same dopant content, the active energy of the ART of Co-doped TiO2film is the least.XRD results show that annealing temperature, annealing time and spin-coating layers catalyze the ART of TiO2films. XRD and Raman results indicate that Co, Ni and Fe doping catalyze the ART. With increasing dopant content, rutile content first increases slowly, then quickly, and finally slowly again, i.e. the evolution of rutile content with dopant content is an "S" shape. The doping decreases the active energy of the ART due to oxygen vacancies and lattice deformation, and then catalyzes the ART. By comparison of influence of Fe, Co and Ni doping on the ART, Co's accelerating effect is the best and Fe's effect is the worst. This may be related to difference between ion radius and valence of doped ions. AFM results show that with increasing dopant content, the surface of the films first become rough and then smooth, finally rough again, which may be related to the expansion and shrinkage of anatase and rutile unit cells.3. Study on BaTiO3/Ti0.9Co0.102/LaNiO3(BTO/TCO/LNO) multilayer film. The existence of TCO layer is good for decreasing dielectric loss.BaTiO3films (BTO) were deposited on Si substrates, and annealing temperature was600,650,700and750℃, respectively. AFM results indicate that with increasing temperature, BTO with perovskite crystallizes well, but the surface of BTO at750℃becomes rough due to big grain. The optical properties of BTO in the wavelength of visible and ultraviolet were investigated by spectroscopic ellipsometry technique (SE). With increasing temperature, OBG decreases from4.21eV to3.69eV due to the increase of grain size and oxygen vacancies. BTO and BTO/TCO films were deposited on LaNiO3(LNO) substrates. Compared with pure BTO thin films, the remnant polarization of multilayer films had no obvious variety, but the coercive voltage of multilayer films had great increase. The dielectric loss of multilayer films was lower than that of pure BTO thin films. A sandwich structure like BTO/TCO/LNO has the smallest dielectric loss.4. Optical properties of MI-TiO2mixed crystalline films. With increasing dopant content, OBG first decreases, then quickly decrease and gradually decrease again in the end. This may be related to phase composition of the MI-TiO2films.Firstly, dielectric function, OBG and thickness of Ni-doped TiO2films on quartz substrates at UV-visible range were investigated by using the transmittance spectrum technique. In general, with increasing photon energy, the real part of dielectric function increases and gradually nears the maximum, and then decreases due to the Van Hove singularities. And the imaginary part of dielectric function nears to zero in the transparent region and sharply increases further increasing photon energy in the absorption region. With increasing Ni content, OBG slowly decrease from3.65eV to3.64eV, then quickly decrease to3.45eV, and gradually decrease again in the end. This may be related to phase composition of the Ni-doped TiO2films. Secondly, dielectric function, OBG and thickness of MI-TiO2mixed crystalline films were investigated by SE. The evolution of dielectric function with photon energy and the change of OBG with dopant content are similar to those of Ni-doped TiO2films.