Ellipsometry Studies On Optical Properties Of Single-phase Epitaxial Transition Metal Oxide LiTi₂O₄ Thin Films

Among the transition metal oxides,LiTi₂O₄ is the only spinel oxide superconductor.In recent years,it has also been widely used as anode of lithium-ions batteries due to Li⁺migration.In this thesis,a series of high quality single-phase LiTi₂O₄ thin films were prepared by pulsed laser deposition（PLD）,and optical transparency,dielectric function,band structure,and the aging effect of optical properties were investigated in depth.The main research contents of this thesis are as follows:（ⅰ）Single-phase LiTi₂O₄ thin films were prepared on Mg Al₂O₄（011）substrates using pulsed laser deposition.As we all know,LiTi₂O₄ is sensitive to moist air,resulting in the disappearance of superconductivity.And then we believe aging effect will also change the optical properties theoretically.Thus,the aging effect of optical properties was investigated by ellipsometry.A high-quality single-phase LiTi₂O₄ thin film were characterized by X-ray diffraction and atomic force microscopy.Subsequently,the prepared sample was exposed to a laboratory atmosphere,and the aging effect of optical properties and band structure was studied by ellipsometry.In the optical modeling process,it was initially modeled using B-spline model,and then we use dispersion model to replace the B-spline model to obtain its dielectric function.It is observed that there are four optical transitions in the fresh sample,located at 2.06,2.39,3.07,and 4.59 e V,respectively.However,in the aged samples,the optical transitions located at 2.06 e V and 2.39 e V are gradually vanished while the transitions located at 3.17 e V,4.59 e V are red-shifted.This is because the double transitions are related to the Jahn-Teller distortion.With the increase of aging time,there is metal-insulator phase transition,the double transitions are disappeared gradually because of oxidized Ti³⁺and the weakened Jahn-Teller distortion.Meanwhile,Li defects and migrates Ti ions in the aged sample are appeared,which leads to the destruction of the basic structure of Ti₂O₄,and increases the Ti 3d orbital hybridization,thus causing the red-shift of the transitions in the high-energy region.（ⅱ）After clarifying the aging effect of optical properties of LiTi₂O₄,the optical properties of single-phase LiTi₂O₄ thin film was further analyzed in depth by Mueller matrix ellipsometry.It reveals weak in-plane optical anisotropy by Mueller-matrix spectroscopic ellipsometry.And we find the in-plane rotational symmetry of LiTi₂O₄ deviates from the dual symmetry,which again proves the existence of in-plane optical anisotropy.We suggest that this optical anisotropy may originate from the cooperative Jahn-Teller distortion,which is associated with the electrical anisotropy of the material.Unfortunately,the in-plane optical anisotropy is too weak to analysis its Mueller-matrix spectra.Therefore,we convert the Mueller-matrix spectra into conventional ellipsometric spectra for analysis.And in order to obtain complex dielectric function,we choose to build the in-plane（a-axis）and out-of-plane（bc-plane）anisotropy in order to remove effect of the epitaxial growth of the film.With the above dispersion model optimization,the in-plane and out-of-plane optical bandgaps of LiTi₂O₄ were estimated to be 4.43 e V and 4.26 e V,respectively.Further,the Standard Critical-Point（SCP）are mostly originated from charge-transfer transitions of Ti related pairs or isolated Ti.Finally,a series of physical parameters such as optical effective mass related to the dielectric function with the classical Drude model,such as quasiparticle effective mass,mobility,free time,etc.And we analyzed the origination of optical transparency of LiTi₂O₄based on the discussion of these parameters.（ⅲ）Because of the limitation that the existing ellipsometry,it can only perform ellipsometric measurements at room temperature,a special cold and hot stage for ellipsometry is used to upgrade the existing ellipsometry.The window error was first calculated theoretically and corrected.Then,the ellipsometric parameters are measured within-175°C to 25°C,and the dielectric function is further obtained.The results show that as the temperature decreases,the real and imaginary part of dielectric function are downward translation,the band gaps are red shift.In addition,there is not phase transition within-175℃-25℃,and it still exhibits metallic characteristics.Finally,it is found that the temperature is positively correlated with the optical resistivity.