High-temperature Dielectric Functions Of Semiconductors And Metals From Ellipsometry Measurements And First-principles Simulations

The accurate acquirement of optical constants of solids is of vital importance for the invstigations of thermal radiative properties in applications,such as energy and chemical engineering,solar energy utilization and aerospace.Nowadays,the experimental data shown in most optical Handbook are measured under room temperature,due to the existence of high-temperature oxidation and self-radiation of samples.There exist limitations in experimental measurements of optical constants at high temperature.However,the acquirement of dielectric functions at high temperature are very important for the structural design and performance prediction of high-temperature optical devices,and thus it is necessary to obtain high-temperaure dielectric functions by theoretical calculations,which can provide basic data for engineering applications.Recently,first-principles calculation has been widely used in the prediction of basic properties of materials,because the input parameters needed are only atom species and atom positions.Compared with experimental study,theoretical simulation can break technical barriers and predict optical properties in high temperature environment.Nowadays,investigations of high-temperature optical constants from fully first-principles are still in the exploratory stage.In this thesis,the electron-phonon interaction is considered in the calculation of electronic band structure on the basis of traditional first-principles calculations.And then the dielectric functions are obtained by electron transiton mechanisms.For semiconductors,the electron-phonon interaction has an obvious influence on dielectric functions,thus the Bethe-Salpeter equation is applied to calculate the temperature effects on the dielectric function for semiconductors in the visible-ultraviolet spectral range.While one still needs to consider the intraband transiton of near free electrons in metals besides interband transition.To verify the accuracy of calculated results,the spectroscopic ellipsometry combined with heating cell are used to measure the optical properties of some semicondutors and metals,and the first-principles calculations are verified.The details of this study are:By applying the visible variable angle spectroscopic ellipsometry combined with heating device,the dielectric functions of III–V semiconductors GaP,InP,GaAs and InAs are measured in the energy range 1.24-6.52 eV?190-1000 nm?for temperature between300-700 K.The experimental results are fitted by the Johs-Herzinger model to extract the peak position and broadening width as a fuction of temperature.It is found that as temperature increases,the peak positions show redshift and the broadening widths increase.Besides,theoretical calculation based on first-principles are performed,and the electron-phonon coupling is taken into account in the calculation of electronic band structure.The electronic eigenvalue and electron lifetime at finite temperature are calculated,and then the finite temperature dielectric function can be calculated according to the Bethe-Salpeter equation.The calculated results show an overall good agreement with experimental measurements.It demonstrates that the redshifts of peak positions are mainly related to the decreases of band gaps,while the broadening width increases.Because as the temperature increases,the scatterings of electrons by phonons are enhanced.Metals are widely used in optoelectronic devices,photocatalysis and optical metamaterials,the realistic representation of high-temperature dielectric functions will provide the fundmental input data for photo-thermal applications.By applying the visible variable angle spectroscopic ellipsometry and infrared variable angle spectroscopic ellipsometry combined with heating device,the dielectric functions of noble metals Au and Ag and transition metals Cr,Mo,W and Ti in the temperature range 300-700 K.The experimental results in the infrared spectral range arising from intraband transition are fitted by the Drude model,and the damping factor and intraband plasma frequency as a function of temperature can be obtained.It is found that the damping factor increases with increasing temperature,but the variation of plasma frequency with temperature dpendent on the atom species.In order to predict the dielectric functions of metals at high temperature from theoretical calculations,the dielectric fcuntions of metals are divided into interband and intraband contributions.The dielectric functions arising from interband transitions are calculated by the Fermi golden role,while the intraband transition is described by the first-principles parameterized Drude model.It is found that the temperature dependence of dielectric function in the infrared spectral range is mainly determined by the electron-phonon interaction.In this work,the electron scattering rate arising from electron-phonon interaction is calculated from first-principles,and then it is used by the Drude model to obtain the high-temperature dielectric function.The calculated dielectric function agrees well with the experimental measurement by VASE.It is found the electron scattering rate of transition metal Cr,Mo and W shows an obvious frequency dependence,which is related to the electron-electron scattering and electron-phonon scattering,and the electron-phonon scattering plays a dominant role.The dielectric functions of transition metal carbide TiC,its nitride TiN and MAX phase ternary carbide Ti₂AlC at high temperatures are calculated by first-principles.It can be concluded from the electronic band structure calculation that all of the three compounds show metallic character.Therefore,the intraband transitions need to be included in the calculation of optical constants.The calculated dielectric functions at room temperature show an overall good agreement with experimental data in literatures.Besides,the dielectric functions at high temperature were calculated,which will provide a guidance for high temperature plasmonic applications.Moreover,the electronic band structures and optical properties of newly discovered 2D MXenes are investigated,and the influence of surface termination on the dielectric functions are calculated,from which the absorbtion coefficient and nomal reflectivity can be obtained.It is found that the 2D MXenes can exhibit semiconducting or metallic properties depending on the functional group.When terminated with F or OH,the 2D MXenes shows metallic properties with good conducting ability and light transmittance in the visible spectral range is high,making it promising for transparent electrodes,while the system exhibits semiconducting property when terminated with O,absorption at visible range is relatively high,which renders its potential applications in photocatalysis.