Ellipsometry Study Of Novel Nanoscale Functional Films

In 21th century,along with the rapid progress in optoelectronic technology and nanotechnology,electronic devices have been achieved the highest degree of integration and miniaturization.With the extensively application requirement for nanoscale functional film materials and devices in the field of aerospace,communications,microelectronics and computer,it is important to explore novel functional film materials and to study theirs applications.Functional films are widely applied in various electronic devices because of their unique electrical,magnetic,optical,thermal properties and physical effects.High-tech electronic devices are the foundation of national high technology research,involving in national defense,energy and information security.The researches of novel functional films,especially for nanoscale films,are the pilot and support of the electronic devices.The properties of nanoscale thin films are different from bulk materials,which make them have broader application prospect.There are many kinds of materials,such as superconducting films,semiconductor films,passivation and protection films,solar films,ferroelectric films,magnetic films and so on.It is significant to prepare novel functional materials and explore their applications in other new fields.In this paper,we introduce several functional films applied in superconducting devices and energy conversion devices.For the high sensitivity and strong anti-interference ability,superconducting devices is used widely for deep space exploration,information confrontation,terahertz imaging.The optical properties of high temperature superconducting films provide technical support for the design and manufacture of superconducting devices.Since Bedonorz and Muller discovered copper oxide high temperature superconductor,its superconducting mechanism is still a mystery.Different from hole doped compounds,the superconducting phase diagram of electron doped compounds La2-xCexCuO4(LCCO)has just been completed in recent years.The pure T'-phase single crystal LCCO is still not available so far,and only a few research groups can successfully fabricate the single-phase LCCO epitaxial thin films.As a result,there are few reports about the optical properties of LCCO films.The optical properties of LCCO are closely related to the electronic states of Cu-O planes.The superconducting mechanism of LCCO is also related to the doping effect in Cu-O planes.Far-infrared optical responses changes dramatically upon entering the superconducting state.While,the visible optical responses provide information about band structures and electronic states.Therefore,it is important to study the optical responses of LCCO.With the increasing composition of Li+,the spinel system Li1+xTi2-xO4(0?x?1/3)occurs metal-semiconductor transition at x?0.1.The ending member Li4Ti5O12 is regarded as an ideal "zero strain" anode material due to its superior electrochemical properties and three-dimensional ion channel.The initial member LiTi2O4 is the only known oxide spinel superconductor.Recently,the researches show that it can be used as Li-ion battery anode material as well.Furthermore,it is more suitable for large current battery than Li4Ti5O12.With the development of the preparation technology,it attracts more attention on the application of LiTi2O4 as a superconducting material as well as an anode material.To good understanding of the physical properties of the spinel system Li1+xTi2-xO4,it is important to systematic investigate and contrast the optical properties of Li4Ti5O12 and LiTi2O4.The crystal structure of transition metal nitrides has combined bonds,covalent bonds and metal bonds,which makes the materials behavior superior electrical conductivity,high density,corrosion resistance and high wear resistance.TiAlON films have been widely used in dry cutting applications.Recently,because of its optical characterization,which has high absorption rate of solar radiation and low thermal radiation loss,TiAlON films have attracted more and more attentions in solar-thermal conversion field.The knowledge of the optical properties of TiAlON films is the necessary information to design solar selective absorbing coatings.Spectral technique is an important technique to study the optical properties of nanoscale films.As a non-destructive technique,spectral analysis can obtain the lattice vibration,band structure,optical constants,phonon mode,interface state and so on.Among all the spectral techniques,spectroscopic ellipsometry(SE)has irreplaceable importance,because of its high precision and convenient operation character.Different from Fourier transform spectrometer,SE can obtain optical constants and thickness of nanoscale films at the same time without Kramers-Kronig calculation.So SE is an important technique in optical high-precision characterization,especially for novel nanoscale films.Based on spectroscopic ellipsometry,our group has studied copper oxide high temperature superconducting films LCCO,transition-metal oxides Li4Ti5O12 and LiTi2O4 and solar selective absorbing film TiAlON,intended to explore the optical constants and dispersion law of the novel thin films.We have discussed the normal-state electronic transitions of LCCO and given its pseudo-dielectric function;Based on the optical investigation of Li4Ti5O12 and LiTi2O4,we systematic contrast the nature of the two films;we have studied the optical properties of TiAlON films,and discussed the roughness layer influences on the optical properties of TiAION.The work of this dissertation will be described in the following aspects:1.The highly c-axis oriented LCCO(x=0.1)films were prepared on(001)SrTiO3 substrates by pulsed laser deposition.The reduced modulus and hardness of LCCO films were studied by nanoindentation test.Based on SE analysis,we discuss the influences of the annealing progress on physical properties.Furthermore,we also discuss the mechanism of the electronic transitions in visible region of the normal state of LCCO.The optical properties of LCCO films were investigated by SE.The single phase La1.9Ce0.1CuO4 superconducting thin films were grown PLD.We proposed a simple and effective method to build the dispersion law of novel films,and given the dispersion law of the LCCO by this method.By using the results of the point-to-point method as a reference,we firstly give a Cauchy+3Lorentz model.The LCCO films with different annealing time were investigated by XRD,AFM and SE.The results show that minor differences in the annealing progress have slight influence on crystalline and electrical properties,but cause a relatively large change in the optical properties of the LCCO films.In addition,the result of the nanoindentation test shows that the mechanical properties of LCCO are mostly affected by the grain size and the surface impurity atoms.For further study,we prepared the large-size LCCO film and extended the spectral range.By applying an appropriate approach,we have determined the pseudo-dielectric function of LCCO film at room temperature by SE in the spectral range from 0.5-4.13eV.The dispersion model is optimized as Drude+4Lorentz model.To obtain the precise energies of the transitions,we subtract the fitted Drude contribution and numerically calculate the third derivative of the Lorentz contribution multiplied by the square of the photon energy E.The pseudo-dielectric function of the LCCO exhibits a typical metallic behavior at energies below 2eV;in this case,this phenomenon is clearly dominated by the free carriers caused by doping.The behavior of electronic structure of LCCO at higher energy range in the normal state has the features of the electron doped cuprates as well as the hole doped cuprates.2.The optical properties of transition-metal oxides Li4Ti5O12 and LiTi2O4 were studied by SE.Combined with Raman spectra and first principle calculation,we discuss the band structure of LiTi2O4 from experimental to theoretical.The transition-metal oxides L14Ti5O12 and LiTi2O4 were characterized by XRD,AFM and SE.Good optical model of SE spectra,which fits for Li4Ti5O12 and LiTi2O4 were achieved using Cauchy dispersion model and Drude-Lorentz dispersion model,respectively.The results of SE spectra confirm the insulating nature of Li4TiO12 and the metallic nature of LiTi2O4.To the best of our knowledge,the optical constants of LiTi2O4 were observed for the first time.We performed first-principles calculations based on the DFT to study the band structures and density of states of LiTi2O4,as implemented by Material Studio.The crystal-field splitting between the eg and the t2g states of the octahedral Ti3+/Ti4+ ion is about 2.09eV,while the theoretical value from the first principle calculation is 2.48eV.The difference between the two values is in the acceptable range.Because of the lack of single phase samples,the experimental data of the single-phase LiTi2O4 are few.Our work enriches the experimental data.3.The thickness-dependence optical properties of TiAlON films were studied by SE.The dispersion models of the amorphous TiAlON films were given.The influence of the roughness and particle size on the optical properties of TiAlON films was discussed qualitatively by the formula.TiAION films with different thicknesses were deposited on glass substrates by magnetron sputtering.The crystalline and component of TiAlON films were analyzed by XRD and XPS.The results demonstrate the amorphous nature of the films and confirm that films are TiAlON.The transmittance and optical constants of TiAlON films were performed by UV spectrophotometer and SE.The surface morphology investigated by SEM and AFM revealed that the films with different thicknesses are significant different in particle size and surface roughness.The results indicated that the transmittance become lower with the smaller particle size.Through formula derivation,we discussed the relationship between roughness and refractive index,and found that the rougher the surface morphology,the lower the corresponding refractive index.