Mo Dang

Generalized simulated annealing algorithm applied in the ellipsometric inversion problem

For the interpretation of an ellipsometric measurement an adaptive optical model has to be assumed because of the lack of inverse equations. Finding the proper parameters of the optical model for multi-layer structure by minimizing the difference (error) between the measured and the computed data becomes a typical global optimization task. The technique of generalized simulated annealing (GSA) was introduced in order to solve inverse problems related to ellipsometry, and was tested in evaluating ellipsometric measurements for lead zirconate titanate films. It is shown that unlike local optimization methods, the convergence of GSA is independent of the initial model and it is capable of producing reliable results in ellipsometry with great computation efficiency.

Ellipsometric Spectra and Refractive Index of Polyaniline

Ellipsometric spectra of polyaniline which is one kind of new conducting polymers are measured. The complex dielectric function, the index of refraction and the absorption coefficient of insulating polyaniline as the functions of photon energy are obtained. The absorption spectra obtained by us and other authors are compared and analyzed.

ELLIPSOMETRIC SPECTRA AND DAMAGE PROFILES OF ION IMPLANTED SILICON

By means of optimization with a multilayer model, the damage profile can be obtained from the measured data by spectroscopic ellipsometry without stripping. The damage profiles of 40keV As+ implanted Si at doses of 4×1013 and 1.4×1014 ions cm-2 are shown and compared with those determined by Rutherford backscattering.

Low Field-Induced Electron Emission Phenomenon Observed from the Carbon Containing Thin Films

Stable cold-cathode electron emission at fields as low as 3 MV/m has been observed from two types of carbon-containing thin film of thickness in submicron scale, i.e., non-doped fullerene C60 and amorphous diamond films. A transparent anode imaging technique was used to record the spatial distribution of individual sites and the total emission current-voltage characteristic of the films. It is found that supplemental measurements of optical spectra and electrical conductivity of the films can provide important evidence useful for understanding the differences in emission behaviour between two types of film.

A FRACTIONAL INTEGRAL REPRESENTATION FOR INTERBAND DIELECTRIC FUNCTION OF CRITICAL POINT ELECTRONS IN VARIOUS DIMENSIONAL CRYSTALS

The dielectric function for direct interband transitions of critical point electrons in an n-dimensional crystal has been derived as the n/2-th iterated integrals of the dielectric function of a harmonic oscillator. A generalized expression for the derivatives of interband dielectric function has also been presented by fractional integration. The results present an explicit physical picture and provide a new approach to the investigations of optical properties of low dimensional and anisotropic crystals.

Dependence of Structure and Haemocompatibility of Amorphous Carbon Films on Substrate Bias Voltage

Tetrahedral amorphous hydrogenated carbon (ta-C:H) films on Si(100) substrates were prepared by using a magnetic-field-filter plasma stream deposition system. Samples with different ratios of sp3-bond to sp2-bond were obtained by changing the bias voltage applied to the substrates. The ellipsometric spectra of various carbon films in the photon energy range of 1.9–5.4 eV were measured. The refractive index n and the relative sp3 C ratio of these films were obtained by simulating their ellipsometric spectra using the Forouhi–Bloomer model and by using the Bruggeman effective medium approximation, respectively. The haemocompatibility of these ta-C:H films was analysed by observation of platelet adhesion and measurement of kinetic clotting time. The results show that the sp3 C fraction is dependent on the substrate bias voltage, and the haemocompatibility is dependent on the ratio of sp3-bond to sp2-bond. A good haemocompatibility material of ta-C:H films with a suitable sp3 C fraction can be prepared by changing the substrate bias voltage.

Ellipsometric Spectra and Optical Properties of Anisotropic SrBi2Ta2O9 Films

Ellipsometric spectra of the SrBi2Ta2O9 (SBT) films of (200) and (0010) predominant orientation are measured and analysed in the range of photon energy from 2 to 5?eV. The results show that the oriented SBT films appear to be strongly anisotropic. The ellipsometric spectra of the (200)-predominant SBT films are different from those of the (0010)-predominant films. We suggest an analysis model for these oriented films and perform the fitting of the anisotropic ellipsometric spectra. The refractive index and the extinction coefficient of the ordinary ray and the extraordinary ray for these oriented SBT films are obtained. The ordinary refractive index is larger than the extraordinary one.

Property of Van Hove Critical Points in Fractional-Dimensional Space

The model of fractional-dimensional space is used to study dielectric function associated with electron interband transitions near a Van Hove critical point in anisotropic systems. Using fractional derivative spectra (FDS) method, it is found that in fractional-dimensional space only a minimum yields a symmetric Lorentzian line shape in FDS but a maximum does not.

Ellipsometric evaluation of the sp(3)-bonded carbon fraction in carbon thin films

We prepared amorphous carbon films on Si (100) substrates by using a magnetic-field-filtered plasma stream deposition system. Various samples with different sp3-bonded carbon fraction were obtained by changing the bias voltage applied to the substrates. We measured the ellipsometric spectra of various carbon film samples in the photon energy range of 2.0-5.0 eV. We also measured the Raman spectra for comparison. Our results show that the ellipsometric spectra are dependent on the sp3 carbon fraction. We analysed the measured ellipsometric spectra by a simple method, and determined the sp3 carbon fraction semi-quantitatively. The results from the ellipsometry and the Raman spectroscopy show the same tendency of the sp3 carbon fraction as a function of bias voltage. We found that the spectroscopic ellipsometry is a relatively simple, non-destructive method to evaluate the sp3 carbon fraction of the amorphous carbon films.

Coherent optical phonon excitation by femtosecond laser pulse in YBa2Cu3O7-δ thin films

With a correlation of nonequilibrium carriers relaxation and coherent phonons displacive excitation, the coherent optical phonon oscillations in YBa2Cu3O7-δ thin films excited by femtosecond laser pulse are simulated theoretically. It is revealed that as the oxygen concentration decreases, the coherent phonon oscillations become easier to be observed due to the decrease of the local coupling between the carriers and the lattice vibrations in the CuO2 plane.