G. Vuye

Making an omnidirectional reflector

The effect of having a finite number of layers on the design of omnidirectional reflectors was investigated. It was shown that the structure should be finished with a low-index layer having a thickness larger than a quarter-wave to increase reflectivity, whereas layers below may remain of quarter-wave optical thickness at normal incidence angle. This general trend has been used for designing and realizing two a-Si-SiO(2) (amorphous silicon and silicon dioxide) omnidirectional reflectors in the near-infrared range on a silicon and a silica substrate, respectively. Owing to the decrease of absorption of recrystallized silicon as compared with a-Si in the visible range, the transmissivity of the structure realized on silica substrate was dramatically increased in the visible range upon annealing, whereas the high reflectivity and the omnidirectional effect were maintained in the near-infrared range.

Determination of refractive-index profiles by a combination of visible and infrared ellipsometry measurements.

Model inhomogeneous silicon oxynitride films were produced by ion-beam sputtering and characterized by ellipsometry in the visible and infrared ranges. These films exhibit strong intentional gradients of the refractive index that cannot be considered linear. A discrete description of the index profile with a few layers or a continuous description with a polynomial are examined; regressions by the use of measurements in the visible only or in the total spectral range (visible and infrared) are performed. Acquisition of data in an extended range is found to be a guarantee of the reliability of the calculated index profiles.

Infrared ellipsometry investigation of SiOxNy thin films on silicon

The dielectric function ˜ε (˜ε = ε(1) + iε(2)) of silicon oxynitride films deposited on silicon wafers by dual ion-beam sputtering is determined by infrared ellipsometry between 580 and 5000 cm(-1). The phase-separation model is unable to reproduce the experimental data. The dependence of ˜ε on stoichiometry is analyzed with the microscopic Si-centered tetrahedron model. The random-bonding model with five SiO(4-j)N(j) (j = 0-4) tetrahedra gives a good description of the spectra, provided the dielectric function of the mixed tetrahedra is carefully chosen.

Optical properties of mixed TiO2 - SiO2 films, from infrared to ultraviolet

Mixed oxides are useful for obtaining the intermediate refractive indices needed in the realization of graded-index thin films. Co-evaporated TiO2-SiO2 mixtures are studied for a large range of concentrations via UV-VIS, IR ellipsometry and XPS. An understanding of the nature of these mixtures and their air exposure stability is important for further applications. At low TiO2 concentrations, Ti4+ ions are inserted into the silica tetrahedral network, as shown by the IR peak at 945 cm-1. At higher concentrations, an evolution from TiO4 tetrahedra to TiO6 octahedra is presumed. The behavior of the O1s core level peak indicates that a least two phases coexist. Comparison between concentration determined using XPS and RBS shows a deficit in TiO2 at the surface of the films, especially at high TiO2 concentrations. The evolution of the mixtures optical constants will be presented in a large wavelength range, going from IR to UV. Particular attention will be paid to the variations with respect to the frequency of the vibration modes in the IR range, to the refractive index in the transparency region, and to the extinction coefficient at he absorption threshold. In addition, AFM measurements show the variation of the grain size as a function of the TiO2 concentration.

Growth of low and high refractive index dielectric films: an in situ ellipsometry study

Dielectric films exhibit very different and complex behaviors during thickness growth, leading generally to a variation of the refractive index in the depth of the film. Several causes of index inhomogeneity are examined: chemical interaction between film and substrate, transition layer, surface layer, etc. Their detection from in situ ellipsometric measurements is presented in the case of CaF2, SiO2 and TiO2 films. Complementary techniques (XPS, HRTEM and AFM) have been used for assessment of realistic optical models.