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.

Infrared ellipsometry investigation of SiO(x)N(y) 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.

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.