S. Fisson

Infrared ellipsometric study of SiO2 films: relationship between LO mode frequency and porosity

Abstract A method for evaluating the pore volume fraction of silica films for different pore connectivities is presented. A series of SiO2 films, evaporated by electron gun with or without ion bombardment, have been studied using visible and infrared ellipsometry. The TO and LO mode frequencies are deduced from the dielectric function calculated from infrared ellipsometric data. The study of both TO and LO mode frequencies is shown to bring independent information on the film microstructure. The TO mode frequency (near 1075 cm−1) varies mainly with the density of the silica matrix. On the other hand, variation in the LO mode frequency (near 1245 cm−1) is mainly due to changes in porosity. In the case of films with pores largely connected, the evaluation of the pore volume fraction from the LO frequency is compared to that obtained from the analysis of the large water absorption band near 3300 cm−1 (H–OH and Si–OH stretching absorption band).

Evolution of the optical properties of Si nanoparticles embedded in SiO2 as function of annealing conditions

The dielectric function of Si nanoparticles embedded in silica has been determined from spectroscopic ellipsometry and photothermal deflexion spectroscopy from 0.7to6eV. The influence of crystalline fraction and diameter of the nanoparticles on their optical properties has been investigated. Above 4nm of diameter, the nanoparticles presented a dielectric function similar to that of fine grained polycrystalline Si (poly-Si) at photon energy higher than 2eV, with the well marked structures associated with the E1 and E2 critical points. In contrast, below 2eV their absorption coefficient was smaller than for poly-Si. Below 2.5nm of diameter, the dielectric function of the nanoparticles drastically changed. The magnitude of the imaginary part of the dielectric function of the nanoparticles near the position of the E1 critical point constantly decreased, whereas it increased at the position of the E2 critical point. These observations can be interpreted as the result of the transfer of the oscillator strength ...

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.

Ellipsometric investigation of the Si/SiO2 interface formation for application to highly reflective dielectric mirrors

We present a study of the Si/SiO 2 interface formation during the growth of Si on SiO 2 using in situ ellipsometry measurements performed at one wavelength (X = 0.4 μm). Variable angle X-ray photoelectron spectroscopy (XPS) is also used to confirm the ellipsometric measurements. In situ ellipsometry and XPS provide evidence for the formation of a SiO x (0 <x < 2) interlayer between the SiO 2 and Si layers. The variation of x with depth is analysed from (cos Δ, tan ψ) trajectories and the results obtained allow reproduction of the spectroscopic ellipsometry spectra too. The effects of the substrate temperature on the growth are investigated and show that the higher the substrate temperature the larger the interface. The utilisation of a SiO 2 /Si multilayer stack for elaborating an omnidirectional reflector, angle-insensitive in the near infrared range, is also presented.

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.