Berreman effect applied to phase characterization of thin films supported on metallic substrates: The case ofTiO2

Abstract

Infrared reflection-absorption spectra of ${\mathrm{TiO}}_{2}$ thin films deposited by plasma-enhanced chemical vapor deposition onto aluminum and by a sol-gel process onto platinum were obtained using s- and p-polarized light and oblique incidence angles. Prominent bands with variable reflection minima position and line shapes, which were shown to be phase dependent, were observed for all samples in the $800\ensuremath{-}900 {\mathrm{cm}}^{\ensuremath{-}1}$ wave number range when p-polarized light and oblique incidence were used. Such bands were attributed to an LO mode of ${\mathrm{TiO}}_{2}$ and their enhancement with the incidence angle is a good example of Berreman effect. Such spectra were analyzed by means of spectral simulation based on the Fresnel equation for a three-layered system. The films' optical constants used in the simulations were obtained through the Kramers-Kr\onig analysis (KKA) of the reflectance spectra of pellets of powdered amorphous ${\mathrm{TiO}}_{2},$ anatase and rutile. Optical constants for hypothetical polycrystalline ${\mathrm{TiO}}_{2}$ systems were also calculated from the dielectric functions of single crystals by means of effective medium theories (EMTs), such as those of Bruggeman, Maxwell-Garnett, and Hunderi. These optical constants were used both for spectral simulation and for understanding the bands observed. However, the optical constants for the powdered standards determined through KKA reproduced experimental results more accurately than those determined through the EMTs. In both experimental and simulated spectra, Berreman effect was very clear-cut and a reliable phase characterization could be carried out.