Berreman effect applied to phase characterization of thin films supported on metallic substrates: The case of TiO2 - art. no. 125404

Infrared reflection-absorption spectra of TiO2 thin films deposited by plas ma-enhanced chemical vapor deposition onto aluminum and by a sol-gel proces s onto platinum were obtained using s- and p-polarized light and oblique in cidence angles. Prominent bands with variable reflection minima position an d line shapes, which were shown to be phase dependent, were observed for al l samples in the 800-900 cm(-1) wave number range when p-polarized light an d oblique incidence were used. Such bands were attributed to an LO mode of TiO2 and their enhancement with the incidence angle is a good example of Be rreman effect. Such spectra were analyzed by means of spectral simulation b ased on the Fresnel equation for a three-layered system. The films' optical constants used in the simulations were obtained through the Kramers-Kronig analysis (KKA) of the reflectance spectra of pellets of powdered amorphous TiO2, anatase and rutile. Optical constants for hypothetical polycrystalli ne TiO2 systems were also calculated from the dielectric functions of singl e crystals by means of effective medium theories (EMTs), such as those of B ruggeman, Maxwell-Gannett, and Hunderi. These optical constants were used b oth for spectral simulation and for understanding the bands observed. Howev er, the optical constants for the powdered standards determined through KKA reproduced experimental results more accurately than those determined thro ugh the EMTs. In both experimental and simulated spectra, Berreman effect w as very clear-cut and a reliable phase characterization could be carried ou t.