Effect of substrate conductivity on infrared reflection spectra of thin TiO2 films

S- and p-polarized middle infrared reflection spectra of a 0.25 μm thick TiO2 film on silicon are measured with beam angle of incidence of φ=45° and 75°. An inversion of the absorption band related to the high frequency maximum of the loss function −1/Im[ϵ(ν)] of TiO2 occurs while changing φ in the spectra measured using a p-polarized infrared beam. This inversion does not take place for TiO2 films grown on aluminum. The phenomenon is studied calculating the spectra with Fresnel and Fuchs–Kliewer reflection coefficients. The former considers real substrates with finite and non-zero conductivity, while the latter considers ideal substrates with infinite and zero conductivity. Our study shows that the effect of the damping factor of the ion oscillations in TiO2 has to be taken into account to explain the observed phenomenon. We conclude that the inversion of the absorption band related to the high frequency maximum of the loss function −1/Im[ϵ(ν)] of TiO2 is due to the finite but low conductivity of the silicon substrate.