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

Abstract S- and p-polarized middle infrared reflection spectra of a 0.25 μm thick TiO 2 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 TiO 2 occurs while changing φ in the spectra measured using a p-polarized infrared beam. This inversion does not take place for TiO 2 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 TiO 2 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 TiO 2 is due to the finite but low conductivity of the silicon substrate.