Interference effects in photoacoustic and reflectance spectroscopies on TiO2/Si structures and TiO2 band gap.

Experimental results of photoacoustic (PAS) and reflectance (RS) spectroscopies of titanium dioxide thin films (TiO2), deposited on Si substrates, are compared in a wide optical range including transparent and absorbent regions of TiO2. Due to the fact that the light modulation frequency f used in the photoacoustic experiments was so low that the thermal diffusion length of the TiO2 (mu = 100 microm) is always larger than the thickness of the studied films, the PAS turns out to be complementary to RS over the entire range. The presence of multiple reflection interference effects makes difficult a direct evaluation of the TiO2 band gap from the PAS signal. However, by employing k(lambda) values, obtained from transmission experiments on equivalent TiO2 films deposited on transparent fused quartz substrates, the PAS spectra for the films deposited on silicon are reconstructed by using those theoretical models that consider multiple reflections. The reasonable agreement of the simulated and experimental PAS spectra allows one to obtain reliable Eg values for the TiO2 films deposited on opaque silicon substrates.