Spatial spectral evolution in pulsed laser deposited lead-germanate thin films by micro-infrared spectroscopy

Lead-germanate thin films were developed on silicon substrates by pulsed laser deposition from bulk glassy targets of composition 0.4PbO-0.6GeO₂, and micro-infrared transmittance measurements were performed to assess the state of the grown films. Measurements across the radius of films revealed surprisingly large spectral changes, reminiscent of lead-oxide variations in corresponding bulk glasses. To search for the origin of this effect, the infrared spectra were simulated by employing the rigorous expression for the transmittance of a bilayer system to take into full account multiple internal reflections in both thin film and substrate. The results showed that the profiles of the experimental spectra can be accurately described by using as input the complex refractive index of the target glassy material and by considering film thickness variations from the center to the edges of the film. This work demonstrates the strong influence of optical effects on the infrared spectra of thin films, and manifests also the effectiveness of infrared spectroscopy when coupled with rigorous calculations to characterize the structure of thin films.