INFRARED OPTICAL RESPONSE FROM Nb ON SiO2 ULTRATHIN FILMS

Oscillations in the infrared reflectance from metallic ultrathin films are described as consequence of quantum size effects. In this contribution, we present experimental evidence of such oscillations for Nb ultrathin films deposited on α-type SiO 2 substrates. Also, it is shown how substrates influence the size effects and the amplitude but not the period of oscillations. Because of the strong influence from the chosen substrate due to absorption, IR reflectivity was fitted to the optical response of our metal-substrate and bare-substrate system by using the three-oscillator model and numerical calculations on the basis of the local field calculation for a single metallic quantum well. Although quantum size effects are well studied in semiconductor compounds, there are few studies of this effect in metallic films where the present investigation has its most important contribution. Measurements for p-polarized reflectance (Rp) are made using a tunable p-polarized CO 2 waveguide laser using wavelengths from the p-branch (9.4 to 9.7 μm) and R-branch (10.0 to 10.4 μm). Nb/SiO 2 ultrathin films were assembled by a conventional RF sputtering technique and tailored thicknesses were deposited from 5.5 to 55 Å.