Spectroellipsometric characterization of plasma-deposited Au/SiO2 nanocomposite films

Nanocomposite films consisting of metal nanoparticles embedded in a dielectric matrix were fabricated by simultaneous sputtering of a gold target and plasma-enhanced chemical vapor deposition of hydrogenated SiO2. The optical constants of the films were determined from spectroscopic ellipsometry measurements and were modeled using the Maxwell–Garnett effective medium theory. The particle size dependence of the free electron absorption was included according to the limited electron mean free path effect using a broadening parameter A=0.16 determined from the comparison of the measured spectra with transmission electron microscopy micrographs. Using bulk interband optical constants for gold, very good agreement was obtained between the model and the measured spectra but only in the narrow particle size range ∼10–20 nm, the latter of which marks the onset of phase retardation effects. For smaller particles, the energy of the surface plasmon resonance was progressively blueshifted with respect to the predicted value. This was interpreted by a size dependence of the interband transitions as a result of strain-induced variation of the lattice constant within the particle.