Optical properties of aluminium clusters embedded in amorphous silicon and silicon nitride films

Abstract

Increasing the optical absorption of thin film solar cells is necessary to increase their efficiency and reduce material costs. Plasmonic scattering nanoparticles are a way to increase absorption in these cells. A lot of research on gold and silver plasmonic nanoparticles has been done, also in the context of solar cells, but these materials are expensive and alternatives are needed. In this thesis the optical properties of aluminium clusters embedded in silicon nitride and amorphous silicon were studied. The aluminium clusters embedded in silicon nitride and amorphous silicon were fabricated by a gas aggregation cluster source combined with a thin film magnetron sputtering source combined in a single vacuum system. The topography of the samples was studied using atomic force microscopy (AFM), while the optical properties of the samples were studied using UV-Vis optical absorption spectroscopy. Finite difference time domain (FDTD) simulations were performed to calculate the optical absorption and the extinction cross section of the cluster-thin-film composites. Aluminium particles with a typical radius of 4-8 nm and a density between 10 and 20 clusters per square micron were made, which were covered with the thin films. Enhanced optical absorption near the red-shifted plasmon frequency in silicon nitride was observed, which was in agreement with the FDTD simulations. Although the FDTD simulations predicted a resonance well matched to the solar spectrum with the amorphous silicon film, no clear plasmon resonance was observed at the expected wavelength. However, a significant difference in optical absorption at wavelengths below 400 nm was observed in the samples with aluminium clusters covered with a 25 nm film of amorphous silicon. Both with silicon nitride and amorphous silicon, the aluminium clusters increased optical absorption.