Influence of Substrate on Plasmon-Induced Absorption Enhancements

A set of periodic plasmonic nanostructures is designed and fabricated as a means to investigate light absorption in single-crystal silicon thin-film structures with silicon-on-insulator (SOI) wafers as a model system. It is shown both computationally and experimentally that plasmon-induced absorption enhancement is remarkably higher for such devices than for thick or semi-infinite structures or for the thin-film amorphous silicon solar cells reported in the literature. Experimental photocurrent enhancements of the orders of 12 and 20 are demonstrated for non-optimized 2200-nm-thick photoconductive and 300-nm-thick photovoltaic test structures, respectively. Theoretical absorption enhancements as high as 80 are predicted to be achievable for the similar structures. The features of the spectral enhancements observed are attributed to several interacting resonance phenomena: not just to the favourable scattering of light by the periodic plasmonic nanoparticle arrays into the SOI device layer and coupling to the waveguide modes interacting with the plasmonic array but also to the Fabry-Pérot type interferences in the layered structure. We show that the latter effect gives a significant contribution to the spectral features of the enhancements, although frequently ignored in the discussions of previous reports.