Double-layer antireflection from silver nanoparticle integrated SiO2 layer on silicon wafer: effect of nanoparticle morphology and SiO2 film thickness

Optical properties of silver nanoparticles (Ag NPs) on SiO2 thin films of variable thickness, as a plasmonic double layer on a plain silicon wafer, are investigated for broadband antireflection. The light confinement into the silicon is found to be sensitive to the SiO2 film thickness of a few nanometers due to an evanescent character of the Ag NPs' near-fields. The Ag NPs' size anisotropy plays a pivotal role in incident light coupling due to the sub-wavelength spatial variation of near-fields at the interface, which leads to reflectance spectrum oscillation behavior in the nanoparticles' surface plasmon resonance and off-resonance regions. With an optimized SiO2/Ag NP double layer, the average reflectance in the 300-1200 nm spectral range is reduced to 14% in comparison to 42% in bare silicon, with a flat minimum reflectance of 3.5% in the 725-1020 nm spectral region. Finite difference time domain calculations are performed for spatial variation of near-fields and their angular distribution of far-fields at different inhomogeneous interfaces (where near-fields exist). The total reflectance from various configurations is simulated theoretically by considering the experimentally optimized physical parameters of the plasmonic double layer to support the observations. To verify the role of SiO2 surface topology apart from the nanoparticle morphology in plasmon near-field coupling, thermally grown SiO2 films are investigated along with the sputtered SiO2 thin films.