Size Dependence of Plasmonic Response in Phosphorus-Doped Si Nanocrystals: Implication for Infrared Plasmonics

Highly doped semiconductor nanocrystals are of great interest for applications in nanophotonics and appear as an exciting alternative for infrared plasmonics with detection and identification of molecules, covering applications in biology, medicine, air quality control, sanitary control, and safety issues. Among the main parameters that influence optical properties and plasmonic response, the nanocrystal size plays a major role. In this work, we report on the influence of the silicon nanocrystal size on the localized surface plasmon resonance obtained in n-type Si nanocrystals embedded in a silicon dioxide matrix. The size control of phosphorus-doped Si nanocrystals was achieved by using a (SiO/SiO2) multilayer architecture. In this study, the nanocrystal diameter is varied from 7 to 16 nm, while the P content is kept constant at 0.9 atom %. Here, we demonstrate that the mid-infrared plasmonic absorption exhibits both a redshift and broadening as the nanocrystal diameter decreases from 16 to 7 nm. The plasmonic response was successfully modeled in the framework of Mie theory, considering the Drude model and size-dependent scattering of free carriers. The redshift of the plasmon is explained not only by size-dependent scattering but also by size-dependent doping efficiency. Both charge carrier mobilities and free carrier densities are found to vary between 17 and 28 cm2 V–1 s–1 and between 1.89 × 1020 and 2.6 × 1020 cm–3, respectively. In this work, we shed light on the key role of the Si/SiO2 interface that needs to be optimized to reach plasmonic properties in very small nanocrystals that could support quantum plasmonics.