Theoretical study of the optical properties of Si1-xGex alloy, and Si/Ge and Ge/Si core/shell nanoparticles

The scattering and absorption efficiencies of light by individual silicon-germanium spherical nanoparticles in air are analysed theoretically in the framework of Lorenz-Mie formalism. We have examined homogeneous Si1-xGex alloy, as well as Si/Ge and Ge/Si core/shell nanoparticles, addressing the influence of particle-diameter and Ge composition on their scattering and absorption efficiencies. Our simulation clearly shows that the Ge composition provides an additional degree of freedom to tailor the optical responses of these nanostructures. The optical properties of Si1-xGex alloy nanoparticles are found to evolve smoothly from those of pure Si to those of pure Ge nanoparticles by increasing x, as expected. The Si/Ge and Ge/Si core/shell nanoparticles show completely different behaviours from that of the alloy nanoparticles as regards as the scattering and absorption of light. In addition, the relative shell thickness has effect only in the case of Ge/Si heterostructure. A Ge-like behaviour is found for Si/Ge core/shell nanoparticles irrespective of the Ge-shell thickness. For Ge/Si core/shell nanoparticles a Si-like behaviour is observed for shell thicknesses strictly greater than the nanoparticle mid-radius; for smaller shell thicknesses, the optical responses evolve and tend to those of pure Ge nanoparticles for ultrathin Si-shells.