Strong Reduction in Ge Film Reflectivity by an Overlayer of 3 nm Si Nanoparticles: Implications for Photovoltaics

We examine the effect of a top nanofilm of 3 nm silicon nanoparticles (Si-NPs) on the spectral features of the reflectivity of germanium (Ge). We use a 450 nm Ge layer grown by low temperature plasma enhanced chemical vapor deposition on a crystalline Si substrate. The Si-NPs are drop casted incrementally on the Ge/Si structure from a particle colloid, forming a nanofilm of increasing thickness up to 30 nm. The stack is characterized using luminescence microscopy, atomic force microscopy, transmission electron microscopy, and reflectivity spectroscopy. The reflectance of the stack is measured over UV/visible and infrared range 4.8–0.8 eV. With the increasing thickness of the Si nanocoating, the results show a strong reduction in the reflectivity of Ge in the visible and blue-UV region of the spectrum with less effect in the infrared, as well as a difference in reflectance at the adjacent lying complex of Λ and L transitions of Ge. The strong reduction in the visible/UV range is understood in terms of a near overlap of the band to band transitions of Si-NPs and Ge. The diminished response in the infrared contributes to shifting of the bandgap of Si-NPs from the infrared to the visible due to quantum confinement. Furthermore, optical properties of Si-NP are estimated and samples are analyzed using Modified Transfer Matrix Method model. The strong reduction of reflectivity of Ge in the visible-UV region is very useful in photovoltaics, with special emphasis on Ge-based multijunction solar cells.