Self-assembled antireflection coatings for light trapping based on SiGe random metasurfaces

Abstract

We demonstrate a simple self-assembly method based on solid state dewetting of ultrathin silicon films and germanium deposition for the fabrication of efficient antireflection coatings on silicon for light trapping. We fabricate SiGe islands with a high surface density, randomly positioned and broadly varied in size. This allows one to reduce the reflectance to low values in a broad spectral range (from 500 nm to 2500 nm) and a broad angle (up to ${55}^{\ensuremath{\circ}})$ and to trap within the wafer a large portion of the impinging light $(\ensuremath{\sim}40%)$ also below the band gap, where the Si substrate is nonabsorbing. Theoretical simulations agree with the experimental results, showing that the efficient light coupling into the substrate is mediated by Mie resonances formed within the SiGe islands. This lithography-free method can be implemented on arbitrarily thick or thin ${\mathrm{SiO}}_{2}$ layers and its duration only depends on the sample thickness and on the annealing temperature.