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

We demonstrate a simple self-assembly method based on solid state dewetting of ultra-thin silicon films and germanium deposition for the fabrication of efficient anti reflection coatings on silicon for light trapping. Via solid state dewetting of ultra-thin silicon on insulator and epitaxial deposition of Ge we fabricate SiGe islands with a high surface density, randomly positioned and broadly varied in size. This allows 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 degrees) and to trap within the wafer a large portion of the impinging light (∼40%) also below the band-gap, where the Si substrate is non-absorbing. Theoretical simulations agree with the experimental results showing that the efficient light coupling into the substrate mediated by Mie resonances formed within the SiGe islands. This lithography-free method can be implemented on arbitrarily thick or thin SiO2 layers and its duration only depends on the sample thickness and on the annealing temperature.