Optical modeling of needle like silicon surfaces produced by an ICP-RIE process

Abstract

We present results of rigorous optical modeling of reactive ion etched crystalline silicon surfaces, so called Black Silicon, for different etching parameters and compare them to experimental data. Reactive ion etching of crystalline silicon with SF6 and O2 can produce a surface consisting of sharp randomly distributed needle like features with a characteristic lateral spacing of about a few hundreds of nanometers and a wide range of aspect ratios depending on the process parameters. Due to the very low reflectance over a broad spectral range such surface textures can be beneficial for photon management in photovoltaic applications. To gain a detailed understanding of the optical properties of Black Silicon surfaces we recovered the full three dimensional geometry of differently etched samples. With these data we calculated the optical response using the finite differences time domain method. From the calculations we will give insight into the magnitude of resonant phenomena within the Black Silicon and the resulting near field enhancement. Furthermore we will present carrier generation profiles which quantify the effect of absorption enhancement due to the nanostructured surface. We also investigate the angular forward scattering distribution into the silicon substrate and the resulting path length enhancement which is crucial for the near band edge absorption especially in thin solar cells.