Wideband mid infrared absorber using surface doped black silicon

Abstract

Black silicon (BSi) is a synthetic nanomaterial with high aspect ratio nano protrusions inducing several interesting properties such as a very large absorptivity of incident radiation. We have recently shown that heavily doping the BSi in volume enables to significantly enhance its mid-infrared absorptivity and tune its spectral range of interest up to 20  μm. In the present letter, we explore the effect of surface doping on BSi radiative properties and its absorptance in particular since surface doping enables reaching even larger dopant concentrations than volume doping but at more limited penetration depths. We considered 12 different wafers of BSi, fabricated with cryogenic plasma etching on n- and p-type silicon wafers and doped using ion-implantation with different dopant types, dosages, and ion beam energies, leading to different dopant concentrations and profiles. The different wafers radiative properties, reflectance, transmittance, and absorptance are experimentally measured using Fourier transform infrared spectroscopy. We show that doping an n-type BSi wafer with phosphorous with a dose of 1017 atm/cm2 and an energy of 100 keV increases its absorptivity up to 98% in the spectral range of 1–5  μm. We propose a simple phenomenological explanation of the observed results based on the dopant concentration profiles and the corresponding incident radiation penetration depth. Obtained results provide simple design rules and pave the way for using ion-implanted BSi for various applications, such as solar energy harvesting, thermo-photovoltaics, and infrared radiation sensing, where both high absorptance and variable dopant concentration profiles are required.