  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 micrometer. In
the present letter, we explore the effect of surface doping on BSi radiative
properties and it 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
measured using Fourier transform infrared spectroscopy. We show that doping an
n-type BSi wafer with Phosphorous with a dose of 10^17 atm/cm2 and an energy of
100 keV increases its absorptivity up to of 98% in the spectral range of 1-5
micrometer. 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.
