The role of laser surface patterning for enhanced optical absorptance of 'black silicon'

Laser surface patterning has recently been used to form black silicon. For photon energies above the band gap energy (1.07 eV corresponding to a wavelength of 1.1 mum) the origin of the very low reflectivity can be accurately modelled as arising from enhanced absorptance from the microstructure on the silicon. However, at lower photon energies the decreased reflectance cannot be attributed to the structured surface alone. For these lower photon energies an excellent theoretical description of the experimentally observed absorptance can be achieved by the addition of an increased imaginary part of the complex permittivity. This term increases linearly with wavelength in the energy range 1.07 eV down to 0.496 eV ( corresponding to a wavelength of 2.5 mum). The implication is that the structuring has introduced a wide range of new surface and sub-surface states ( potentially from impurities) which absorb the light.