  Textured silicon surface structures, in particular black silicon (b-Si), open
up possibilities for Si-based solar cells and photodetectors to be extremely
thin and highly sensitive owing to perfect light-trapping and anti-reflection
properties. However, near-infrared (NIR) performance of bare b-Si is limited by
Si band gap of 1.12 eV or 1100 nm. This work reports a simple method to
increase NIR absorption of b-Si by $in$ $vacuo$ silicidation with magnesium.
Obtained Mg$_2$Si/b-Si heterostructure has a complex geometry where b-Si
nanocones are covered by Mg$_2$Si shells and crowned with flake-like Mg$_2$Si
hexagons. Mg$_2$Si formation atop b-Si resulted in 5-fold lower reflectivity
and optical absorption to be no lower than 88\% over 200-1800 nm spectral
range. More importantly, Mg$_2$Si/b-Si heterostructure is more adjusted to
match AM-1.5 solar spectrum with theoretically higher photogenerated current
density. The maximal advantage is demonstrated in the NIR region compared to
bare b-Si in full accordance with one's expectations about NIR sensitive narrow
band gap ($\sim$0.75 eV) semiconductor with high absorption coefficient, which
is Mg$_2$Si. Results of optical simulation confirmed the superiority of
Mg$_2$Si/b-Si NIR performance. Therefore, this new wide-band optical absorber
called black silicide proved rather competitive alongside state-of-the-art
approaches to extend b-Si spectral blackness.
