Mg$_2$Si is the new black: introducing a black silicide with $>$95% average absorption at 200-1800 nm wavelengths

Abstract

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 ( ∼ 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 stateof-the-art approaches to extend b-Si spectral blackness.