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

TL;DR

This paper introduces Mg$_2$Si/b-Si heterostructures that significantly enhance near-infrared absorption and reduce reflectivity in black silicon, improving its suitability for solar and photodetector applications.

Contribution

It presents a simple in-vacuo silicidation method to create Mg$_2$Si shells on black silicon, boosting NIR absorption and spectral matching with solar spectrum.

Findings

Achieved at least 88% absorption from 200-1800 nm

Reduced reflectivity by five times compared to bare b-Si

Enhanced NIR performance aligned with Mg$_2$Si's narrow band gap

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 ($\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.