Absorptivity enhancement of black silicon using electroless Cu plating

In this work, lightly-doped black silicon (BSi) is fabricated using cryogenic DRIE in a maskless manner and its transmittance and reflectance are measured using an integrating sphere and a spectrometer in the near infrared (NIR) wavelength range of 1300 nm - 2500 nm. Then, the surface is cleaned and copper (Cu) is deposited on the BSi using the wet deposition technique of electroless plating, enabling high throughput coating. The copper ions are deposited on the BSi surface in a Cu sulphate solution, taking advantage of the conformity of the plating to the nano/micro structures of the BSi targeting lower reflectance and higher absorptivity. The Cu-plated BSi is measured and observed to have a minimum reflectance of 10% compared to 30% in the case of BSi, and a minimum transmittance of 10% compared to 40% in the bare black silicon. Thus, the Cu-plated BSi has a maximum absorptivity of about 80% compared to 30% in the bare BSi. The absorptivity is found to decrease with increasing the wavelength. This enhancement using the electroless Cu plating further qualifies the BSi as a candidate for NIR thermal light sources.