Conformal Al2O3 coatings on black silicon by thermal ALD for surface passivation

Upon inductive coupled plasma reactive ion etching (ICP-RIE) of Si surfaces needle-like nanostructures with aspect ratios up to 10 emerge showing excellent anti-reflection and light-trapping properties with absorption over 97% throughout the UV and VIS spectral range. In addition, the absorption at the band edge of silicon is enormously enhanced due to scattering. In this work we report on the feasibility to deposit conformal Al2O3 dielectric layers on these very rough silicon surfaces which enable adequate surface passivation. Lifetimes over 170 s have been measured on deep structured b-Si substrates. The optical properties of black silicon (b-Si) and the possible influence of applied alumina passivation layers as well as their passivation performance are discussed.