Characterization of MAE-textured nanoporous silicon for solar cells application: Optics and surface passivation

We present a three-step metal-assisted chemical etching (MAE) texturing technique to fabricate nanoporous Si (MAE nSi). The relationship between the resulting surface morphology, optical properties, and surface recombination of the MAE nSi are presented and analyzed. We also show that the solar weighted reflectance R_w of unencapsulated MAE nSi is less than 12% for all MAE nSi samples investigated in this paper. We show that the texture morphology has a near-isotropic surface reflectance for nonnormal light incidence that is similar to the isotexture morphology. The angular distribution of the reflected light suggests that most of the losses due to the surface reflectance can be recovered after encapsulation, and this has been confirmed experimentally in this paper. Intriguingly, despite an approximately threefold increase in the surface area (compared with a planar sample), following texturing, the increase in the surface recombination velocity eff for samples passivated by atomic layer deposited Al₂O₃ is found to be relatively small. This suggests that for the ALD-Al₂O₃ passivation scheme, neither the local curvature nor the predominant crystallographic orientation causes additional recombination at the MAE nSi surface.