Effect of MACE Parameters on Electrical and Optical Properties of ALD Passivated Black Silicon

Metal-assisted chemical etching (MACE) enables efficient texturing of diamond-wire sawn multicrystalline silicon wafers. However, the excellent optics are often sacrificed by polishing the surface to achieve better surface passivation with chemicalvapor-deposited (CVD) silicon nitride (SiN
_X
). In this paper, we show that a polishing step is not required when CVD SiN
_X
is replaced with atomic-layer-deposited (ALD) aluminum oxide (Al
₂
O
₃
). Indeed, while polishing increases reflectance, it has in general only very modest effect on surface recombination velocity of ALD-passivated b-Si. Furthermore, since ALD Al
₂
O
₃
is compatible with various surface morphologies due to its excellent conformality, the MACE parameters can be more freely adjusted. First, the concentration of silver nitrate (AgNO
₃
) in AgNO
₃
/H
₂
O solution that is used to deposit Ag nanoparticles is shown to affect the final b-Si morphology. Instead of needle-shaped b-Si produced by 5 mmol/L AgNO
₃
concentration, two orders of magnitude lower AgNO
₃
concentration produces porous structures, which are more challenging to passivate. Additionally, we demonstrate that a separate Ag nanoparticle removal step in nitric acid (HNO
₃
) is not a prerequisite for high carrier lifetime. Instead, Ag nanoparticles present during polishing in a HF/HNO
₃
/H
₂
O solution affect the final b-Si morphology by accelerating the etching of Si. The results demonstrate that no tradeoffs are necessary between optical and electrical properties of MACE b-Si when using ALD.