Passivation of c-Si surfaces by ALD tantalum oxide capped with PECVD silicon nitride

We demonstrate effective passivation of a variety of crystalline silicon (c-Si) surfaces by thermal atomic layer deposited (ALD) tantalum oxide (Ta₂O₅) underneath a capping silicon nitride (SiN_x) layer by plasma enhanced chemical vapor deposited (PECVD). Surface recombination is investigated as a function of Ta₂O₅ thickness for p- and n-type Si substrates, both with and without boron (p⁺) or phosphorus (n⁺) diffusions. It is found that the recombination decreases markedly with increasing Ta₂O₅ thickness on p, n and p⁺ c-Si surfaces, but it follows an opposite trend on n⁺ c-Si surfaces. In all four cases, the surface recombination velocity plateaus at a Ta₂O₅ thickness of 12 nm. The thermal stability of surface passivation by Ta₂O₅/SiN_x is examined by subjecting p⁺ and n⁺ diffused wafers to a typical solar cell metallization firing process, finding that it is essentially stable on p⁺ diffusions, but not on n⁺ ones, regardless of Ta₂O₅ thickness. We also evaluate the passivating properties of the Ta₂O₅/SiN_x stack on planar {100}, planar {111}, and textured n-type undiffused silicon surfaces, finding that (i) planar {111} Si exhibits a 4.6-fold higher recombination than planar {100} Si, and (ii) recombination at a textured surface is approximately equivalent to that at a planar {111} after surface area correction. Furthermore, the area-corrected recombination ratio of textured to planar {100} boron diffused p⁺ regions is shown to be 2.2 for three different diffusions with sheet resistances at 56, 122, and 214 Ω/sq. Finally, optical simulation reveals a low reflection and negligible absorption loss for the Ta₂O₅/SiN_x stack. The Ta₂O₅/SiN_x stack is thus demonstrated to be an excellent surface passivation and antireflection coating for high efficiency silicon solar cells.