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

Abstract

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 (SiNx) 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 12nm. The thermal stability of surface passivation by Ta2O5/SiNx 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/SiNx 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 214Ω/sq. Finally, optical simulation reveals a low reflection and negligible absorption loss for the Ta2O5/SiNx stack. The Ta2O5/SiNx stack is thus demonstrated to be an excellent surface passivation and antireflection coating for high efficiency silicon solar cells.