Thermal stability of spatial ALD deposited Al2O3 capped by PECVD SiNx for the passivation of lowly- and highly-doped p-type silicon surfaces

Al2O3 deposited by atomic layer deposition (ALD) is known to provide an excellent passivation on lowly- and highly-doped p-type silicon surfaces even after contact firing processes. Therefore, Al2O3 layers are of great interest for the passivation of the rear side of p-type solar cells as well as the boron-doped emitter of n-type solar cells. In this work we studied the passivation quality of fired Al2O3/SiNx layer stacks on lowly- and highly-doped p-type silicon surfaces applying an in-line capable, spatial ALD tool for the Al2O3 deposition. On both surfaces we observed a very high level of surface passivation for stacks with 4 nm Al2O3 after firing. For lowly-doped p-type wafers, we measured effective lifetimes up to ~2.8 ms, which correspond to an upper limit for the surface recombination velocity of ~3 cm/s. On textured p+np+ samples diffused with a shallow boron doped emitter, we obtained emitter saturation current densities around 50 fA/cm2, allowing an open-circuit voltage potential in the range of 700 mV. Spatially resolved lifetime images revealed a very uniform passivation over the whole wafer for both types of samples. These results demonstrate that Al2O3/SiNx stacks are well suited for the industrial fabrication of p-type and n-type solar cells, without the necessity of an extra thermal out-gassing step between Al2O3 and SiNx deposition.