Comparison of the thermal stability of single Al2O3 layers and Al2O3/SiNx stacks for the surface passiviation of silicon

We measure surface recombination velocities (SRVs) below 10 cm/s on low-resistivity (1.4 cm) p-type crystalline silicon wafers passivated with plasma-assisted and thermal atomic layer deposited (ALD) aluminium oxide (Al2O3) films. Ultrathin Al2O3 films (< 5 nm) are particularly relevant for the implementation into solar cells, as the deposition rate of the ALD process is very low compared to e.g. plasma-enhanced chemical vapor deposition (PECVD). Hence, we examine the passivation quality of a stack consisting of an ultrathin Al2O3 passivation layer deposited by ALD and a SiNx capping layer deposited by PECVD. Our experiments show a substantial improvement of the thermal stability during firing at 810 °C for the Al2O3/SiNx stacks compared to a single Al2O3 layer. We report on a „regeneration effect‟ observed for Al2O3 single layers after firing, where the degraded passivation is significantly improved after annealing at 400 °C and also by illumination at room temperature using a halogen lamp. Nevertheless, for Al2O3/SiNx stacks we measure SRVs < 15 cm/s after firing, whereas for Al2O3 single layers the regenerated SRVs are in the range of 10-30 cm/s. Al2O3/SiNx stacks are hence ideally suited for the implementation into industrial-type silicon solar cells, although „regenerated‟ Al2O3 single layers should result in a comparable cell performance.