Silicon surface passivation by ultrathin Al2O3 films and Al2O3/SiNx stacks

We show that aluminum oxide (Al₂O₃) layers deposited by thermal as well as by plasma-assisted atomic layer deposition (ALD) are very well suited for the effective surface passivation of p-type silicon wafers. Surface recombination velocities (SRVs) well below 10 cm/s are measured for both ALD variants. The SRV strongly increases with decreasing film thickness if the Al₂O₃ films are <;10 nm for thermal ALD and <;5 nm for plasma ALD. Firing at 830°C in a conveyor-belt furnace deteriorates the passivation quality, in particular for ultrathin Al₂O₃ films. For Al₂O₃ films ≤10 nm the thermal stability of the Al₂O₃ was significantly improved by depositing a 75-nm capping layer of PECVD-SiN_x onto the Al₂O₃. Application of Al₂O₃ to the rear of PERC solar cells shows that high V_oc values above 660 mV and J_sc values of 40 mA/cm² are achievable, irrespective of the ALD variant applied. However, generally slightly higher efficiencies are obtained for Al₂O₃ deposited by plasma ALD. The plasma ALD layers result in the highest efficiency of 21.1%.