High speed atmospheric pressure ALD for industrial scale solar cell passivation

A next generation material for surface passivation is atomic layer deposition (ALD) Al2O3. However, conventional time-resolved ALD is limited by its low deposition rate. Therefore, an experimental high-deposition-rate prototype ALD reactor based on the spatially-separated ALD principle has been developed. This reactor leads to deposition rates up to 1.2 nm Al2O3/s. In this work, the passivation quality and uniformity of the experimental spatially-separated ALD Al2O3 films are evaluated and compared to conventional temporal ALD Al2O3, by use of quasi-steady-state photo-conductance (QSSPC) and carrier density imaging (CDI). It is shown that spatially-separated Al2O3 films of increasing thickness provide an increasing surface passivation level. Moreover, on p-type CZ Si, 10 and 30 nm spatial ALD Al2O3 layers can achieve the same level of surface passivation as equivalent temporal ALD Al2O3 layers. In contrast, on n-type FZ Si, spatially-separated ALD Al2O3 samples generally do not reach the same optimal passivation quality as equivalent conventional temporal ALD Al2O3 samples. Nevertheless, after "firing", 30 nm of spatially-separated ALD Al2O3 on 250 µm thick n-type (1-5 O.cm) FZ Si wafers can lead to effective surface recombination velocities as low as 2.9 cm/s, compared to 1.9 cm/s in the case of 30 nm of temporal ALD Al2O3.