Spatially separated atomic layer deposition of Al2O3, a new option for high-throughput Si solar cell passivation

Abstract

A next generation material for surface passivation of crystalline Si is Al2O3. It has been shown that both thermal and plasma-assisted (PA) atomic layer deposition (ALD) Al2O3 provide an adequate level of surface passivation for both p- and n-type Si substrates. 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 and Al2O3 deposition rates up to 1.2 nm/s have been demonstrated. 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 mu m thick n-type (2.4 Omega 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. Copyright (C) 2011 John Wiley & Sons, Ltd.