Effect of ALD reactants on blistering of aluminum oxide films on crystalline silicon

Abstract

Atomic layer deposited aluminum oxide (Al 2 O 3 ) has in recent years proven to be a promising surface passivation material for crystalline silicon solar cells. However, blistering in Al 2 O 3 films is a common problem deteriorating the surface passivation quality. Here, blistering is studied from material aspects including film thickness, film composition and postdeposition heat treatment. We show how thicker films, higher annealing temperatures and longer annealing times lead to more severe blistering and demonstrate how blistering can be avoided by using either O 3 as the oxidant or depositing a thin TiO 2 layer at the silicon interface.

Effect of substrate pretreatments on the atomic layer deposited Al2O3 passivation quality

“…It is well known that so called blisters (local film delamination from the substrate) can appear in Al 2 O 3 films if water is used as an oxidant in the ALD reaction and the film thickness exceeds 10 nm. 17 The blisters are known to deteriorate the surface passivation, 13 and could therefore explain our lifetime degradation in HF dipped samples after firing as seen in Fig. 2.…”

“…19 It is likely that in the case of thin chemical SiO 2 layer resulting from SC2 cleaning, there are less hydrogen residues at the interface, which decreases the amount of released hydrogen thus preventing the blistering. 17…”

Avoiding blistering in Al2O3 deposited on planar and black Si

“…In addition, the quality of passivation of Al2O3 is likely to be affected by the presence of blisters. A few studies reported variations of effective surface recombination velocity (Seff) with varying amount of blistering [32,35], based on effective lifetime (eff) measurements using by quasi-steady state photoconductance method, and therefore averaged on full size wafers. In addition, experimental direct evidence that the local quality of the passivation is degraded at the blisters is difficult to obtain, although such an effect is expected.…”

Effective passivation of p+ and n+ emitters using SiO2/Al2O3/SiNx stacks: Surface passivation mechanisms and application to industrial p-PERT bifacial Si solar cells

“…A thicker Al2O3 (e.g. >30 nm) can improve firing stability to some extent; such thick Al2O3 films, however, suffer from blistering [108,109] issues and are not cost-efficient.…”