Competitive gettering of iron in silicon photovoltaics: Oxide precipitates versus phosphorus diffusion

J D Murphy,R J Falster,R E Mcguire,K Bothe,V V Voronkov

doi:10.1063/1.4892015

Abstract

Experiments have been conducted to understand the behaviour of iron in silicon containing oxide precipitates and associated defects (dislocations and stacking faults), which is subjected to phosphorus diffusion gettering. Injection-dependent minority carrier lifetime measurements are analysed to provide quantitative information on the degree to which the precipitates and associated defects are decorated with iron impurities. These data are correlated with bulk iron measurements based on the photodissociation of FeB pairs. Iron in the vicinity of oxide precipitates in samples with relatively low levels of bulk iron contamination (&lt; 5 × 1012 cm−3) can be gettered to some extent. Higher levels of bulk iron contamination (&gt; 1.2 × 1013 cm−3) result in irreversible behaviour, suggesting iron precipitation in the vicinity of oxide precipitates. Bulk iron is preferentially gettered to the phosphorus diffused layer opposed to the oxide precipitates and associated defects.

Highlights

Silicon wafers for photovoltaics may contain extended crystal defects
Experiments have been conducted to understand the behaviour of iron in silicon containing oxide precipitates and associated defects, which is subjected to phosphorus diffusion gettering
We have systematically studied the contamination of oxide precipitate-containing silicon with iron, and how the material subsequently responds to phosphorus diffusion gettering (PDG)

Summary

Introduction

Silicon wafers for photovoltaics may contain extended crystal defects. In multicrystalline silicon (mc-Si), these include dislocations, precipitates, and grain boundaries. Gettering is routinely used in solar cell processing to remove impurities. Gettering processes, such as phosphorus diffusion gettering (PDG) and aluminium gettering, are highly effective in silicon wafers free of extended defects.. Gettering processes, such as phosphorus diffusion gettering (PDG) and aluminium gettering, are highly effective in silicon wafers free of extended defects.6,7 They are less effective at improving lifetime in the vicinity of decorated extended defects.. They are less effective at improving lifetime in the vicinity of decorated extended defects.8–10 This suggests that the interaction between impurities and extended defects is either not fully overcome during gettering processes, or that the impurities re-segregate to the extended defects during cooling. More experimental work is needed to ascertain whether or not impurities can be gettered away from different types of extended defect

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Discussion

Conclusion