Improving doped polycrystalline silicon passivating contacts with magnesium fluoride

Passivating contacts based on doped polysilicon (poly-Si) and ultrathin silicon oxide offer outstanding surface passivation and low contact resistance. The passivation quality of poly-Si contacts can be further improved by hydrogenating the oxide interface, often achieved via thermal annealing with hydrogen-containing dielectric coatings such as silicon nitride (SiNx) or aluminium oxide (AlOx). In this work, we present an alternative method using magnesium fluoride (MgFx) coatings to improve the recombination properties of doped poly-Si layers. It is found that MgFx coatings can yield comparable benefits as AlOx and SiNx on n-type poly-Si, enhance the passivation effect of SiNx on p-type poly-Si when used as an additional capping layer, and facilitate the recovery of firing-induced-degradation in n-type poly-Si. Besides improving passivation, MgFx exhibits some inherent advantages over other dielectric films in that it is more conductive and has a low work function. Utilising these properties, we have demonstrated the use of 10 nm thick MgFx films as an interlayer to improve contact formation between n-type poly-Si and evaporated aluminium, achieving a contact resistivity of 1.6mΩ∙cm2 while preserving the passivation. Our results highlight a possible pathway to use MgFx to simultaneously improve the passivation and metallisation of doped poly-Si contacts in industrial production.