Temporal and spatial atomic layer deposition of Al-doped zinc oxide as a passivating conductive contact for silicon solar cells

Recently, stacks consisting of an ultrathin SiO2 coated with atomic-layer-deposited (ALD) Al-doped zinc oxide (ZnO:Al) have been shown to yield state-of-the-art passivation of n-type crystalline silicon surfaces and provide low contact resistivities to n(+)-doped Si and poly-Si surfaces. Key for achieving good surface passivation are an intentionally-grown SiO2 interlayer, an aluminum oxide (Al2O3) capping layer and a post-deposition anneal, whereas n-type doping of the ZnO is required to achieve a low contact resistivity. In this work, we present the latest results and insights obtained for this contact stack. This includes a study of the minimum required thicknesses of both the ZnO and the Al2O3 capping layer to achieve a high passivation level after post-deposition anneal. Also, we provide details on how to remove the Al2O3 capping layer selectively from the ZnO:Al after the post-deposition anneal using a pH-controlled wet-etch, such that the ZnO:Al can be contacted by a metal. Whereas previous work was based on lab-scale temporal ALD, in this work we highlight the industrialization potential by demonstrating that these layers can be prepared by spatial ALD, yielding good passivation levels on both undiffused n-type and n(+)-diffused c-Si surfaces. Finally, we demonstrate the capability of ALD to deposit ZnO:Al layers selectively on oxidized regions of an otherwise HF-last treated c-Si surface. Such area-selective deposition opens up potential pathways for local, self-aligned contact formation. Altogether, this work provides valuable insights into the working mechanism and practical aspects of ZnO:Al-based passivating contacts.

Automated summary

Recently, it has been shown that stacks consisting of ultrathin SiO2 coated with ALD Al-doped zinc oxide (ZnO:Al) can achieve excellent passivation and low contact resistivity. This study investigates the required thicknesses of ZnO and Al2O3 capping layer for high passivation levels, and demonstrates selective removal of Al2O3 layer from ZnO:Al. Furthermore, it highlights the potential of spatial ALD for industrial applications and the selective deposition of ZnO:Al on oxidized regions of a c-Si surface.