Title:	Stress-Induced De-passivation of Poly-Si/SiNx Layer Stacks
Author(s):	J. Hoß, J. Lossen, J. Linke, R. Kopecek, F. Buchholz
Keywords:	Annealing, Degradation, Passivation, Polycrystalline, Hydrogen
Topic:	Silicon Materials and Cells
Subtopic:	High Temperature Route for Si Cells
Event:	8th World Conference on Photovoltaic Energy Conversion
Session:	1AO.5.4
Pages:	22 - 28
ISBN:	3-936338-86-8
Paper DOI:	10.4229/WCPEC-82022-1AO.5.4
Price:	0,00 EUR
Document(s):	paper

Abstract/Summary:

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The present study investigates the passivation of layer stacks consisting of an interfacial oxide, LPCVD deposited polycrystalline silicon (poly-Si) and PECVD deposited silicon nitride (SiNx) layers when subjecting them to high temperature processes after the deposition of SiNx. We show that the passivation of such poly-Si/SiNx layer stacks can be severely degraded by high temperature processes. Further, we found that the passivation can be recovered after removing the annealed SiNx layer and subsequent annealing of the c-Si/SiO2/poly-Si interface. We conclude that thermal stress, induced by the densification of SiNx contributes to the loss of passivation at the c-Si/SiO2/poly-Si interface. Comparing symmetric and asymmetric sample structures, where the poly-Si/SiNx stack is present on both and on only one side respectively, shows that stress induced de-passivation and the re-passivation of poly-Si/SiNx stacks cannot be appropriately studied with symmetric test structures under certain circumstances. The importance of these findings is underlined when developing cell concepts that require the presence of such stacks in high temperature process steps. As an example, results for TOPCon cell precursors fabricated with an alternative process sequence with poly-Si deposition before BBr3 emitter diffusion are presented. Although the poly-Si layer was capped with SiNx during the emitter diffusion, cell precursors with iVoc values of up to 709 mV could be produced with this process. We further propose to consider stress-induced de-passivation as potential root-cause for the degradation of the passivation of polySi/SiNx stacks during the firing process of such samples.