The fundamental properties of SiNx:H that determine its passivating qualities

Structural properties of SiN_x:H layers deposited with N₂+SiH₄ and NH₃+SiH₄ are related to the passivation properties. It is shown that the Si-N bond density in the layers is an important parameter for the passivation of mc-Si solar cells. The best bulk and thermally most stable surface passivation is observed for Si-N bond densities around 1.1·10²³ cm^-3. Lower bond densities result in a more open structure and will probably lead to H₂ formation during annealing. These H₂ molecules do not contribute to the passivation but will effuse from the layers into the ambient. Higher Si-N bond densities will result in too dense layers with a lower diffusivity of H and less bulk passivation during short anneals like contact firing. This explains why the bulk passivating properties of SiN^x:H layers does not depend on the initial H concentration in those layers.