Some controversial points related to transport in microcrystalline silicon.
Résumé
We have compared the results of effective medium theories with the d.c. dark electrical conductivity for a number of undoped thin film Si series crossing the amorphous/microcrystalline transition. All series exhibited the percolation threshold at 70 – 80 % crystallinity, i.e., at values higher than expected for a random mixture. We explain this observation by a resistive coating of the crystalline grains, which presents a limiting step for transport. We identify this coating with the Large Grain Boundaries (LGB) from our previously formulated model of transport. The LGBs consist of an amorphous Si based tissue with a band gap increased due to H and O alloying, to which also most of the defects concentrate. The apparent paradox of higher room temperature conductivity resulting from the formation of LGBs is explained, too. Our microscopic measurements of conductivity with an AFM tip are consistent with this model if care is taken to avoid artifacts related to tip induced oxidation of the sample surface.
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