Abstract
The results of effective medium theories are compared with the dc 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. This observation is explained in terms of a resistive coating of the crystalline grains, which presents a limiting step for transport. The coating is identified with the large grain boundaries (LGBs) from a 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 also explained. 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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