Impurity Conduction along the Least Resistance Paths of Granular Insulating Thin Films

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Abstract
The electrical charge transfer is investigated through Au—FeF3—Au thin‐film structures as a function of the crystallization degree of the insulating layers. For a constant growth rate, the texture of the samples, prepared by sublimation under vacuum, depends essentially on the substrate temperature during the condensation of the vapours (Ts). Between 450 and 650 K, the FeF3 thin films are granular. The crystallized grains, the average diameter of which increases with Ts, are embedded in a disordered matter. Under these conditions, the dc conductivity of the structure greatly depends on the preparation parameter Ts. However, in all cases, as for many other insulators, the charge transfer throughout FeF3 is due to the presence of localized electronic states situated into its band gap. These states correspond to a weak lack of fluorine and are situated at 1.1 eV below the conduction energy. Now it is demonstrated that the corresponding Coulombic wells in interaction are more particularly concentrated in the intergranular zones of the layers. If their density remains constant in the whole, it grows with Ts in these regions when the volume of disordered matter decreases, which defines the least resistance paths. Along these percolation ways, the average height of the potential barriers separating two consecutive states decreases when Ts increases. In consequence, if the dc conductivity results from thermally activated jumps of electrons above these potential barriers at high temperature, below a temperature which increases with Ts a hopping process predominates.