Background:The process of electromigration is still not quantitatively understood. We showed recently that it can be used reliably for formation of single atomic point contacts in pre-structured Ag nanostructures. Results:The process of formation of nanocontacts by electromigration (EM) down to a single atomic point contact was investigated for ultrathin (5 nm) Ag structures at 100\,K. In this paper, we compare the structures with constrictions below the average grain size of Ag layers (15 nm), where the contribution of a single grain dominates, with structures of much larger constrictions of around 150 nm with multiple grains at the centre constriction during the initial steps of EM. The latter initially form filamentous structures. Despite these clear morphological differences, the conductance traces of both types of structures suggest that finally, i.e., in the quantized conduction regime, only one atomic point contact was formed. To analyse the thinning process within the semi-classical regime in detail, we used experimental conductance histograms in the range between 2 G0 and 15G0 and their corresponding Fourier transforms (FT). The FT analysis of the conductance histograms exhibits a clear preference for thinning along the  direction. Using well-established models, both atom-by-atom steps and ranges of stability, presumably caused by electronic shell effects, can be discriminated. A large range (5 to 14G0) of unstable conductance values was found in these electromigrated contacts that has not been reported by other techniques. It was observed irrespective of the initial geometry. Conclusion: Although the directional motion of atoms during EM leads to specific properties like the instabilities mentioned, similarities to mechanically opened contacts with respect to cross sectional stability were found.
Keywords: electromigration; nanostructures; silver; Si substrate; focussed ion beam
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Chatterjee, A.; Tegenkamp, C.; Pfnür, H. Beilstein Arch. 2019, 2019149. doi:10.3762/bxiv.2019.149.v1
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