We investigate nanocontact formation by thermally assisted electromigration of gold nanowires. An automatic cycling process allows us to follow a line of constant power dissipated at the nanocontact up to resistances corresponding to 10–20 conductance quanta. The contacts are thinned in a controlled way by voltage-induced heating. In the ballistic regime, oscillations of the conductance histograms show oscillations typical for atomic discreteness.

We show scanning probe microscopy measurements of metallic nanocontacts between controlled electromigration cycles. The nanowires used for the thinning process are fabricated by shadow evaporation. The highest resolution obtained using scanning force microscopy is about 3 nm. During the first few electromigration cycles the overall slit structure of the nanocontact is formed. The slit first passes along grain boundaries and then at a later stage vertically splits grains in the course of consuming them. We find that first the whole wire is heated and later during the thinning process as the slit forms the current runs over several smaller contacts which needs less power.Comment: 4 pages, 4 figure

The electrical resistance R of metallic nanocontacts subjected to controlled cyclic electromigration in ultra-high vacuum has been investigated in-situ as a function of applied voltage V . For sufficiently small contacts, i.e., large resistance, a decrease of R(V ) while increasing V is observed.This effect is tentatively attributed to the presence of contacts separated by thin vacuum barriers in parallel to ohmic nanocontacts. Simple model calculations indicate that both thermal activation or tunneling can lead to this unusual behavior. We describe our data by a tunneling model whose key parameter, i.e., the tunneling distance, changes because of thermal expansion due to Joule heating and/or electrostatic strain arising from the applied voltage. Oxygen exposure during electromigration prevents the formation of negative R(V ) slopes, and at the same time enhances the probability of uncontrolled melting, while other gases show little effects. In addition, indication for field emission has been observed in some samples.

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We have observed an aggregation of carbon or carbon derivatives on platinum and natively oxidized silicon surfaces during STM measurements in ultra-high vacuum on solvent-cleaned samples previously structured by e-beam lithography. We have imaged the aggregated layer with scanning tunneling microscopy (STM) as well as scanning electron microscopy (SEM). The amount of the aggregated material increases with the number of STM scans and with the tunneling voltage. Film thicknesses of up to 10 nm with five successive STM measurements have been obtained.