Henriquez Correa, Ricardo Andres

The electrical transport mechanism of non-percolated copper ultrathin films was studied. For this purpose, resistance behavior was measured during sample growth, aging in vacuum and oxidation in air, and contrasted with a model based on tunnel current and on film’s morphology. In addition, the electrical characterization of chromium and gold ultrathin films was performed and compared with that obtained for copper. All films were grown on muscovite mica through thermal evaporation under high vacuum conditions. Electrical characterization throughout films’ growth, aging and oxidation was performed in situ and in real time. Finally, to address the transport mechanism of non-percolated oxidized copper films, samples were put into a cryostat in which electrical resistance was measured changing the temperature between 35 and 300 K. It was found that the three materials present an almost constant resistance decay during growth. This resistance decrease was studied for copper films by fitting a tunnel transport model which considered islands’ distance as a function of film thickness, indicating a resistance reduction given by coalescing islands. During aging, the resistance of copper and gold ultrathin films increases without reaching a saturation value after 30 min, with a behavior independent of the material or the initial resistance. The theoretical model applied to copper film resistance explains the increment by further formation of 3D structures, mainly conducted by atom diffusion on the substrate. Finally, a change in the resistance behavior is observed during the oxidation of copper ultrathin films, electrical transport is mediated by two mechanisms a semi-conductor type, resembling that of oxidized chromium layers, and a tunnel conduction type, observed in gold films. The first mechanism dominates when temperature is above 200 K, while tunneling is the main process for temperatures below 150 K.

The predominant role of surface scattering, in electronic transport through nanoscale thin films, was examined by measuring and modeling the change in resistivity induced by the adsorption of thiols on the surface. For this purpose, gold ultrathin films on sapphire, using chromium as surfactant, were prepared through thermal evaporation, seeking to maximize the surface induced dispersion. A maximum resistivity increase of 13.5% was observed, in an 8 nm Au/Cr/Sapphire sample. This is the highest reported value for such films to date. The morphology of the samples was measured by STM and characterized through height-difference correlation function. A fractal self-affine representation of the surface was found, and it was modified to account the thiol effect: the scattering center due to the adsorption of a molecule was modeled as a void in the surface. This change was related to the electrical transport of the film through two quantum models: the Palasantzas and Barnas (PB) theory and the Sheng, Xing and Wang theory, extended by Munoz et al. (mSXW). For our samples, the mSXW theory did not predict a resistivity change, while the PB theory provides a good description of the experimental resistivity increase due to thiols adsorption as function of thickness.