Electron density changes and the surface resistivity of thin metal films: oxygen on Cu(1 0 0)

Through a study of dc resistance and infrared reflectance changes induced in epitaxial Cu(1 0 0) films by adsorbed oxygen, we show that standard surface resistivity models based on free electrons and point scatterers are inadequate, even if adsorbate-induced changes in conduction electron density are considered. Previous experiments showed that the reflectance–resistance change ratio varies among adsorbates on identically prepared samples, in contradiction of free-electron scattering models. Electron density changes were proposed to account for the variation. In the present work the ratio for adsorbed oxygen is found to vary with the conductivity of the clean film, which differs from sample to sample. Interpreting these results within a free electron model would require that each adsorbate localize an unreasonably large number of conduction electrons. Significant modification of the prevailing free-electron models, perhaps including energy-dependent scattering, will be necessary to explain the experimental results.