Charge transfer and surface scattering at Cu-C-60 planar interfaces

Thin-film planar structures of Cu and C-60 have been sequentially deposited onto sapphire substrates in high vacuum and studied using in situ resistivity measurements during deposition together with ex situ atomic force microscopy characterization of surface topography. Two different regimes of behavior are identified. In the first of these. the thin-film limit in which the Cu is thin enough to be in the coalescence regime with an islanded morphology, the presence of an adjacent C-60 monolayer, doped by charge transfer from the metal, creates a shunting path and a corresponding pronounced decrease in resistance. The sheet resistance of overlying doped monolayers is found to be similar to 8000 Omega, with a corresponding room-temperature resistivity that is a factor of 2 less than that of the three-dimensional alkali-metal-doped compounds A(3)C(60) (A=K, Rb). The enhanced conductivity of an underlying monolayer of C-60 is sufficient to reduce the critical thickness at which an overlying Cu film becomes conducting by almost a factor of 2 even though the roughness of such films is enhanced over that of Cu films deposited directly on the substrate. In the second regime of behavior, the continuous film limit in which the Cu is thick enough to have a size-effect resistivity proportional to the reciprocal of the him thickness. the presence of an adjacent C-60 monolayer gives rise to an increase in resistance. Measurements on a number of samples with different thicknesses reveal that this resistance increase is best described by diffuse surface scattering. A scattering cross section of 5 Angstrom(2) resulting from a fit to this model represents the contact area under each C-60 molecule.