CHARGE-TRANSFER AND SURFACE SCATTERING AT CU-C-60 PLANAR INTERFACES

Thin-film planar structures of Cu and C-60 have been sequentially depo sited onto sapphire substrates in high vacuum and studied using in sit u resistivity measurements during deposition together with ex situ ato mic force microscopy characterization of surface topography. Two diffe rent regimes of behavior are identified. In the first of these. the th in-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 m onolayer, 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 800 0 Omega, with a corresponding room-temperature resistivity that is a f actor 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 underly ing monolayer of C-60 is sufficient to reduce the critical thickness a t 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 beh avior, the continuous film limit in which the Cu is thick enough to ha ve a size-effect resistivity proportional to the reciprocal of the him thickness. the presence of an adjacent C-60 monolayer gives rise to a n increase in resistance. Measurements on a number of samples with dif ferent thicknesses reveal that this resistance increase is best descri bed by diffuse surface scattering. A scattering cross section of 5 Ang strom(2) resulting from a fit to this model represents the contact are a under each C-60 molecule.