Temperature Dependence of the Structural Disorder of Artificially Roughened Silver Surfaces.

Artificial roughness on the surface of expitaxial (111) silver films is produced by low temperature vacuum desposition of an overlay of a few atomic layers. With heating the overlay anneals to become an integrated part of the underlying film between 200(DEGREES)K and 300(DEGREES)K. This transition has been monitored through the changes in surface scattering of conduction electrons as manifested in the conductance of silver films 100(ANGSTROM) to 800(ANGSTROM) thick. This conductance decreases sharply during the low temperature deposition of the overlay because of increased surface scattering. As the temperature is raised, the conductance at first decreases reversibly, but beyond a certain temperature it begins to increase irreversibly, signalling the onset of annealing of the overlay. The conductance data have been analyzed using a generalization of size effect theory that includes scattering throughout the depth of the disordered overlay as well as on its surface. Such interpretation leads to surface layer order correlation lengths varying from 20(ANGSTROM) to 400(ANGSTROM) as the transition proceeds. The initial state is characteristic of that of amorphous metals, while the final state of the fully annealed surface, still exhibits a finite order correlation length. The transition between these two end states goes via a series of metastable states at intermediate temperatures characterized by increasing structural order. The surface order correlation lengths vs. temperature, as found, are independent of the thickness of the underlying silver film, for film thicknesses above 300(ANGSTROM), and show diffusion active at a much lower temperatures than in the bulk. Effects in thinner films, and in the presence of inert atmosphere, are also explored.