Interfacial roughness of

Abstract

X-ray-reflectivity techniques have been used to characterize the (001) surface of Au and Au bicrystals containing different [001] twist grain boundaries. Measurements of the specular and nonspecular reflectivities of single-crystal films grown by fast evaporation of Au wire onto NaCl single crystals indicate that the Au surfaces are very rough. Similar measurements of the bicrystal specimens suggest that the grain boundaries, which were produced by hot pressing together the single-crystal films, are also rough. Direct evidence for the existence of diffuse scattering from a grain boundary is presented in an in situ x-ray study of the growth and destruction of a grain boundary. Grain-boundary growth and destruction were found to occur on a relatively short time scale. This observation suggests that reflectivity and diffraction measurements of grain boundaries are reliable only when the formation of the grain boundary is continuously monitored. Since specular-reflectivity measurements are only sensitive to the magnitude of surface or interfacial roughness, the nonspecularly reflected diffuse scattering must be measured in order to fully characterize the roughness of a surface or interface, e.g., its fluctuation height and texture. The strength of diffuse scattering from grain boundaries was found to depend upon the geometrical arrangement of their surroundings; thus, the roughness of a grain boundary may be partially determined by its surroundings.The form of the nonspecular diffuse scattering from the single-crystal film and bicrystals could be explained by representing the free surface of the Au single crystal as a self-affine rough surface, and the grain-boundary region as a combination of self-affine rough interfaces. Using this model, details about the fluctuation heights and textures of the rough surface and interfaces, as well as the densities of three different twist grain boundaries could be determined. These results should motivate the development of computer simulations of grain-boundary structures that include self-affine interfacial roughness. A difficulty was encountered in the combination of the intensity profile of the diffuse scattering with that of the specularly reflected radiation from the single-crystal film. This difficulty might stem from a breakdown of the distorted-wave Born approximation, which may occur when a specimen producing copious quantities of diffuse scattering is examined with a strongly interacting probe. Alternatively, the difficulty may be caused by multiple sources of roughness, which are not yet included in theoretical treatments of diffuse scattering.