Impurity segregation at well-defined interfaces: sulphur at (111)Cu/(110)Mo and (111)Ag/(110)Mo interfaces

Ag/Mo and Cu/Mo interfaces are created by evaporating a silver (or copper) thin film onto a (110) molybdenum single crystal. Growth takes place according to epitaxial relations and the interface is expected to be as coherent and clean as possible. Both samples are stable at all temperatures, in vacuum or in H2, even under fast quenching or very long annealing. Under exposition to H2S/H2 mixtures, sulfur adsorbs on silver (or copper), diffuses through the thin film, and segregates at the interface. The segregated quantity, as measured by radiotracer techniques through the thin film, is found to be about one sulfur monolayer (7 x 1014 at/cm2) in both cases at low sulfur chemical potential. Such quantities, and the very accuracy of the results, indicate that segregated atoms are located precisely at the interface and even organized as a true monolayer, bound to silver (or copper) and molybdenum on its both sides. The presence of sulfur at Cu/Mo interfaces then induces a decomposition of the film into small crystallites which wet only partially the molybdenum substrate. Under classical assumptions, contact angle measurements lead to values of the shift of the interfacial energy caused by the segregated sulfur layer