Field emission and field ion microscope studies of the epitaxial growth of nickel on tungsten

Single crystals of nickel have been grown on tungsten field emitter tips by vapour deposition in UHV, using substrate temperatures in the range 425–700° K, and nickel fluxes of the order of 0.1–10 atom layers per minute. The orientation relationship is {111} Ni ∥ {110} W, with 〈211〉 Ni ∥ 〈110〉 W. The directional misfit is 3.5%. The crystals grow from initial nuclei formed on the {110} step ledges at multilayer coverages.To permit direct FIM study of the crystals, some specimens were grown on nearly perfect, field evaporated tungsten surfaces, held at 500°K. Examination at 78°K, using hydrogen and helium as the imaging gases, gave information regarding the structure, form and degree of perfection of the crystals. An interfacial reaction was observed, which produced considerable disordering at the tungsten surface. The mechanism of this reaction apparently involved the substitutional replacement of tungsten atoms by nickel, in the interface lattice. The implications of these observations with respect to the theory of epitaxial growth are discussed.