X-ray photoelectron forward scattering studies of surface segregation in epitaxial Ni–Cu–Ni(100) sandwich structures

Abstract

The forward scattering of the Cu 2p3/2 core level photoelectrons by overlying Ni lattice atoms which occurs in the photoemission process has been used to follow the segregation of Cu to the surface of epitaxial Ni–Cu–Ni(100) sandwich structures. The results indicate that when 1 monolayer (ML) of Cu is covered by 1–2 ML of Ni, surface diffusion processes constitute the mechanism by which the segregation occurs, whereas if 1 ML of Cu is covered by 10 ML or more of Ni the segregation involves the usual bulk (vacancy) diffusion mechanism. Since these processes are quite general, it appears that the top few atomic layers of a binary alloy can equilibrate with respect to surface segregation at a temperature significantly lower than that which is required for bulk equilibration. This combination of x-ray photoelectron spectroscopy (XPS) forward scattering and epitaxial sandwich structures thus provides both more clear-cut data and a more specific understanding of segregation phenomena than is usually possible by conventional XPS and Auger studies on random alloys.