Epitaxy of Au on Ag(111) Studied by High-Energy Ion Scattering

Abstract

The first stage of epitaxial growth of Au on Ag(lll) has been studied by MeV ion scattering. The monolayer-by-monolayer (Frank-van der Merwe) growth mode for this system is confirmed by measurements of (1) the Ag surface peak, (2) the Au-Au shadowing effect, and (3) the Ag Auger intensity.PACS numbers: 68.55.+ b, 68.48.+ f, 79.20.Fv Epitaxial growth of thin films has been divided into three categories: (1) the Volmer-Weber mode, in which the surface energy of the film material is large compared with that of the substrate, i.e., Au on NaCl; (2) the Frank-van der Merwe mode in which the surface energy of the film is less than or comparable to that of the substrate and there is low strain energy in the film, and (3) the Stranski-Krastanov mode in which the surface energy of the film is less than or comparable to that of the substrate but there is a high strain energy in the film. 1 The Frankvan der Merwe mode is characterized as layerby-layer growth (as opposed to three-dimensional island formation) and is generally applicable in cases of autoepitaxy (i.e., Si on Si) or in cases of "almost autoepitaxy" (i.e., metal A on metal B with a close match in lattice constant). The initial stages of growth in this mode are necessarily studied by the surface-science techniques of lowor medium-energy electron diffraction (LEED or MEED) and Auger analysis.In this Letter we describe the first systematic study of Au epitaxial growth on a Ag(lll) surface using high-energy ion scattering. We show that the growth mode in this system is clearly of a Frank-van der Merwe type (as expected) and that ion scattering techniques are particularly suited to these studies.Experiments were carried out in an UHV system equipped with LEED and Auger apparatus and a Au deposition source. 2 The UHV system is directly coupled to a Van de Graaf accelerator so that high-energy ion scattering experiments can be performed in situ. Prior to Au deposition, the bulk Ag(lll) samples were cleaned via sputtering and annealing. They showed essentially no trace impurities either in the Auger scan or in the ion scattering spectra; the LEED pattern displayed a sharp, well-defined (lxl) pattern. The magnitude of the surface peak in the normal (111) direction and the off-normal (110) direction was in good agreement with calculations assuming a "bulklike" surface. Analyses of this data indicated that both horizontal and vertical displacements of the surface atoms in the clean Ag surface are <0.1 A, in good agreement with LEED analyses. 3 Au deposition was monitored primarily through the ion scattering spectra, although Auger spectra were checked following Au deposition to affirm that light impurity contamination was negligible. Measurements and evaporation were done with the substrate at room temperature and at 140 °K.The first indication of epitaxy in the scattering experiments is the reduction of the substrate surface peak (SP). 4 For a clean crystal the SP results from the interaction of the aligned ion beam with the first monolayer(s) of the single crystal soli...

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