1. Alloy Formation at the Epitaxial Interface for Ag Films Deposited on Al(001) and Al(110) Surfaces at Room Temperature* N.R. Shivaparan, M.A. Teter, and R.J. Smith, Physics Dept., Montana State University, Bozeman MT 59717  Goal: Understand epitaxial growth of metals on metals  Contributing factors include: lattice matching, surface energies, formation energies, strain energy  Related work: "small" atoms (Ni, Fe, Pd, Co) form alloys at the Al surface (300 K); "large" atoms ( Ti ) form overlayer  What about Ag ? Alloy or overlayer?  1% lattice mismatch with Al  metallic radius slightly larger than Al  surface energy of Ag larger than for Al  small, negative formation energy ( AgAl 2 )  Previous work: Ag forms epitaxial 1x1 overlayer; long-range order lost at low coverage; ordered compact islands at higher coverage (Egelhoff, App. SS 11/12, 761 (1982) )  Present work: XPS, HEIS, LEED for Ag on Al(001) and Al(110) Figure 1. (left) HEIS Channeling spectra for 4.6 ML Ag Observe decrease of Al surface peak so Ag atoms shadow Al atoms Coverage of Ag is determined by analysis of Ag peak Figure 2. (right) Rocking curves for HEIS Channeling with ion beam incident normal to the surface.  Minimum yield for clean Al surface indicates good quality crystal  Dip in the Ag yield for 9 ML Ag film is evidence for epitaxial ordering of Ag film, aligned with substrate Figure 3. (left) HEIS Al surface peak vs. Ag coverage  Decrease in Al surface peak area means Al atoms are shadowed by Ag  Amount of Ag shadowing is less than that expected for pseudomorphic overlayer (solid line from computer simulations)  Growth of Ag islands (calculated dashed line) also not observed Figure 4. (right) HEIS Ag surface peak vs. Ag coverage  At higher coverages Ag atoms are shadowed by other atoms in the overlayer  Ag yield follows closely that calculated for pseudomorphic Ag overlayer (  )  Straight line shows Ag scattering yield for non-channeling incident direction Figure 5. (far right) LEED patterns for Ag on Al(001)  (a) Clean Al(001)  (b) 0.5 ML Ag gives two domains of (5x1) and possible quasi-hexagonal patches  (c) 2.5 ML diffuse background; long-range order is lost  (d) 3.6 ML (1x1) pattern begins to return  (e) 30 ML (1x1) pattern still present (end of experiment)  (f) 30 ML film heated to 200 o C - order is lost; alloy forms Figure 6. (right) XPS Intensity for Al 2p and Ag 3d 5/2  Al intensity normalized to clean surface value; Ag normalized to thick film value  Al yield is attenuated, but somewhat slower than expected for layered growth of Ag (dashed line)  Ag grows rapidly suggesting an overlayer (modeled for two different attenuation lengths); possible island growth at larger coverages Figure 7. (far right) XPS Binding Energy for Ag 3d 5/2  Chemical shift of Ag indicates alloying of Ag with Al  Range of chemical shift similar to that seen for Ag-Al alloys (Ag sol. sol. at 368.9 eV; AlAg 2 at 368.4 eV)* * Fuggle, et al., Phys. Rev. B16, 750 (1977) Comments:  Larger surface energy for Al favors outward diffusion of Al  Negative Al-Ag formation energy favors alloying  Good lattice matching results in very little strain energy  Up to 25% solid solubility of Al in Ag at higher temperatures with negligible change in lattice constant favors alloying  Growth of ordered Ag overlayer after 4 ML Ag (from LEED and HEIS shadowing) Conclusions:  Alloy forms at the interface for up to 10 ML Ag  Larger surface energy of Ag compensated by small formation energy of alloy (XPS)  Negligibly small lattice strain results in nearly perfect epitaxy (HEIS shadowing; LEED)  Complex LEED pattern at low coverage ( < 1ML Ag) similar to other fcc-fcc systems (e.g. Ag on Cu(001) )  Similar conclusions reached for Ag on Al(110) surface (not shown here) *Work supported by NSF Grant DMR-9710092 and by NASA EPSCoR grant NCCW-0058 A copy of this poster is available at http://www.physics.montana.edu/Ionbeams/ionbeams.html