View
214
Download
0
Tags:
Embed Size (px)
Citation preview
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
a b
c d
e f
Goal: Understand epitaxial growth of metals on metalsContributing factors include: lattice matching, surface
energies, formation energies, strain energyRelated work: "small" atoms (Ni, Fe, Pd, Co) form alloys at
the Al surface (300 K); "large" atoms ( Ti ) form overlayerWhat about Ag ? Alloy or overlayer?
1% lattice mismatch with Almetallic radius slightly larger than Alsurface energy of Ag larger than for Alsmall, negative formation energy ( AgAl2 )
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 crystalDip 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 oC - order is lost; alloy forms
•Figure 6. (right) XPS Intensity for Al 2p and Ag 3d5/2
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 3d5/2
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; AlAg2 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