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Epitaxial Overlayers vs Alloy Formation at Aluminum-Transition Metal Interfaces

Epitaxial Overlayers vs Alloy Formation at Aluminum-Transition Metal Interfaces. Richard J. Smith Physics Department Montana State University Bozeman MT 59715. Acknowledgements. Ph.D students: Adli Saleh,V. Shuthanandan, N. Shivaparan

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Epitaxial Overlayers vs Alloy Formation at Aluminum-Transition Metal Interfaces

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  1. Epitaxial Overlayers vs Alloy Formation at Aluminum-Transition Metal Interfaces Richard J. Smith Physics Department Montana State University Bozeman MT 59715

  2. Acknowledgements • Ph.D students: Adli Saleh,V. Shuthanandan, N. Shivaparan • Dr. Yong-wook Kim (from ASSRC) • National Science Foundation • http://www.physics.montana.edu/Ionbeams/ionbeams.html KVS-July 1999

  3. Finding a better growth model... • Motivation: Try to understand metal-metal interface formation (A/B); overlayer growth vs. alloy formation • Consider the following mechanisms: • Surface energy (broken bonds) • Chemical formation energy • Strain energy B A KVS-July 1999

  4. Systems studied... • Substrates: Al(111), Al(100), Al(110) • Metal overlayers studied so far: • Fe, Ni, Co, Pd (atomic size smaller than Al) • Ti, Ag, Zr (atomic size larger than Al) • All have surface energy > Al surface energy • All form Al compounds with Hform < 0 • Use resistively heated wires ( ~ML/min) • Deposit on substrate at room temperature KVS-July 1999

  5. Techniques used... • High-energy ion scattering and channeling (HEIS) • X-ray photoemission - intensities and chemical shifts in binding energy (XPS) • X-ray photoelectron diffraction (XPD) • Low-energy electron diffraction (LEED) • Low-energy ion scattering (LEIS) KVS-July 1999

  6. MSU Ion Beam Laboratory KVS-July 1999

  7. 2 MV van de Graaff Accelerator KVS-July 1999

  8. Scattering chamber • High precision sample goniometer • Hemispherical VSW analyzer (XPS, ISS) • Ion and x-ray sources • LEED • Metal wires for film deposition KVS-July 1999

  9. Overview of High Energy Ion Scattering (HEIS) • MeV He+ ions • Yield = Q   (Nt) • Ni peak for coverage • Al peak for structure KVS-July 1999

  10. Angular Yield (Channeling dip) • 1 MeV He+ • Al bulk yield • Ag surface peak • inc = 0o • det = 105o • ~1015 ions/cm2 • min = 3.6% KVS-July 1999

  11. 1. Ti on Al(100) surface peaks • Surface peaks (SP) • Decrease in Al SP area • Ti shadows Al atoms • FCC KVS-July 1999

  12. HEIS: Al surface peak area vs. Ti coverage • Decrease in Al SP (o) to 5.5 ML • Simulation (•) for flat Ti layer in FCC Al sites • Critical thickness of 5 ML ~ 4.4% lattice mismatch • Increase > 5 ML Ti layer relaxation KVS-July 1999

  13. Ti on Al(100): XPS intensity vs Ti coverage • Attenuation follows flat film model (solid line) after 2 ML • No decrease of intensity for first monolayer • Possible Ti-Al interchange at top layer KVS-July 1999

  14. Ti on Al(100): XPD angular scans • Enhanced Al 2p emission at 0o, 45o • Forward scattering for FCC lattice • Ti 2p photopeaks show enhanced emission along same directions • FCC Ti film ! KVS-July 1999

  15. 2: Ag on Al(100):Al surface peak • Ag shadows Al surface atoms • Shadowing not like that for flat Ag overlayer • Not Ag islands on FCC lattice • Small strain at interface(0.9%) KVS-July 1999

  16. Ag on Al(100): Ag surface peak • Ag atoms are shadowed at high coverage • Well-ordered Ag film • Confirmed by LEED KVS-July 1999

  17. Ag on Al(100) LEED patterns A. Clean B. 0.5 ML C. 2.5 ML D. 3.6 ML E. 30 ML F. 30 ML heated to 250 oC KVS-July 1999

  18. Ag on Al(100): XPS intensities • Rapid decrease of Al peak • Rapid growth of Ag peak • Growth of Ag islands for high coverage • Flat film grows at first but not pure Ag KVS-July 1999

  19. Ag on Al(100): Ag binding energy (BE) • Ag 3d energy decreases gradually • Ag 4d (VB) also changes • BE shift is similar to bulk Al-Ag alloys • Al moves up into Ag film AlAg2 Al+dilute Ag KVS-July 1999

  20. 3: Ni on Al(110) Al surface peaks • Al SP area increases with Ni coverage • 3 regions with different slopes (2) (0.35) (~0) • No LEED spots • Interface alloy forms at room temperature KVS-July 1999

  21. Ni on Al(110):XPS chemical shifts • Shifts in BE • Shifts in satellite • Compare with XPS for bulk alloys to identify surface composition NiAl3 1.05eV Ni2Al 0.75eV (8.0 eV) NiAl 0.2 eV (7.2 eV) Ni3Al 0.0 eV (6.5 eV) Ni 0.0 eV (5.8 eV) KVS-July 1999

  22. Snapshots from MC simulations • MC (total energy) using EAM potentials for Ni, Al (Voter) • Equilibrate then add Ni in 0.5 ML increments (solid circles) • Ion scattering simulations (VEGAS) Clean Al(110) Al(110)+0.5 ML Ni Al(110)+2.0 ML Ni KVS-July 1999

  23. Ni on Al(110):HEIS simulations using the snapshots • Measured (o) Simulation () • Slopes agree • Change at 2 ML correct • Use snapshots for insight • Ni atoms move below the surface KVS-July 1999

  24. Conclusions: • Combined HEIS, XPS, LEED to study film structures on solid-solid interfaces • Ti/Al(100) epitaxial fcc overlayer up to 5 ML • Ag/Al(100) epitaxial overlayer with some alloying of Al into the Ag overlayer • Ni/Al(110) disordered alloy formation for deposition at room temperature KVS-July 1999

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