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Evolution of Ni-Al interface alloy for Ni deposited on Al surfaces at room temperature

Evolution of Ni-Al interface alloy for Ni deposited on Al surfaces at room temperature. R. J. Smith Physics Department, Montana State Univ. Work supported by NSF (DMR) http://www.physics.montana.edu. Metal-metal Interface Structure. Understand overlayer growth and alloy formation

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Evolution of Ni-Al interface alloy for Ni deposited on Al surfaces at room temperature

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  1. Evolution of Ni-Al interface alloy for Ni deposited on Al surfaces at room temperature R. J. Smith Physics Department, Montana State Univ. Work supported by NSF (DMR) http://www.physics.montana.edu 7th Sino-Korean Symp June 2000

  2. Metal-metal Interface Structure • Understand overlayer growth and alloy formation • Chemical composition and structure of the interface • Applications: magnetoresistive devices, spin electronics • Surface energy (broken bonds) • Chemical formation energy • Strain energy B interface A 7th Sino-Korean Symp June 2000

  3. Metal-metal 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 7th Sino-Korean Symp June 2000

  4. 2 MV van de Graaff Accelerator 7th Sino-Korean Symp June 2000

  5. MSU Ion Beam Laboratory 7th Sino-Korean Symp June 2000

  6. Ion scattering chamber • High precision sample goniometer • Hemispherical VSW analyzer (XPS, ISS) • Ion and x-ray sources • LEED • Metal wires for film deposition 7th Sino-Korean Symp June 2000

  7. Overview of High Energy Ion Scattering (HEIS) • MeV He+ ions • Yield = Q   (Nt) • Ni peak for coverage • Al peak for structure 7th Sino-Korean Symp June 2000

  8. Angular Yield (Channeling dip) • 1 MeV He+ • Al bulk yield • Ag surface peak • inc = 0o • det = 105o • ~1015 ions/cm2 • min = 3.6% 7th Sino-Korean Symp June 2000

  9. HEIS: Al yield vs Ni coverage • 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 7th Sino-Korean Symp June 2000

  10. HEIS: Al yield vs Fe coverage • Al SP area increases with Fe coverage • 3 regions with different slopes (3.2)(0.96)(~0) • Interface alloy forms at room temperature 7th Sino-Korean Symp June 2000

  11. HEIS: Al yield vs Ti coverage • Ti atoms shadow Al atoms and reduce Al yield • Critical thickness at ~5 ML • Simulation () for flat Ti layer in FCC Al sites • Film relaxes for coverage > 5 ML 7th Sino-Korean Symp June 2000

  12. XPS chemical shifts for Ni 2p • Shifts in BE • Shifts in satellite • Compare with XPS for bulk alloys (BE) (sat) 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) 7th Sino-Korean Symp June 2000

  13. 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 7th Sino-Korean Symp June 2000

  14. Ion scattering simulations using VEGAS and the MC snapshots • Measured (o) Simulation () • Slopes agree • Change of slope at 2 ML correct • Good agreement so use snapshots for more insight 7th Sino-Korean Symp June 2000

  15. Composition profiles using the snapshots for Al(110) + Ni • Ni atoms go into surface • Al atoms move out • Make dense NiAl layer • Process changes after 2ML 7th Sino-Korean Symp June 2000

  16. Layer-resolved ion scattering yield using the snapshots of Al(110) + Ni • ~1Al/Ni top 15 layers • ~1Al/Ni next 15 layers! • Ni atoms and dense interface structure cause dechanneling below the surface 7th Sino-Korean Symp June 2000

  17. XPS: Comparison of Calculated and Measured Intensities at 30 C • XPS intensity vs Ni coverage • Best agreement with data for Ni = 5.2 Å Al = 15 Å • Universal curve Ni = 13.5 Å Al = 20.2 Å • Equilibrium? 7th Sino-Korean Symp June 2000

  18. Conclusions • Combined HEIS, XPS, EAM to study Ni-Al interface • Ni-Al interface alloy forms in two stages at 30 oC • 0-2ML Ni atoms move down into the surface and form a relatively dense NiAl compound • 2-8 ML Outdiffusion of Al is reduced, Ni-rich alloy (Ni3Al) forms; eventually covered by Ni metal • At 250oC Ni atoms diffuse into the bulk - no surface compounds form • More study is needed for abrupt interface formation 7th Sino-Korean Symp June 2000

  19. HEIS: Deposition of Ni at 250 C • Ni peak is now very broad • Very little Ni at the surface • Ni has diffused ~ 400 Å into the substrate • Increased dechanneling in substrate 7th Sino-Korean Symp June 2000

  20. XPS: Comparison of Calculated and Measured Intensities at 250 C • XPS intensity vs Ni coverage • Coverage from RBS • Ni diffuses into substrate beyond range of XPS • See no chemical shift for Ni 2p 7th Sino-Korean Symp June 2000

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