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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 and V. Shutthanandan* Physics Department, Montana State Univ. *Currently at EMSL, PNNL, Richland WA Work supported by NSF http://www.physics.montana.edu. Metal-metal Interface Structure.

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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 and V. Shutthanandan* Physics Department, Montana State Univ. *Currently at EMSL, PNNL, Richland WA Work supported by NSF http://www.physics.montana.edu NWAPS-May 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 NWAPS-May 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 NWAPS-May 2000

  4. Ion scattering chamber • High precision sample goniometer • Hemispherical VSW analyzer (XPS, ISS) • Ion and x-ray sources • LEED • Metal wires for film deposition NWAPS-May 2000

  5. Overview of High Energy Ion Scattering (HEIS) • MeV He+ ions • Yield = Q   (Nt) • Ni peak for coverage • Al peak for structure NWAPS-May 2000

  6. 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 NWAPS-May 2000

  7. XPS chemical shifts for Ni 2p • 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) NWAPS-May 2000

  8. 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 NWAPS-May 2000

  9. Ion scattering simulations using VEGAS and the MC snapshots • Measured (o) Simulation () • Slopes agree • Change of slope at 2 ML correct • Use snapshots for more insight NWAPS-May 2000

  10. 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 NWAPS-May 2000

  11. Layer-resolved 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 NWAPS-May 2000

  12. Conclusions • Combined HEIS, XPS, EAM to study Ni-Al interface • Ni-Al interface alloy forms in two stages • 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 NWAPS-May 2000

  13. MSU Ion Beam Laboratory NWAPS-May 2000

  14. 2 MV van de Graaff Accelerator NWAPS-May 2000

  15. Angular Yield (Channeling dip) • 1 MeV He+ • Al bulk yield • Ag surface peak • inc = 0o • det = 105o • ~1015 ions/cm2 • min = 3.6% NWAPS-May 2000

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