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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 : 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 A B interface
Metal-metal systems studied... : 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
2 MV van de Graaff Accelerator : 2 MV van de Graaff Accelerator
MSU Ion Beam Laboratory : MSU Ion Beam Laboratory
Ion scattering chamber : Ion scattering chamber High precision sample goniometer
Hemispherical VSW analyzer (XPS, ISS)
Ion and x-ray sources
LEED
Metal wires for film deposition
Overview of High Energy Ion Scattering (HEIS) : Overview of High Energy Ion Scattering (HEIS) MeV He+ ions
Yield = Q (Nt)
Ni peak for coverage
Al peak for structure
Angular Yield (Channeling dip) : Angular Yield (Channeling dip) 1 MeV He+
Al bulk yield
Ag surface peak
inc = 0o
det = 105o
~1015 ions/cm2
min = 3.6%
HEIS: Al yield vs Ni coverage : 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
HEIS: Al yield vs Fe coverage : 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
HEIS: Al yield vs Ti coverage : 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
XPS chemical shifts for Ni 2p : 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)
Snapshots from MC simulations : Snapshots from MC simulations Al(110)+0.5 ML Ni Clean Al(110) Al(110)+2.0 ML Ni 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)
Ion scattering simulations using VEGAS and the MC snapshots : 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
Composition profiles using the snapshots for Al(110) + Ni : 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
Layer-resolved ion scattering yield using the snapshots of Al(110) + Ni : 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
XPS: Comparison of Calculated and Measured Intensities at 30 C : 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?
Conclusions : 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
HEIS: Deposition of Ni at 250 C : 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
XPS: Comparison of Calculated and Measured Intensities at 250 C : 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
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