Transcript The Nobel Laboratory Experiments Revisited

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
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Metal-metal Interface Structure
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Understand overlayer growth and alloy formation
Chemical composition and structure of the interface
Applications: magnetoresistive devices, spin electronics
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Surface energy (broken bonds)
B
 B   A   int  0
interface
Chemical formation energy
A
A  B  AB  formationenergy
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Strain energy
E(dobs )  E(dequil )  strainenergy
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Metal-metal systems studied...
Substrates: Al(111), Al(100), Al(110)
 Metal overlayers studied so far:
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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
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Ion scattering chamber
High precision
sample goniometer
 Hemispherical VSW
analyzer (XPS, ISS)
 Ion and x-ray sources
 LEED
 Metal wires for film
deposition
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Overview of High Energy Ion
Scattering (HEIS)
MeV He+ ions
 Yield = Q   (Nt)
 Ni peak for coverage
 Al peak for structure
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HEIS: Al yield vs Ni coverage
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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
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XPS chemical shifts for Ni 2p
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Shifts in BE
Shifts in satellite
Compare with XPS for
bulk alloys to identify
surface composition
NiAl3 1.05eV
Ni2Al 0.75eV
NiAl 0.2 eV
Ni3Al 0.0 eV
Ni
0.0 eV
(8.0 eV)
(7.2 eV)
(6.5 eV)
(5.8 eV)
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Snapshots from MC simulations
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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
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Ion scattering simulations using
VEGAS and the MC snapshots
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Measured (o)
Simulation ()
Slopes agree
Change of slope
at 2 ML correct
Use snapshots for
more insight
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Composition profiles using the
snapshots for Al(110) + Ni
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Ni atoms go into surface
Al atoms move out
Make dense NiAl layer
Process changes after 2ML
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Layer-resolved scattering yield
using the snapshots of Al(110) + Ni
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~1Al/Ni top 15 layers
~1Al/Ni next 15 layers!
Ni atoms and dense interface
structure cause dechanneling
below the surface
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Conclusions
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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
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MSU Ion Beam Laboratory
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2 MV van de Graaff Accelerator
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Angular Yield (Channeling dip)
1 MeV He+
 Al bulk yield
 Ag surface peak
 inc = 0o
 det = 105o
 ~1015 ions/cm2
 min = 3.6%
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