7th Sino-Korean Symp June 2000 1 Evolution of Ni-Al interface alloy for Ni deposited on Al surfaces at room temperature R. J. Smith Physics Department,

7th Sino-Korean Symp June 20007th Sino-Korean Symp June 200011

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 R. J. Smith Physics Department, Montana State Univ.Physics Department, Montana State Univ.

Work supported by NSF (DMR) Work supported by NSF (DMR) http://www.physics.montana.eduhttp://www.physics.montana.edu

7th Sino-Korean Symp June 20007th Sino-Korean Symp June 2000 22

Metal-metal Interface Structure

Understand overlayer growth and alloy formationUnderstand overlayer growth and alloy formation Chemical composition and structure of the interfaceChemical composition and structure of the interface Applications: magnetoresistive devices, spin electronicsApplications: magnetoresistive devices, spin electronics

Surface energy (broken bonds)Surface energy (broken bonds)

Chemical formation energyChemical formation energy

Strain energyStrain energy

A

B0int AB

energyformation ABBA

energystrain )()( equilobs dEdE

interface


Metal-metal systems studied...

Substrates: Al(111), Al(100), Al(110)Substrates: Al(111), Al(100), Al(110) Metal overlayers studied so far:Metal overlayers studied so far:

Fe, Ni, Co, Pd (atomic size smaller than Al)Fe, Ni, Co, Pd (atomic size smaller than Al) Ti, Ag, Zr (atomic size larger than Al)Ti, Ag, Zr (atomic size larger than Al)

All have surface energy > Al surface energyAll have surface energy > Al surface energy All form Al compounds with All form Al compounds with HHformform < 0 < 0

Use resistively heated wires ( ~ML/min)Use resistively heated wires ( ~ML/min) Deposit on substrate at room temperatureDeposit on substrate at room temperature


2 MV van de Graaff Accelerator2 MV van de Graaff Accelerator


MSU Ion Beam LaboratoryMSU Ion Beam Laboratory


Ion scattering chamber Ion scattering chamber

High precision High precision sample goniometersample goniometer

Hemispherical VSW Hemispherical VSW analyzer (XPS, ISS)analyzer (XPS, ISS)

Ion and x-ray sourcesIon and x-ray sources LEEDLEED Metal wires for film Metal wires for film

depositiondeposition


Overview of High Energy Ion Scattering (HEIS)

MeV HeMeV He++ ions ions Yield = Q Yield = Q (Nt) (Nt) Ni peak for coverageNi peak for coverage Al peak for structureAl peak for structure


Angular Yield (Channeling dip)

1 MeV He1 MeV He++

Al bulk yieldAl bulk yield Ag surface peakAg surface peak incinc = 0 = 0oo

detdet = 105 = 105oo

~10~101515 ions/cm ions/cm22

min min = 3.6%= 3.6%


HEIS: Al yield vs Ni coverageHEIS: Al yield vs Ni coverage

Al SP area Al SP area increases with increases with Ni coverage Ni coverage

3 regions with 3 regions with different slopes different slopes (2) (0.35) (~0)(2) (0.35) (~0)

No LEED spotsNo LEED spots Interface alloy Interface alloy

forms at room forms at room temperaturetemperature


HEIS: Al yield vs Fe coverageHEIS: Al yield vs Fe coverage

Al SP area Al SP area increases with increases with Fe coverage Fe coverage

3 regions with 3 regions with different slopes different slopes (3.2)(0.96)(~0)(3.2)(0.96)(~0)

Interface alloy Interface alloy forms at room forms at room temperaturetemperature


HEIS: Al yield vs Ti coverageHEIS: Al yield vs Ti coverage

Ti atoms shadow Ti atoms shadow Al atoms and Al atoms and reducereduce Al yield Al yield

Critical thickness Critical thickness at ~5 MLat ~5 ML

Simulation (Simulation () for ) for flat Ti layer in flat Ti layer in FCC Al sitesFCC Al sites

Film relaxes for Film relaxes for coverage > 5 MLcoverage > 5 ML


XPS chemical shifts for Ni 2pXPS chemical shifts for Ni 2p

Shifts in BEShifts in BE Shifts in satelliteShifts in satellite Compare with XPS for Compare with XPS for

bulk alloys bulk alloys

(BE) (sat)(BE) (sat)NiAlNiAl33 1.05eV 1.05eV

NiNi22Al 0.75eV (8.0 eV)Al 0.75eV (8.0 eV)

NiAl 0.2 eV (7.2 eV)NiAl 0.2 eV (7.2 eV)

NiNi33Al 0.0 eV (6.5 eV)Al 0.0 eV (6.5 eV)

Ni 0.0 eV (5.8 eV)Ni 0.0 eV (5.8 eV)


Snapshots from MC simulationsSnapshots from MC simulations

Al(110)+0.5 ML Ni Al(110)+0.5 ML Ni Clean Al(110)Clean Al(110) Al(110)+2.0 ML Ni Al(110)+2.0 ML Ni

MC (total energy) using EAM potentials for Ni, Al (Voter)MC (total energy) using EAM potentials for Ni, Al (Voter) Equilibrate then add Ni in 0.5 ML increments (solid circles)Equilibrate then add Ni in 0.5 ML increments (solid circles) Ion scattering simulations (VEGAS)Ion scattering simulations (VEGAS)


Ion scattering simulations using VEGAS and the MC snapshotsIon scattering simulations using VEGAS and the MC snapshots

Measured (o) Measured (o) Simulation (Simulation ())

Slopes agreeSlopes agree Change of slope Change of slope

at 2 ML correct at 2 ML correct Good agreement Good agreement

so use snapshots so use snapshots for more insight for more insight


Composition profiles using the snapshots for Al(110) + NiComposition profiles using the snapshots for Al(110) + Ni

Ni atoms go into surfaceNi atoms go into surface Al atoms move outAl atoms move out Make dense NiAl layerMake dense NiAl layer Process changes after 2MLProcess changes after 2ML


Layer-resolved ion scattering yield using the snapshots of Al(110) + NiLayer-resolved ion scattering yield using the snapshots of Al(110) + Ni

~1Al/Ni top 15 layers~1Al/Ni top 15 layers ~1Al/Ni next 15 layers!~1Al/Ni next 15 layers! Ni atoms and dense interface Ni atoms and dense interface

structure cause dechanneling structure cause dechanneling below the surface below the surface


XPS: Comparison of Calculated and Measured Intensities at 30 CXPS: Comparison of Calculated and Measured Intensities at 30 C

XPS intensity vs XPS intensity vs Ni coverageNi coverage

Best agreement Best agreement with data for with data for NiNi

= 5.2 Å = 5.2 Å AlAl = =

15 Å15 Å Universal curve Universal curve

NiNi = 13.5 Å = 13.5 Å

AlAl = 20.2 Å = 20.2 Å

Equilibrium?Equilibrium?


ConclusionsConclusions

Combined HEIS, XPS, EAM to study Ni-Al interfaceCombined HEIS, XPS, EAM to study Ni-Al interface Ni-Al interface alloy forms in two stages at 30 Ni-Al interface alloy forms in two stages at 30 ooCC 0-2ML Ni atoms move down into the surface and 0-2ML Ni atoms move down into the surface and

form a relatively dense NiAl compoundform a relatively dense NiAl compound 2-8 ML Outdiffusion of Al is reduced, Ni-rich alloy 2-8 ML Outdiffusion of Al is reduced, Ni-rich alloy

(Ni(Ni33Al) forms; eventually covered by Ni metalAl) forms; eventually covered by Ni metal

At 250At 250ooC Ni atoms diffuse into the bulk - no surface C Ni atoms diffuse into the bulk - no surface compounds form compounds form

More study is needed for abrupt interface formationMore study is needed for abrupt interface formation


HEIS: Deposition of Ni at 250 CHEIS: Deposition of Ni at 250 C

Ni peak is now Ni peak is now very broadvery broad

Very little Ni at Very little Ni at the surfacethe surface

Ni has diffused Ni has diffused ~ 400 Å into ~ 400 Å into the substrate the substrate

Increased Increased dechanneling in dechanneling in substratesubstrate


XPS: Comparison of Calculated and Measured Intensities at 250 CXPS: Comparison of Calculated and Measured Intensities at 250 C

XPS intensity vs XPS intensity vs Ni coverageNi coverage

Coverage from Coverage from RBSRBS

Ni diffuses into Ni diffuses into substrate beyond substrate beyond range of XPSrange of XPS

See no chemical See no chemical shift for Ni 2pshift for Ni 2p

Documents

7th Sino-Korean Symp June 2000 1 Evolution of Ni-Al interface alloy for Ni deposited on Al surfaces at room temperature R. J. Smith Physics Department,