Epitaxial Overlayers vs Alloy Formation at Aluminum- Transition Metal Interfaces Richard J. Smith Physics Department Montana State University Bozeman MT.

Epitaxial Overlayers vs Alloy Formation at Aluminum-

Transition Metal Interfaces

Epitaxial Overlayers vs Alloy Formation at Aluminum-

Transition Metal Interfaces

Richard J. SmithRichard J. Smith

Physics DepartmentPhysics Department

Montana State UniversityMontana State University

Bozeman MT 59715Bozeman MT 59715

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Acknowledgements

Ph.D students: Adli Saleh,V. Shuthanandan,Ph.D students: Adli Saleh,V. Shuthanandan,

N. ShivaparanN. Shivaparan Dr. Yong-wook Kim (from ASSRC)Dr. Yong-wook Kim (from ASSRC)

National Science Foundation National Science Foundation

http://www.physics.montana.edu/Ionbeams/ionbeams.htmlhttp://www.physics.montana.edu/Ionbeams/ionbeams.html

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Finding a better growth model...

Motivation: Try to understand metal-metal interface Motivation: Try to understand metal-metal interface formation (A/B); overlayer growth vs. alloy formationformation (A/B); overlayer growth vs. alloy formation

Consider the following mechanisms:Consider the following mechanisms: Surface energy (broken bonds)Surface energy (broken bonds)

Chemical formation energyChemical formation energy

Strain energyStrain energy

A

B0int AB

energyformation ABBA

energystrain )()( equilobs dEdE

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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

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Techniques used...Techniques used...

High-energy ion scattering and channeling (HEIS)High-energy ion scattering and channeling (HEIS) X-ray photoemission - intensities and chemicalX-ray photoemission - intensities and chemical

shifts in binding energy (XPS)shifts in binding energy (XPS) X-ray photoelectron diffraction (XPD)X-ray photoelectron diffraction (XPD) Low-energy electron diffraction (LEED)Low-energy electron diffraction (LEED) Low-energy ion scattering (LEIS)Low-energy ion scattering (LEIS)

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MSU Ion Beam LaboratoryMSU Ion Beam Laboratory

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2 MV van de Graaff Accelerator2 MV van de Graaff Accelerator

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Scattering chamber 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

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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

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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%

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1. Ti on Al(100) surface peaks 1. Ti on Al(100) surface peaks

Surface Surface peaks (SP)peaks (SP)

Decrease Decrease in Al SP in Al SP areaarea

Ti shadows Ti shadows Al atomsAl atoms

FCCFCC

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HEIS: Al surface peak area vs. Ti coverageHEIS: Al surface peak area vs. Ti coverage

Decrease in Al SP Decrease in Al SP (o) to 5.5 ML(o) to 5.5 ML

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

Critical thickness Critical thickness of 5 ML ~ 4.4% of 5 ML ~ 4.4% lattice mismatchlattice mismatch

Increase > 5 ML Increase > 5 ML Ti layer relaxationTi layer relaxation

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Ti on Al(100): XPS intensity vs Ti coverageTi on Al(100): XPS intensity vs Ti coverage

Attenuation Attenuation follows flat film follows flat film model (solid model (solid line) after 2 MLline) after 2 ML

No decrease of No decrease of intensity for intensity for first monolayerfirst monolayer

Possible Ti-Al Possible Ti-Al interchange at interchange at top layertop layer

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Ti on Al(100): XPD angular scansTi on Al(100): XPD angular scans

Enhanced Al 2p Enhanced Al 2p emission at 0emission at 0oo, 45, 45oo

Forward scattering Forward scattering for FCC latticefor FCC lattice

Ti 2p photopeaks Ti 2p photopeaks show enhanced show enhanced emission along emission along same directionssame directions

FCC Ti film !FCC Ti film !

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2: Ag on Al(100):Al surface peak2: Ag on Al(100):Al surface peak

Ag shadows Al Ag shadows Al surface atomssurface atoms

Shadowing not Shadowing not like that for flat like that for flat Ag overlayerAg overlayer

Not Ag islands Not Ag islands on FCC latticeon FCC lattice

Small strain at Small strain at interface(0.9%)interface(0.9%)

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Ag on Al(100): Ag surface peakAg on Al(100): Ag surface peak

Ag atoms are Ag atoms are shadowed at shadowed at high coveragehigh coverage

Well-ordered Well-ordered Ag filmAg film

Confirmed by Confirmed by LEEDLEED

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Ag on Al(100) LEED patternsAg on Al(100) LEED patterns

A. CleanA. Clean

B. 0.5 MLB. 0.5 ML

C. 2.5 MLC. 2.5 ML

D. 3.6 MLD. 3.6 ML

E. 30 MLE. 30 ML

F. 30 ML F. 30 ML heated to heated to 250 250 ooCC

a b c

d e f

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Ag on Al(100): XPS intensitiesAg on Al(100): XPS intensities

Rapid decrease Rapid decrease of Al peakof Al peak

Rapid growth of Rapid growth of Ag peakAg peak

Growth of Ag Growth of Ag islands for high islands for high coveragecoverage

Flat film grows Flat film grows at first but not at first but not pure Agpure Ag

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Ag on Al(100): Ag binding energy (BE)Ag on Al(100): Ag binding energy (BE)

Ag 3d energy Ag 3d energy decreases decreases graduallygradually

Ag 4d (VB) Ag 4d (VB) also changesalso changes

BE shift is BE shift is similar to bulk similar to bulk Al-Ag alloysAl-Ag alloys

Al moves up Al moves up into Ag filminto Ag film

AlAg2

Al+dilute Ag

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3: Ni on Al(110) Al surface peaks3: Ni on Al(110) Al surface peaks

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

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Ni on Al(110):XPS chemical shiftsNi on Al(110):XPS chemical shifts

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

bulk alloys to identify bulk alloys to identify surface compositionsurface composition

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)

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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)

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Ni on Al(110):HEIS simulations using the snapshotsNi on Al(110):HEIS simulations using the snapshots

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

Slopes agreeSlopes agree Change at 2 ML Change at 2 ML

correctcorrect Use snapshots for Use snapshots for

insight insight Ni atoms move Ni atoms move

below the surfacebelow the surface

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Conclusions:Conclusions:

Combined HEIS, XPS, LEED to study film Combined HEIS, XPS, LEED to study film structures on solid-solid interfacesstructures on solid-solid interfaces

Ti/Al(100) epitaxial fcc overlayer up to 5 MLTi/Al(100) epitaxial fcc overlayer up to 5 ML Ag/Al(100) epitaxial overlayer with some Ag/Al(100) epitaxial overlayer with some

alloying of Al into the Ag overlayeralloying of Al into the Ag overlayer Ni/Al(110) disordered alloy formation for Ni/Al(110) disordered alloy formation for

deposition at room temperaturedeposition at room temperature