vs Alloy Formation at Aluminum-Transition Metal Interfaces Richard J. Smith Richard J. Smith Physics Department Physics Department Montana State University Montana State University Bozeman MT 59715 Bozeman MT 59715
Dec 20, 2015
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