Ch Kor Symp00

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