1. 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, Montana State Univ. Work supported by NSF (DMR) http://www.physics.montana.edu

2. 7th Sino-Korean Symp June 20002 Metal-metal Interface Structure ä Understand overlayer growth and alloy formation ä Chemical composition and structure of the interface ä Applications: magnetoresistive devices, spin electronics ä Surface energy (broken bonds) ä Chemical formation energy ä Strain energy A B interface

3. 7th Sino-Korean Symp June 20003 Metal-metal systems studied... ä Substrates: Al(111), Al(100), Al(110) ä Metal overlayers studied so far: ä Fe, Ni, Co, Pd (atomic size smaller than Al) ä Ti, Ag, Zr (atomic size larger than Al) ä All have surface energy > Al surface energy ä All form Al compounds with  H form < 0 ä Use resistively heated wires ( ~ML/min) ä Deposit on substrate at room temperature

4. 7th Sino-Korean Symp June 20004 2 MV van de Graaff Accelerator

5. 7th Sino-Korean Symp June 20005 MSU Ion Beam Laboratory

6. 7th Sino-Korean Symp June 20006 Ion scattering chamber ä High precision sample goniometer ä Hemispherical VSW analyzer (XPS, ISS) ä Ion and x-ray sources ä LEED ä Metal wires for film deposition

7. 7th Sino-Korean Symp June 20007 Overview of High Energy Ion Scattering (HEIS) ä MeV He + ions ä Yield = Q   (Nt) ä Ni peak for coverage ä Al peak for structure

8. 7th Sino-Korean Symp June 20008 Angular Yield (Channeling dip) ä 1 MeV He + ä Al bulk yield ä Ag surface peak ä  inc = 0 o ä  det = 105 o ä ~10 15 ions/cm 2 ä  min = 3.6%

9. 7th Sino-Korean Symp June 20009 HEIS: Al yield vs Ni coverage ä Al SP area increases with Ni coverage ä 3 regions with different slopes (2) (0.35) (~0) ä No LEED spots ä Interface alloy forms at room temperature

10. 7th Sino-Korean Symp June 200010 HEIS: Al yield vs Fe coverage ä Al SP area increases with Fe coverage ä 3 regions with different slopes (3.2)(0.96)(~0) ä Interface alloy forms at room temperature

11. 7th Sino-Korean Symp June 200011 HEIS: Al yield vs Ti coverage ä Ti atoms shadow Al atoms and reduce Al yield ä Critical thickness at ~5 ML ä Simulation (  ) for flat Ti layer in FCC Al sites ä Film relaxes for coverage > 5 ML

12. 7th Sino-Korean Symp June 200012 XPS chemical shifts for Ni 2p ä Shifts in BE ä Shifts in satellite ä Compare with XPS for bulk alloys (BE) (sat) (BE) (sat) NiAl 3 1.05eV Ni 2 Al 0.75eV (8.0 eV) NiAl 0.2 eV (7.2 eV) Ni 3 Al 0.0 eV (6.5 eV) Ni 0.0 eV (5.8 eV)

13. 7th Sino-Korean Symp June 200013 Snapshots from MC simulations Al(110)+0.5 ML Ni Clean Al(110) Al(110)+2.0 ML Ni ä MC (total energy) using EAM potentials for Ni, Al (Voter) ä Equilibrate then add Ni in 0.5 ML increments (solid circles) ä Ion scattering simulations (VEGAS)

14. 7th Sino-Korean Symp June 200014 Ion scattering simulations using VEGAS and the MC snapshots ä Measured (o) Simulation (  ) ä Slopes agree ä Change of slope at 2 ML correct ä Good agreement so use snapshots for more insight

15. 7th Sino-Korean Symp June 200015 Composition profiles using the snapshots for Al(110) + Ni ä Ni atoms go into surface ä Al atoms move out ä Make dense NiAl layer ä Process changes after 2ML

16. 7th Sino-Korean Symp June 200016 Layer-resolved ion scattering yield using the snapshots of Al(110) + Ni ä ~1Al/Ni top 15 layers ä ~1Al/Ni next 15 layers! ä Ni atoms and dense interface structure cause dechanneling below the surface

17. 7th Sino-Korean Symp June 200017 XPS: Comparison of Calculated and Measured Intensities at 30 C ä XPS intensity vs Ni coverage ä Best agreement with data for Ni = 5.2 Å Al = 15 Å ä Universal curve Ni = 13.5 Å Al = 20.2 Å ä Equilibrium?

18. 7th Sino-Korean Symp June 200018 Conclusions ä Combined HEIS, XPS, EAM to study Ni-Al interface ä Ni-Al interface alloy forms in two stages at 30 o C ä 0-2ML Ni atoms move down into the surface and form a relatively dense NiAl compound ä 2-8 ML Outdiffusion of Al is reduced, Ni-rich alloy (Ni 3 Al) forms; eventually covered by Ni metal ä At 250 o C Ni atoms diffuse into the bulk - no surface compounds form ä More study is needed for abrupt interface formation

19. 7th Sino-Korean Symp June 200019 HEIS: Deposition of Ni at 250 C ä Ni peak is now very broad ä Very little Ni at the surface ä Ni has diffused ~ 400 Å into the substrate ä Increased dechanneling in substrate

20. 7th Sino-Korean Symp June 200020 XPS: Comparison of Calculated and Measured Intensities at 250 C ä XPS intensity vs Ni coverage ä Coverage from RBS ä Ni diffuses into substrate beyond range of XPS ä See no chemical shift for Ni 2p