1. NWAPS-May 2000 1 Evolution of Ni-Al interface alloy for Ni deposited on Al surfaces at room temperature R. J. Smith and V. Shutthanandan* Physics Department, Montana State Univ. *Currently at EMSL, PNNL, Richland WA Work supported by NSF http://www.physics.montana.edu

2. NWAPS-May 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. NWAPS-May 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. NWAPS-May 20004 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

5. NWAPS-May 20005 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

6. NWAPS-May 20006 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

7. NWAPS-May 20007 XPS chemical shifts for Ni 2p ä Shifts in BE ä Shifts in satellite ä Compare with XPS for bulk alloys to identify surface composition 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)

8. NWAPS-May 20008 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)

9. NWAPS-May 20009 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 Use snapshots for more insight

10. NWAPS-May 200010 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

11. NWAPS-May 200011 Layer-resolved 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

12. NWAPS-May 200012 Conclusions ä Combined HEIS, XPS, EAM to study Ni-Al interface ä Ni-Al interface alloy forms in two stages 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

13. NWAPS-May 200013 MSU Ion Beam Laboratory

14. NWAPS-May 200014 2 MV van de Graaff Accelerator

15. NWAPS-May 200015 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%