1. Epitaxial Overlayers vs Alloy Formation at Aluminum- Transition Metal Interfaces Richard J. Smith Physics Department Montana State University Bozeman MT 59715

2. KVS-July 19992 Acknowledgements ä Ph.D students: Adli Saleh,V. Shuthanandan, N. Shivaparan N. Shivaparan ä Dr. Yong-wook Kim (from ASSRC) ä National Science Foundation ä http://www.physics.montana.edu/Ionbeams/ionbeams.html

3. KVS-July 19993 Finding a better growth model... ä Motivation: Try to understand metal-metal interface formation (A/B); overlayer growth vs. alloy formation ä Consider the following mechanisms: ä Surface energy (broken bonds) ä Chemical formation energy ä Strain energy A B

4. KVS-July 19994 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

5. KVS-July 19995 Techniques used... ä High-energy ion scattering and channeling (HEIS) ä X-ray photoemission - intensities and chemical shifts in binding energy (XPS) shifts in binding energy (XPS) ä X-ray photoelectron diffraction (XPD) ä Low-energy electron diffraction (LEED) ä Low-energy ion scattering (LEIS)

6. KVS-July 19996 MSU Ion Beam Laboratory

7. KVS-July 19997 2 MV van de Graaff Accelerator

8. KVS-July 19998 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

9. KVS-July 19999 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

10. KVS-July 199910 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%

11. KVS-July 199911 1. Ti on Al(100) surface peaks ä Surface peaks (SP) ä Decrease in Al SP area ä Ti shadows Al atoms ä FCC

12. KVS-July 199912 HEIS: Al surface peak area vs. Ti coverage ä Decrease in Al SP (o) to 5.5 ML ä Simulation () for flat Ti layer in FCC Al sites ä Critical thickness of 5 ML ~ 4.4% lattice mismatch ä Increase > 5 ML Ti layer relaxation

13. KVS-July 199913 Ti on Al(100): XPS intensity vs Ti coverage ä Attenuation follows flat film model (solid line) after 2 ML ä No decrease of intensity for first monolayer ä Possible Ti-Al interchange at top layer

14. KVS-July 199914 Ti on Al(100): XPD angular scans ä Enhanced Al 2p emission at 0 o, 45 o ä Forward scattering for FCC lattice ä Ti 2p photopeaks show enhanced emission along same directions ä FCC Ti film !

15. KVS-July 199915 2: Ag on Al(100):Al surface peak ä Ag shadows Al surface atoms ä Shadowing not like that for flat Ag overlayer ä Not Ag islands on FCC lattice ä Small strain at interface(0.9%)

16. KVS-July 199916 Ag on Al(100): Ag surface peak ä Ag atoms are shadowed at high coverage ä Well-ordered Ag film ä Confirmed by LEED

17. KVS-July 199917 Ag on Al(100) LEED patterns A. Clean B. 0.5 ML C. 2.5 ML D. 3.6 ML E. 30 ML F. 30 ML heated to 250 o C

18. KVS-July 199918 Ag on Al(100): XPS intensities ä Rapid decrease of Al peak ä Rapid growth of Ag peak ä Growth of Ag islands for high coverage ä Flat film grows at first but not pure Ag

19. KVS-July 199919 Ag on Al(100): Ag binding energy (BE) ä Ag 3d energy decreases gradually ä Ag 4d (VB) also changes ä BE shift is similar to bulk Al-Ag alloys ä Al moves up into Ag film AlAg 2 Al+dilute Ag

20. KVS-July 199920 3: Ni on Al(110) Al surface peaks 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

21. KVS-July 199921 Ni on Al(110):XPS chemical shifts 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)

22. KVS-July 199922 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)

23. KVS-July 199923 Ni on Al(110):HEIS simulations using the snapshots ä Measured (o) Simulation (  ) ä Slopes agree ä Change at 2 ML correct ä Use snapshots for insight ä Ni atoms move below the surface

24. KVS-July 199924 Conclusions: ä Combined HEIS, XPS, LEED to study film structures on solid-solid interfaces ä Ti/Al(100) epitaxial fcc overlayer up to 5 ML ä Ag/Al(100) epitaxial overlayer with some alloying of Al into the Ag overlayer ä Ni/Al(110) disordered alloy formation for deposition at room temperature