Richard J. Smith Physics Department Montana State University Bozeman MT 59715

1. Influence of Substrate Surface Orientation on the Structure of Ti Thin Films Grown on Al Single-Crystal Surfaces at Room Temperature Richard J. Smith Physics Department Montana State University Bozeman MT 59715

2. Acknowledgements • Ph.D students: Adli Saleh,V. Shuthanandan, N. Shivaparan, G. A. White • Dr. Yong-wook Kim (from ASSRC) • National Science Foundation • http://www.physics.montana.edu/Ionbeams/ionbeams.html Thin Film Symp -July 1999

3. Finding a better growth model... • Motivation: Try to understand metal-metal interface formation (A/B); overlayer growth vs. alloy formation • Consider the following mechanisms: • Surface energy (broken bonds) • Chemical formation energy • Strain energy B A Thin Film Symp -July 1999

4. Overview of properties for films grown on Al substrates • Substrates: Al(111), Al(001), Al(110) • Ni, Pd, Fe, Co alloy with Al surface at 300 K • Ti and Ag form epitaxial overlayers on Al • All have surface energies > Al surface energy • All form Al compounds with Hform < 0 • Bulk Ti has hcp structure; Al is fcc. • Use resistively heated wires ( ~ML/min) • Deposit on substrate at room temperature Thin Film Symp -July 1999

5. Techniques used... • High-energy ion scattering and channeling (HEIS) • X-ray photoemission - intensities and chemical shifts in binding energy (XPS) • X-ray photoelectron diffraction (XPD) • Low-energy electron diffraction (LEED) • Low-energy ion scattering (LEIS) Thin Film Symp -July 1999

6. MSU Ion Beam Laboratory Thin Film Symp -July 1999

7. 2 MV van de Graaff Accelerator Thin Film Symp -July 1999

8. Scattering chamber • High precision sample goniometer • Hemispherical VSW analyzer (XPS, ISS) • Ion and x-ray sources • LEED • Metal wires for film deposition Thin Film Symp -July 1999

9. Overview of High Energy Ion Scattering (HEIS) • MeV He+ ions • Yield = Q   (Nt) • Ni peak for coverage • Al peak for structure Thin Film Symp -July 1999

10. Ti on Al(001): HEIS surface peaks • Surface peaks (SP) • Decrease in Al SP area • Ti shadows Al atoms • FCC Thin Film Symp -July 1999

11. Ti on Al(001): HEIS Al surface peak area vs Ti coverage • Decrease in Al SP (o) to 5.5 ML • Simulation (•) for flat Ti layer in FCC Al sites • Critical thickness of 5 ML ~ 4.4% lattice mismatch • Increase > 5 ML Ti layer relaxation Thin Film Symp -July 1999

12. Ti on Al(001): XPS intensity vs Ti coverage • Attenuation follows flat film model (solid line) after 2 ML • No decrease of intensity for first monolayer • Possible Ti-Al interchange at top layer Thin Film Symp -July 1999

13. Ti on Al(001): XPD angular scans • Enhanced Al 2p emission at 0o, 45o • Forward scattering for FCC lattice • Ti 2p peaks show enhanced emission along same directions • FCC Ti film ! Thin Film Symp -July 1999

14. Ti on Al(001): HEIS Channeling • Channeling along (101) shows outward relaxation for Ti layer • a = 2.07 Å HEIS • a = 2.12Å XPD • FCC Ti was also confirmed by LEED IV analysis Thin Film Symp -July 1999

15. Ti on Al(110): HEIS Al surface peak vs Ti coverage • Shadowing of Al atoms by Ti same as for Al(001) • Critical thickness at ~5 ML • Simulation () for flat Ti layer in FCC Al sites • Film relaxes for coverage > 5 ML Thin Film Symp -July 1999

16. Ti on Al(110): XPS intensity vs Ti coverage • Attenuation of Al 2p peak is very rapid at first • Ti on top of Al surface • Short attenuation length may be the result of the reduced XPD at normal exit angle Thin Film Symp -July 1999

17. Ti on Al(111): ISS spectra • Recent work includes ISS to determine surface composition • Peaks for Al, Ti, O • ISS (detecting ions) is very surface sensitive • Still see some Al for 9 ML of Ti Thin Film Symp -July 1999

18. Ti on Al(111): ISS intensity vs Ti coverage • Rapid decrease of Al and fast increase of Ti intensity below 2 ML • Indicates growth of a flat Ti overlayer • Slower rates of change after 2 ML • Indicates growth of Ti islands after 2 ML Ti Thin Film Symp -July 1999

19. Ti on Al(111): XPS intensity vs Ti coverage • Intensity decreases like flat film growth model to ~2 ML • Then follows model of island growth • Stronski-Krastinov growth Thin Film Symp -July 1999

20. Ti on Al(111): LEED patterns • LEED pattern becomes weak around 1.5 ML • LEED gradually returns by ~ 4ML • LEED shows hexagonal pattern from 4 to 11 ML (10.4 ML Ti here) Thin Film Symp -July 1999

21. Ti on Al(111): XPD peaks for Al • XPD scans for Al 2p • Forward scattering at 0o, 22o, 39o for fcc Al(111) • Intensity decreases as add Ti overlayers but still have ordered Al substrate Thin Film Symp -July 1999

22. Ti on Al(111): XPD peaks for Ti • XPD scans for Ti 2p • No forward scattering at low Ti coverage • Intensity builds at 32o typical of hcp structure • Reduced yield at fcc peak locations Thin Film Symp -July 1999

23. Ti on Al(111): XPD Ti and Al • Ti and Al emission are clearly different and consistent with the hcp and fcc structures • Ti grows on Al(111) with hcp structure • Fcc stacking in Al does not prevent hcp stacking in Ti • Recent XRD studies on Ti stacking in Ti-Al multilayers still under discussion Thin Film Symp -July 1999

24. Ti on Al(111): HEIS/Channeling • Channeling dips at normal incidence • Ti is shadowed so have ordered film on Al surface • Ti and Al axes are aligned normal to surface Thin Film Symp -July 1999

25. Ti on Al(111): HEIS Al SP vs Ti coverage • Al SP area initially decreases • Ti shadows Al atoms • After 2 ML the Al SP increases as if Al is disordered or alloyed • XPD, LEED, ISS, XPS indicate ordered Ti film on ordered Al • Dechanneling ? Thin Film Symp -July 1999

26. Conclusions: • Ti films on Al(110) and Al(001) grow with an epitaxial fcc structure up to 5 ML and then relax to a new structure, probably hcp. • Ti films on Al(111) exhibit SK growth mode, initial flat layer followed by hcp island growth • Possible strain relief by alloying at surface for Ti/Al(001) • Unexplained channeling results for Ti/Al(111) Thin Film Symp -July 1999