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Influence of Substrate Surface Orientation on the Structure of Ti Thin Films Grown on Al Single- Crystal Surfaces at Room Temperature Richard J. Smith.

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Presentation on theme: "Influence of Substrate Surface Orientation on the Structure of Ti Thin Films Grown on Al Single- Crystal Surfaces at Room Temperature Richard J. Smith."— Presentation transcript:

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 Thin Film Symp -July Acknowledgements ä Ph.D students: Adli Saleh,V. Shuthanandan, N. Shivaparan, G. A. White N. Shivaparan, G. A. White ä Dr. Yong-wook Kim (from ASSRC) ä National Science Foundation ä

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

5 Thin Film Symp -July 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 Thin Film Symp -July MSU Ion Beam Laboratory

7 Thin Film Symp -July MV van de Graaff Accelerator

8 Thin Film Symp -July 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 Thin Film Symp -July 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 Thin Film Symp -July Ti on Al(001): HEIS surface peaks ä Surface peaks (SP) ä Decrease in Al SP area ä Ti shadows Al atoms ä FCC

11 Thin Film Symp -July Ti on Al(001): 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

12 Thin Film Symp -July Ti on Al(001): 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

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

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

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

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

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

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

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

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

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

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

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

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

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

26 Thin Film Symp -July Conclusions: ä 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. ä Ti films on Al(111) exhibit SK growth mode, initial flat layer followed by hcp island growth ä Possible strain relief by alloying at surface for Ti/Al(001) ä Unexplained channeling results for Ti/Al(111)


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