1. Salamanca.ppt, © Thomas Schwarz-Selinger, 03. Juni 2008 G. S. Oehrlein*, T. Schwarz-Selinger, K. Schmid, M. Schlüter and W. Jacob Interaction of Deuterium Atoms with Hard a-C:H Films: Isotope Exchange, Soft-layer Formation and Steady-state Erosion * work performed 2007 during sabbatical leave from: Department of Materials Science & Engineering and Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, Maryland 20742 9 th International Workshop on Hydrogen Isotopes in Fusion Reactor Materials, Salamanca, Spain, 3 June 2008

2. Salamanca.ppt, © Thomas Schwarz-Selinger, 03. Juni 2008 outline motivation: motivation: fundamental understanding of H/C interaction (tritium removal from carbon surfaces with H 0, D 0 ) strategy: strategy:exposure of a model system (a-C:H, a-C:D) to thermal H 0, D 0 beams at RT  surface modification and erosion: ellipsometry  isotope exchange: ion beam analysis  absolute values (cross sections, penetration depth…)

3. Salamanca.ppt, © Thomas Schwarz-Selinger, 03. Juni 2008 experimental 1. film preparation:  U sb = -300 V  H/(H+C) = 0.33  n C = 9 ·10 22 cm -3, (1.8 g/cm 3 ) Si 001 rf plasma (CH 4, 2Pa)  exposure of plasma deposited, hard a-C:H and a-C:D to thermal atomic H 0 or D 0 beams under UHV

4. Salamanca.ppt, © Thomas Schwarz-Selinger, 03. Juni 2008 experimental exposure of plasma deposited, hard a-C:H and a-C:D to thermal atomic H 0 or D 0 beams under UHV W. Jacob et al., Review of Scientific Instruments 74, 5123- 5136 (2003). ellipsometry (633 nm) H 2 or D 2 T=2100K H 0, D 0 2. exposure:  T substrate  320 K  j H,D = 1.3 · 10 15 cm -2 s -1

5. Salamanca.ppt, © Thomas Schwarz-Selinger, 03. Juni 2008 experimental 3. ex-situ analysis: nuclear reaction analysis D( 3 He,p) 4 He V.Kh. Alimov et al., Nucl. Instr. Meth. B234, 169 (2005). exposure of plasma deposited, hard a-C:H and a-C:D to thermal atomic H 0 or D 0 beams under UHV 3 He p@ 690 keV, 5 µC p, 4 He @ 135° 0.15 sr mylar

6. Salamanca.ppt, © Thomas Schwarz-Selinger, 03. Juni 2008 erosion of a-C:H with H 0 Vietzke and Philipps, Fusion Technol. 15, 108 (1989). Schlüter et al., J. Nuclear Mater. 136, 33 (2008). Horn et al., J. Chem. Phys. Lett. 231, 193 (1994). present study

7. Salamanca.ppt, © Thomas Schwarz-Selinger, 03. Juni 2008 model system a-C:H Schwarz-Selinger et al., J. Appl. Phys. 86 (7), 3988 (1999). present study ’hard a-C:H’ growth from CH 4, C 2 H 2, C 2 H 4, C 2 H 6, C 3 H 8, C 4 H 10 … film properties like - hydrogen content - density - refractive index are closely correlated

8. Salamanca.ppt, © Thomas Schwarz-Selinger, 03. Juni 2008 model system a-C:H Schwarz-Selinger et al., J. Appl. Phys. 86(7), 3988 (1999). film properties like - hydrogen content - density - refractive index are closely correlated density drop / thickness increase by factor of 2 growth from CH 4, C 2 H 2, C 2 H 4, C 2 H 6, C 3 H 8, C 4 H 10 …

9. Salamanca.ppt, © Thomas Schwarz-Selinger, 03. Juni 2008 ellipsometry 2 layer model erosion growth 

10. Salamanca.ppt, © Thomas Schwarz-Selinger, 03. Juni 2008 ellipsometry 2 layer model erosion growth 

11. Salamanca.ppt, © Thomas Schwarz-Selinger, 03. Juni 2008 ellipsometry 3 layer model erosion growth  hard soft

12. Salamanca.ppt, © Thomas Schwarz-Selinger, 03. Juni 2008 ellipsometry 3 layer model erosion growth  hard soft

13. Salamanca.ppt, © Thomas Schwarz-Selinger, 03. Juni 2008 ellipsometry 3 layer model erosion growth  hard soft

14. Salamanca.ppt, © Thomas Schwarz-Selinger, 03. Juni 2008 ellipsometry 3 layer model erosion growth  hard soft

15. Salamanca.ppt, © Thomas Schwarz-Selinger, 03. Juni 2008 ellipsometry 3 layer model erosion growth  hard soft

16. Salamanca.ppt, © Thomas Schwarz-Selinger, 03. Juni 2008 ellipsometry

17. Salamanca.ppt, © Thomas Schwarz-Selinger, 03. Juni 2008 ellipsometry

18. Salamanca.ppt, © Thomas Schwarz-Selinger, 03. Juni 2008 NRA results: D 0 (H 0 ) areal density vs. exposure time total uptake of D initial thickness: 20 nm expected D content of a 14 Å thick soft layer: 6·10 15 D 0 /cm 2

19. Salamanca.ppt, © Thomas Schwarz-Selinger, 03. Juni 2008 NRA results: D 0 (H 0 ) areal density vs. exposure time loss of D (isotope exchange) total uptake of D initial thickness: 20 nm initial D content of a 14 Å thick hard layer: 5·10 15 D 0 /cm 2

20. Salamanca.ppt, © Thomas Schwarz-Selinger, 03. Juni 2008 NRA results: D 0 (H 0 ) areal density vs. exposure time deuteration of extra sites loss of D (isotope exchange) total uptake of D initial thickness: 20 nm

21. Salamanca.ppt, © Thomas Schwarz-Selinger, 03. Juni 2008 ellipsometry trajectories for D and H erosion switching between H and D: steady state erosion  no difference in optical response erosion @ 650 K  ellipsometry does not see isotope exchange

22. Salamanca.ppt, © Thomas Schwarz-Selinger, 03. Juni 2008 change in D area density and modified layer thickness uptake of D without optical response  isotope exchange

23. Salamanca.ppt, © Thomas Schwarz-Selinger, 03. Juni 2008 for a flux density j = 1.3·10 15 D 0 /cm 2 s an initial hydrogen density n H = 0.33 · 12.1·10 22 /cm 3 an uptake of n D = 3·10 15 D/cm 2 in the first 5 minutes: σ = 2·10 -18 cm 2 = 0.02 Å 2 analysis for simple isotope exchange we have: Küppers et al.: abstraction of bonded H: 0.05 Å 2 cm 2

24. Salamanca.ppt, © Thomas Schwarz-Selinger, 03. Juni 2008 interaction of H with a-C:H known from literature * : abstraction addition thermal activated erosion  * = 0.05 Å 2  * = 1.3 Å 2 (4.5 Å 2 ) *J. Küppers, Surf. Science Reports 22, 249 (1995)

25. Salamanca.ppt, © Thomas Schwarz-Selinger, 03. Juni 2008 1.4 nm thickness, deuteration of extra sites –A) based on total number of C atoms in layer –B) formation of C-D2 from C-D i.e., comparable to D-exchange Both steps must be going on. analysis σ extra sites = 6·10 -19 cm 2 = 0.006 Å 2 σ extra sites = 1.3·10 -18 cm 2 = 0.013 Å 2

26. Salamanca.ppt, © Thomas Schwarz-Selinger, 03. Juni 2008 3 stages of D interaction with a-C:H (at 50 °C) 1.isotope exchange complete after ~ 30 min ( j = 2·10 18 D 0 /cm 2 ) comparison of the cross-section for this process with literature values for H interaction with C:H shows that this corresponds to the cross-section of hydrogen abstraction from the C:H surface 2.creation of new C-D bonds soft a-C:D layer formation occurs up to ~ 200 min ( j = 2·10 19 D 0 /cm 2 ) 3.erosion of a-C:H, a soft a-C:D layer “remains” on the substrate, with roughly constant thickness (1.4 nm)

27. Salamanca.ppt, © Thomas Schwarz-Selinger, 03. Juni 2008

28. interaction of H with a-C:H known from literature * : thermal activated erosion k = f(T) erosion !  H fluence ! *J. Küppers, Surf. Science Reports 22, 249 (1995)

29. Salamanca.ppt, © Thomas Schwarz-Selinger, 03. Juni 2008 sp 2 Hydration and erosion circle: Horn et al., Chem. Phys. Lett. 231, 193 (1994) Zecho et alJ. Phys. Chem. B 105 (2001). Chemical Erosion: microscopic model 1)chemisorption of H on sp 2 site H  = 1.3 Å 2 H sp x sp 3 H  = 1.3 Å 2 H CH 3 2)chemisorption of H on sp x site (hydration) 3)abstraction of H to form H 2 H H 2  = 0.05 Å 2 CH 3 E act =1.7 eV 5)relaxation back to sp 2 above 750 K 6)direct thermal decomposition to sp 2 above 900 K with E act =2.4 eV 4 a) thermal release of CH 3 radicals from activated sites above 400 K E act =1.7 eV CH 3 4 b) chemisorption of H on sp x site  = 1.3 Å 2 H

30. Salamanca.ppt, © Thomas Schwarz-Selinger, 03. Juni 2008 Chemical erosion: structure dependence E. Vietzke et al., Surf. Coat. Technol. 47 (1991) 156-161 Total Y of H o on films of:  a-C:H (plasma-deposited),  pre-irradiated graphite,  graphite and  diamond disorder order 1000 x  reactivity of the surface depends critically on the surface structure

31. Salamanca.ppt, © Thomas Schwarz-Selinger, 03. Juni 2008 mechanistic picture

32. Salamanca.ppt, © Thomas Schwarz-Selinger, 03. Juni 2008

33. ellipsometry

34. Salamanca.ppt, © Thomas Schwarz-Selinger, 03. Juni 2008 ellipsometry

35. Salamanca.ppt, © Thomas Schwarz-Selinger, 03. Juni 2008 change in D area density and modified layer thickness uptake of D without optical response  isotope exchange

36. Salamanca.ppt, © Thomas Schwarz-Selinger, 03. Juni 2008 for a flux density j = 1.3·10 15 D 0 /cm 2 s an initial hydrogen density n H = 0.33 · 12.1·10 22 /cm 3 an uptake of n D = 3·10 15 D/cm 2 in the first 5 minutes: σ = 2·10 -18 cm 2 = 0.02 Å 2 analysis for simple isotope exchange we have: Küppers et al.: abstraction of bonded H: 0.02 Å 2 cm 2