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Slow positron implantation spectroscopy – a tool to characterize vacancy-type damage in solids

Slow positron implantation spectroscopy – a tool to characterize vacancy-type damage in solids G. Brauer Institut für Ionenstrahlphysik und Materialforschung, Forschungszentrum Dresden-Rossendorf Postfach 510119, D-01314 Dresden, Germany. See also:

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Slow positron implantation spectroscopy – a tool to characterize vacancy-type damage in solids

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  1. Slow positron implantation spectroscopy– a tool to characterize vacancy-type damage in solids G. Brauer Institut für Ionenstrahlphysik und Materialforschung, Forschungszentrum Dresden-Rossendorf Postfach 510119, D-01314 Dresden, Germany See also: # G.Brauer W. Anwand, P.G. Coleman, W. Skorupa Slow positron annihilation spectroscopy – a tool to characterize vacancy-type damage in ion-implanted 6H-SiC Vacuum 78 (2005) 131-136 # R.I. Grynszpan, W. Anwand, G. Brauer, P.G. Coleman Positron depth profiling in solid surface layers Annales de Chimie - Science des Materiaux (2007, in press) (18 pp)

  2. Where do positrons come from ? - (1) radioactive decay - (2) Bremsstrahlung / pair production Institut für Ionenstrahlphysik und Materialforschung

  3. β+-decay: p  n + e+ + ν 22Na Emean = 225 keV Emax = 542 keV n(E)dE Emean E - pair production: Eγ 2 m0c² Institut für Ionenstrahlphysik und Materialforschung

  4. G. Dlubek, PhD 1975, U Halle Distribution of positrons from ²²Na in solids (experiment) ρLi = 0,535 g cm-3 ρSiC = 3,217 g cm-3 In good approximation holdsfor e+from ²²Na r zeff = 100 mg cm-2 ρ zmax = 200 mg cm-2 Example: ρFe = 7,841 g cm-3 zeff = 128µm zmax = 255 µm Institut für Ionenstrahlphysik und Materialforschung

  5. Positron energy vs. Time Institut für Ionenstrahlphysik und Materialforschung

  6. increasing preference Positron trapping by open volume defects vacancyagglomerate grain boundary edge dislocation monovacancy Institut für Ionenstrahlphysik und Materialforschung

  7. TRAPPING MODEL • rate equation approach (vacancies, dislocations) • diffusion-limited approach (vacancy agglomerates, shape of the trapping site!) defectconcentrationNd sensitivity range monovacancies dislocations 0,1 – 200 ppm 1012 – 1015 m-2 In metals: larger sizes seen: agglomerated mono- vacancies 2 – 50 k = µdefect Nd k = 4p D+ Nd Institut für Ionenstrahlphysik und Materialforschung

  8. Principal methods of Positron Annihilation Spectroscopy (PAS) Institut für Ionenstrahlphysik und Materialforschung

  9. Eg = m0c² + 0.5 c x pparallel pparallel ... electron momentum component parallel to emission direction of g-quantum Definition of the line shape parameters S and Win DB low electron momentum parameter, annihilation with valence electrons S = A1 / A W= (B1 + B2) / A high electron momentum parameter,annihilation with core electrons Institut für Ionenstrahlphysik und Materialforschung

  10. Cartoon of a slow positron beam Institut für Ionenstrahlphysik und Materialforschung

  11. Cartoon of a positron moderator E ~ 3eV W (110), negative workfunction for positrons, ~ 3eV Institut für Ionenstrahlphysik und Materialforschung

  12. „SPONSOR“ SlowPOsitroNSystemOfRossendorf • direction of • fast e+ • γ-rays Positron energy: 30 eV ... 36 keV Beam diameter: ~ 4 mm at all energies Since 1999: (1,09 ± 0,01) keV FWHM Institut für Ionenstrahlphysik und Materialforschung

  13. „SPONSOR“ Institut für Ionenstrahlphysik und Materialforschung

  14. Depth distribution of thermalized positrons in SiC • “natural” positrons from 22Na (b) mono-energetic positrons of • energy E as indicated • N(z) is the distribution function obtained by P(E,z) is the distribution function of positrons • integration of P(E,z) over all energies having the energy E • (0-542 keV) of positrons from 22Na.

  15. Goal of Slow Positron Implantation Spectroscopy (SPIS) depth (nm) Institut für Ionenstrahlphysik und Materialforschung

  16. First step: from S(E) to S(d) plot Institut für Ionenstrahlphysik und Materialforschung

  17. D+ … Diffusion coefficientkeff ... Part of the positrons annihilating in defects n(z) ...Positron density at depth z First step:from S(E) to S(d) plot(mathematical background) Numerical solution of the positron diffusion equation (one dimensional) (Software „VEPFIT“: van Veen u.a. in AIP Conf. Proc. 218 (1990) 171) Makhovian distribution of the implanted positrons z0 = zmean/Γ/(1/m+1) zmean = A/ρ*En ... mean penetration depth of positrons A, m, n ... experimental parameters Mean positron depth zmean : nm ρ : g cm-3 E : keV Institut für Ionenstrahlphysik und Materialforschung

  18. Second step:theoretical calculation of positron lifetimes positron lifetime - specific for bulk and every defect - independent from defect concentration see e.g. G. Brauer, W. Anwand, P.G. Coleman, A.P. Knights, F. Plazaola, Y. Pacaud, W. Skorupa, J. Störmer, P. Willutzki, Positron studies of defects in ion implantated SiC, Phys. Rev. B54 (1996) 3084-3092 Institut für Ionenstrahlphysik und Materialforschung

  19. Problems to identify a defect by positron lifetime in a compound semiconductor • Experiment: • - already native (grown in) defects can exist on both sublattices • - defects may be charged • mostly impossible to create a certain defect on one sublattice only, • e.g. by irradiation Theory: - calculations performed so far for neutral defects only - different approaches to include electron-positron interaction available - adjustment of calculation to reality somehow necessary - possible lattice relaxation around a defect • Theoretical methods in use: • ATSUP (atomic superposition method): • rigid lattice positions, large defects, gives positron binding energy • (b) LMTO (linear muffin tin orbital method): • ab initio calculation, small defect configurations only, gives positron affinity and positron binding energy

  20. Third step:lifetime measurements (pulsed positron beam, Munich) Result:scaling curve S(N) see e.g. W. Anwand, G. Brauer, P.G. Coleman, W. Skorupa, MRS Proc. Vol. 504 (1998) 135-140 Institut für Ionenstrahlphysik und Materialforschung

  21. W. Anwand, G. Brauer, P.G. Coleman, R. Yankov, W. Skorupa, Appl. Surf. Sci. 149 (1999) 140-143 Fourth step:defect size depth distribution N(d) W. Anwand, G. Brauer, W. Skorupa, Appl. Surf. Sci. 184 (2001) 247-251 depth d (nm) Institut für Ionenstrahlphysik und Materialforschung

  22. Evolution of ion implantation-caused vacancy-type defects in 6H-SiC Motivation: • ion beam synthesis of a buried (SiC)1-x(AlN)x layer • layer between 80 nm and 210 nm with x~0.2 to adjust the band gap between 3.0 eV (6H-SiC) and 6.2 eV (2H-AlN) Experimental details: • {0001}-oriented, n-type 6H-SiC wafer • Fourfold implantation necessary: Al+ implantation:100 keV (5.0x1016 cm-2) and 160 keV (1.3x1017 cm-2) N+ implantation:65 keV (5.0x1016 cm-2) and 120 keV (1.3x1017 cm-2) • substrate temperature during implantation: 800°C Institut für Ionenstrahlphysik und Materialforschung

  23. Results of TRIM / SRIM calculations F. Ziegler, J.P. Biersack, The stopping and range of ions in matter, http://www.SRIM.org Al+100 keV Al+160 keV Institut für Ionenstrahlphysik und Materialforschung

  24. Results of TRIM / SRIM calculations F. Ziegler, J.P. Biersack, The stopping and range of ions in matter, http://www.SRIM.org N+65 keV N+120 keV

  25. 6H-SiCion-implanted(800°C) +annealed(1.200°C, 10 min) Institut für Ionenstrahlphysik und Materialforschung

  26. 6H-SiCion-implanted (800°C) + annealed (1.650°C, 10 min) ? Incomplete defect annealing – a surface problem ? Institut für Ionenstrahlphysik und Materialforschung

  27. Detection capabilities for various microprobe techniques (a) defect concentration (b) defect size (PAS refers to all positron annihilation techniques)

  28. Solved... • more and more sophisticated data collection possible • ambitious theoretical calculations available Further improvements needed... • increase of available positron beam intensity • depth dependant positron lifetime measurements routinely • combination of PAS results with those from other methods State – of – the – art in this field was reviewed at SLOPOS – 10 Doha / Qatar, March 19 – 25, 2005. Results see at: Applied Surface Science, Vol. 252 (Feb 2006) Next meeting: SLOPOS – 11 atOrleans / France, July 9 -13, 2007 Institut für Ionenstrahlphysik und Materialforschung

  29. Coincidence Doppler Broadening measurements Principle Institut für Ionenstrahlphysik und Materialforschung

  30. CDB – results Institut für Ionenstrahlphysik und Materialforschung

  31. Institut für Ionenstrahlphysik und Materialforschung

  32. EPOS scheme For a description of the project, see also: R. Krause-Rehberg, S. Sachert, G. Brauer, A. Rogov, K. Noack Appl. Surf. Sci. 252 (2006) 3106

  33. The End Institut für Ionenstrahlphysik und Materialforschung

  34. Detection capabilities for various microprobe techniques (a) defect concentration (b) defect size (PAS refers to all positron annihilation techniques)

  35. EPOS scheme For a description of the project, see also: R. Krause-Rehberg, S. Sachert, G. Brauer, A. Rogov, K. Noack Appl. Surf. Sci. 252 (2006) 3106

  36. Fourth step:defect size depth distribution N(d) see e.g. W. Anwand, G. Brauer, W. Skorupa, Appl. Surf. Sci. 149 (1999) 140-143 Institut für Ionenstrahlphysik und Materialforschung

  37. Depth distribution of thermalized positrons in SiC • “natural” positrons from 22Na (b) mono-energetic positrons of • energy E as indicated • N(z) is the distribution function obtained by P(E,z) is the distribution function of positrons • integration of P(E,z) over all energies having the energy E • (0-542 keV) of positrons from 22Na.

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