Presentation is loading. Please wait.

Presentation is loading. Please wait.

Slow positron implantation spectroscopy – a tool to characterize vacancy-type damage in solids G. Brauer Institut für Ionenstrahlphysik und Materialforschung,

Similar presentations


Presentation on theme: "Slow positron implantation spectroscopy – a tool to characterize vacancy-type damage in solids G. Brauer Institut für Ionenstrahlphysik und Materialforschung,"— Presentation transcript:

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 Institut für Ionenstrahlphysik und Materialforschung - (2) Bremsstrahlung / pair production

3 - pair production: E γ 2 m 0 c² n(E)dE E mean E 22 Na E mean = 225 keV E max = 542 keV β + -decay: p n + e + + ν Institut für Ionenstrahlphysik und Materialforschung

4 Distribution of positrons from ²²Na in solids (experiment) ρ Li = 0,535 g cm -3 ρ SiC = 3,217 g cm -3 In good approximation holds for e + from ²²Na z eff = 100 mg cm -2 ρ z max = 200 mg cm -2 Example: ρ Fe = 7,841 g cm -3 z eff = 128µmz max = 255 µm Institut für Ionenstrahlphysik und Materialforschung G. Dlubek, PhD 1975, U Halle

5 Institut für Ionenstrahlphysik und Materialforschung Positron energy vs. Time

6 Positron trapping by open volume defects increasing preference grain boundaryedge dislocationmonovacancy vacancy agglomerate Institut für Ionenstrahlphysik und Materialforschung

7 TRAPPING MODEL -rate equation approach (vacancies, dislocations) -diffusion-limited approach (vacancy agglomerates, shape of the trapping site!) = µ defect N d = 4 D + N d MethodParameter DB AC LT S, W H, mean sensitivity range defect concentration N d 0,1 – 200 ppm 10 12 – 10 15 m -2 monovacancies dislocations In metals: larger sizes seen: 2 – 50 agglomerated mono- vacancies Institut für Ionenstrahlphysik und Materialforschung

8 Principal methods of Positron Annihilation Spectroscopy (PAS)

9 Definition of the line shape parameters S and W in DB Institut für Ionenstrahlphysik und Materialforschung E = m 0 c² + 0.5 c x p parallel p parallel... electron momentum component parallel to emission direction of -quantum S = A 1 / A W= (B 1 + B 2 ) / A low electron momentum parameter, annihilation with valence electrons high electron momentum parameter,annihilation with core electrons

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

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

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

13 SPONSOR

14 (a)natural positrons from 22 Na (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 energieshaving the energy E (0-542 keV) of positrons from 22 Na. Depth distribution of thermalized positrons in SiC

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 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) D + … Diffusion coefficient k eff... Part of the positrons annihilating in defects n(z)... Positron density at depth z Makhovian distribution of the implanted positrons z 0 = z mean /Γ/(1/m+1) z mean = A/ρ*E n... mean penetration depth of positrons A, m, n... experimental parameters z mean : nm ρ: g cm -3 E: keV Institut für Ionenstrahlphysik und Materialforschung Mean positron depth

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

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) Institut für Ionenstrahlphysik und Materialforschung Experimental details: -{0001}-oriented, n-type 6H-SiC wafer -Fourfold implantation necessary: Al + implantation: 100 keV (5.0x10 16 cm -2 ) and 160 keV (1.3x10 17 cm -2 ) N + implantation: 65 keV (5.0x10 16 cm -2 ) and 120 keV (1.3x10 17 cm -2 ) -substrate temperature during implantation: 800°C

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

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

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

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

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 Institut für Ionenstrahlphysik und Materialforschung 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 at Orleans / France, July 9 -13, 2007

29 Institut für Ionenstrahlphysik und Materialforschung C oincidence D oppler B roadening measurements Principle

30 CDB – results Institut für Ionenstrahlphysik und Materialforschung

31

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

35

36

37

38

39

40

41

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

43 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

44 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

45 (a)natural positrons from 22 Na (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 energieshaving the energy E (0-542 keV) of positrons from 22 Na. Depth distribution of thermalized positrons in SiC


Download ppt "Slow positron implantation spectroscopy – a tool to characterize vacancy-type damage in solids G. Brauer Institut für Ionenstrahlphysik und Materialforschung,"

Similar presentations


Ads by Google