Quantitative Low Energy Ion Scattering: achievements and challenges

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1 Quantitative Low Energy Ion Scattering: achievements and challenges Peter Bauer Institute for Experimental Physics Atomic Physics and Surface Science Johannes Kepler University Linz, Austria

2 Intro Low energy ion scattering ESA : ions only Layout Achievements: surface composition analysis o Scattering potential o Charge exchange o information depth Challenges subsurface information o stopping power reionization probability TOF-LEIS

3 Intro Low Energy Ion Scattering projectiles: noble gas ions, large scattering angle (no grazing collisions) I 0 E 0 = keV detector ( + ) J + J I i 0 P i c i N s d i d N i a b q Surface composition analysis J i+ detected ion current (ions/sec) I 0 primary ion current (ions/sec) c i atomic surface concentration N s atomic surface density (atoms/cm 2 ) P + i ion fraction of atom i d i /d scattering cross section (atom i) detector solid angle + detector efficiency (incl. transmission)

4 Intro Low Energy Ion Scattering ESA: only ions detected surface sensitivity energy spectrum of detected ions (arb.u.) kev He + Ta & O J + Ta O J + O ESA spectrum secondary ions Ta J I i 0 Sensitivity factor S i + P i c i N s d i d final energy E (ev) (B. Bruckner, 201 5)

5 Intro Low Energy Ion Scattering ESA: only ions detected surface sensitivity energy spectrum of detected ions (arb.u.) kev He + Ta & O J + Ta O J + O ESA spectrum secondary ions Ta Relative yield A i final energy E (ev) (B. Bruckner, 201 5) A P A P Ta O 1 Ta O C

6 Intro Scattering cross section Screened Coulomb potential: r V r Φ r a V C (r/a) screening function d/d depends on (r/a) model Φr a bi exp ci r a 3 i1 1 He Au 1,25 1,20 He Au (d scr /d)/(d R /d) 0,1 d TFM /d Rutherford d ZBL /d Rutherford ratio d ZBL /d TFM 1,15 1,10 1,05 d ZBL d d d TFM d ZBL /d TFM 0, Energy E [kev] 1, Energy E [kev]

ESA: only ions detected surface sensitivity Surface composition analysis energy spectrum of detected ions (arb.u.) 10000 1000 3 kev He + Ta & O A + Ta O A + O ESA spectrum secondary ions Ta A j + constant?

7 ESA: only ions detected surface sensitivity Surface composition analysis energy spectrum of detected ions (arb.u.) kev He + Ta & O A + Ta O A + O ESA spectrum secondary ions Ta A j + constant? charge exchange! (B. Bruckner, 2015) final energy E (ev)

8 Charge exchange (He ions) Auger neutralization (AN) is possible at any ion energy E & at any surface atom Resonant charge exchange (reionization, res. neutralization) (reionization & resonant neutralization) E > threshold energy E th R min (E th,) < R crit Quasi resonant neutralization (qrn) resonant levels at atom and ion ( quantum oscillations, difficult quantification)

9 Charge exchange (He ions) Auger neutralization (AN) is possible at any ion energy E & at any surface atom Resonant charge exchange (reionization, res. neutralization) (reionization & resonant neutralization) E > threshold energy E th R min (E th,) < R crit Quasi resonant neutralization (qrn) resonant levels at atom and ion ( quantum oscillations difficult quantification) difficult How to quantification) do quantitative composition analysis? How to obtain surface sensitivity?

10 charge exchange Auger Neutralization (AN) I 0 Auger transition rate A J +, J 0 (H. Hagstrum, 1954) A depends on electron density parameter r s A (r s (x,y,z)) in front of a surface Typically, A /s

11 charge exchange Auger Neutralization (AN) I 0 J +, J 0 Rate equation (1D) for survival probability P + dp P z survival probability P + = exp(-v c /v ) (He + remains He + ) A dt P dz v with v c = A (z)dz 0 v c characteristic velocity AN efficiency v c m/s 0.1 a.u. Typically, z 1Å information depth due to AN

12 charge exchange Energy spectrum of scattered ions in AN regime ke 0-43 ev (collisional inelastic loss) ion spectrum N + (E) (arb.u.) kev He + Cu poly surface peak (survivals) all detected ions are survivals no charge exchange energy (ev) ke 0

13 charge exchange P + (1/v ) in the AN regime He + Cu: for E < 2.1 kev, only AN is possible ion signal: 1 st atomic layer dominates He - Cu variation of geometry variation of energy v c = m/s P + = exp(-v c /v ) (H.H. Brongersma et al., Surf.Sci.Rep., 2007)

14 charge exchange Summary AN regime Information depth high AN rate and long dwell time information depth 1 ML Quantitative composition analysis AN depends on DOS matrix effects to be expected not first choice for composition analysis

15 charge exchange Resonant charge exchange in a close collision R min < R crit overlap of orbitals electron promotion (atomic collision) is active for E > E th adiabatic He + Al RN RI surface resonant reionization neutralization RI RN R crit AN diabatic (N.P. Wang et al., 2001) R min < R crit E > E th A Cu: R crit 0.1 Å E th 2 kev AN dominant Ta: R crit 0.5 Å E th 0.4 kev RN, RI dominant Al: R crit 0.5 Å E th 0.2 kev

16 charge exchange Resonant charge exchange in a close collision P Pin (1 PRN ) Pout (1 Pin ) P RI P out Survivals (no AN, no RN) reionized projectiles (AN+RI) RI surface reionization A

17 charge exchange Resonant charge exchange in a close collision P Pin (1 PRN ) Pout (1 Pin ) P RI P out Survivals (no AN, no RN) reionized projectiles (AN+RI) AN: P + = exp(-v c /v ) (survival probability) RN: P RN = 1 - exp(-v RN /v) neutralization probability due to rate RN RI: P RI = exp(-v RI /v) reionization probability due to rate RI rates RI, RN :??? RN, RI scale with velocity v

18 P + : Variation of geometry E = E th : P RI = P RN = 0 E > E th : P RI >0, P RN > kev He + Au-poly (polar scan) AN, v c = m/s 0.4 P (D. Roth, 2016) model calc.: P RI = 0.45 P RN = x x x x x10-6 1/v

19 P + : Variation of geometry probability 1,0 0,8 0,6 0,4 He + Au-poly, CuAu(100) P RN (CuAu(100), Primetzhofer et al., 2009) P RI (CuAu(100), Primetzhofer et al. 2009) P RN (Au-poly) P RI (Au-poly) P RN (fit) PRI (fit) P x10 5 m/s He + Au-poly polar scans Au-poly CuAu(100) Au-poly ref. P + (a) E 0 = 1.2 kev P + (a) E 0 = 2 kev P + (a) E 0 = 4 kev P + (a) E 0 = 6 kev P + (a) E 0 = 8 kev P + model fit 0, , primary ion energy, kev x x x x10-5 (D. Roth, 2016) 1/v, s/m

20 charge exchange Reionization He + - Al poly is active for E > E th (threshold energy) 1 consequences? He + Al empirical fact: variation of E P + = e -v R/v Ion fraction expt. fit to expt. AN (theory) fit to AN 0 1x10-5 2x10-5 3x10-5 1/v (s/m) P + expt << P + AN RN dominant v R >> v c (S. Rund et al., 2011) Low ion signal charge state 0

21 charge exchange 4 kev He + Ta: ion spectrum kev Ta surface peak (survivals and reionized proj.) N(E) E th reionization background (subsurface backscattering) E [ev] (Barbara Bruckner, 2014)

22 charge exchange Information depth in reionization regime penetration to deeper layers & surface : information depth is due to P + out (no AN on way out) J + RI (1) = (R crit )P RI P + out J + RI (2) = (R crit ) P RI (P + out) 3 = J + RI (1) P + out 2 J + RI (3) = (R crit ) P RI (P + out) 5 = J + RI (1) P + out 4 J + RI = J + RI(1) (1 + (P + out) 2 + (P + out) 4 + ) st J layer RI 1 J 2 1 P RI J RI 1 out st layer

23 MC-simulations and charge exchange modeling the reionzation background in TRIM by introducing a minimum number of additional parameters P RN and P RI probabilties 4 kev He Cu: Good agreement for A = /s P RN P RI present work Draxler et al. Primetzhofer et al. present work Draxler et al. Primetzhofer et al. He + Cu Counts keV He + Cu Experiment MC - simulation (K.Khalal-Kouache) Energy (kev) K. Khalal-Kouache et al., (2016) Energy (ev)

24 Surface composition analysis P + (He Si) influence of oxygen exposure Relat. ion yield J + /(d/d)/e He + Si He + SiO 2 (Barbara Bruckner, 2014) (T. Jansens et al., 2003) (Hidde Brongersma, 1992 (?)) A P A P Si O 1 Si O C

25 charge exchange P + (He Al) influence of oxygen vc = 2.2x105 m/s He + Al metal He + Al 2 O 3 He + Al (metal): v R = m/s v R >> v c P He Al He Al 2 O 3 He + Al in Al 2 O 3 : v R = m/s 1/v 0: P ,0 5,0x10-6 1,0x10-5 1,5x10-5 2,0x10-5 2,5x10-5 3,0x10-5 (Barbara Bruckner, 2014) 1/v

26 Surface composition analysis P + (He Ta) P + (He Ta 2 O 5 ) 10 0 He Ta He Ta 2 O 5 Ta 2 O 5 1 kev He + Ta - O 10-1 P + Ta 10-2 cleanta x x x x x10-5 1/v [m/s] (Barbara Bruckner, 2014) Linear dependence signal - concentration! Reionization regime is best suited for composition analysis But: physics of reionization is not yet understood!

27 Summary reionization regime Information depth polycrystals: surface peak information depth 1 ML quantitative surface composition analysis probabilities P RN, P RI : depend only weakly on band structure absence of matrix effects! 1/v

28 Achievements Characterization of graphene 3 kev He + CH x / graphene / metals? /Si (Stan Prusa et al, Langmuir, 2015)

29 Achievements Characterization of graphene layers 3 kev He + CH x / graphene / metals? /Si (Stan Prusa et al, Langmuir, 2015)

30 Achievements Characterization of graphene layers 3 kev He + CH x / graphene / metals? /Si (Stan Prusa et al, Langmuir, 2015)

31 challenges Challenges: subsurface information He + subsurface Hf: E x Required input: de/dx in Al 2 O 3 multiple scattering reionization at surface (Philipp Brüner, 2014)

32 challenges Reionization subsurface information kev Ta surface peak (survivals) N(E) 600 Final energy penetration depth? 400 E th reionization background (subsurface backscattering) E [ev] (Barbara Bruckner, 2014)

33 challenges TRBS + charge exchange experiment kev He + Ta TRBS experiment: 800 N expt (E)*E N expt (E)*E - sputtered ions N expt (E) = N TRBS (E)p RI,eff N + (E) N TRBS (E)*P RI,eff surface peak 50A Ta (TRBS) p RI,eff = P RI P + out 200 p RI,eff is a surface property E [ev] Electronic stopping Multiple scattering Path length increase

34 Summary quantification Reionization: best suited for composition analysis no matrix effects! charge exchange still lacks basic understanding Auger regime: not recommended for composition analysis band structure (matrix effects) effects to be expected quasi resonant neutralization: not recommended for composition analysis P + oscillation amplitudes of a factor 3, P + depends on band structure

35 Summary information depth Reionization regime: 1 ML for polycrystals (depending on E) may be larger for single crystals (focusing collisions) Auger regime: 1 ML quasi resonant neutralization: neutralizes much more effective than AN information depth = 1 ML

36 TOF-LEIS application: Cu/PET N ( E ) [ a r b. u. ] 60 3keV He q= thick Cu 4nm Cu/PET 4nm Marlowe ~3nm final energy [ev]

37 TOF-LEIS application: Ag clusters/pet Small clusters: buried in PET 3.5 nm Larger clusters: on PET, covered 11 nm (J M Flores-Camacho, 2011)

38 TOF-LEIS: growth Au on B 400 Au 0.1nm nominal Au 0.4nm nominal Au 1.6nm nominal Au 6.4nm nominal Au 0.2nm nominal Au 0.8nm nominal Au 3.2nm nominal 30 yield (arb. units) energy (ev) D. Primetzhofer et al., APL (2008)

39 Growth of Au on B TOF-LEIS: 1-10 kev He + Au nanostructures on B Information on coverage and height? D. Primetzhofer et al., APL (2008)

40 TRBS-Simulations: 1 Å kev He + Au (d = 1Å) f = 1 exp. (1Å) energy spectra (arb.units) TRBS (8Å) TRBS (8Å)*0.18 h f h f < final energy (ev) D. Primetzhofer et al., APL (2008)

41 Acknowledgements Dietmar Roth Daniel Primetzhofer Acknowledgments: Stefanie Rund Hidde Brongersma Roland Steinberger Carmina Monreal Karima Khalal-Kouache Edmund Taglauer Dominik Göbl Barbara Bruckner

42 Thank you!

43 charge exchange (c) Quasiresonant neutralization d-electrons (e.g., of Ge) are quasi resonant with He 1s level quantum oscillations! (Hidde Brongersma et al., Surf.Sci.Rep. 62(2007) 63)

44 charge exchange Quantum oscillations d-electrons (e.g., of Ge) are quasi resonant with He 1s level quantum oscillations! (Erickson et al. (1975))

45 charge exchange Quantum oscillations Way in: at mixing distance R M the projectile forgets its charge state collision: phase difference evolves between the two paths (V 1, V 2 ) until projectile passes R M again qrn atomic collision: No dependence on a, b, no 1/v scaling! b a 1 R M 1 V ( R) V ( t) dt dr v( R) from known V 1 (R), V 2 (R) R 0 R 0 R M I + = a + + b cos 2( /2), I 0 = a 0 + b sin 2( /2) I + oscillations are equidistant as f(1/v)

46 charge exchange Interplay AN qrn RI Threshold energy for reionization: E f 1200 ev for E f < 1200 ev only Auger neutralization quasi-resonant neutral. Dominik Göbl et al., J. Phys.: Conden. Matt. (2013)

47 charge exchange Quantum oscillations d-electrons are quasi resonant with He 1s level quantum oscillations! He Pb He Ge He Bi (Erickson et al., 1975) (Smith et al., 1974) (Zartner et al., 1978)

48 charge exchange Quantitative P + for He + Ge P + << 1: qrn is very effective qrn works one-way : He + He 0 AN for Cu --- P + surv = e -v qrn/v 0 qrn (He 0 He + is not possible!) No reionization up to 1.3 kev P + = qrn-surviving probability P + = e -P qrn = e -v qrn/v P + 1 kev (v = 0.1 a.u.) v qrn 10 6 m/s 5 v c (Goebl et al., 2013) qrn dominates over AN Information depth = 1 ML! (without reionization) Oscillation amplitude factor 2: quantification

49 charge exchange Quantitative P + for He + Ge AN --- P + surv = e -v qrn/v 0 AN is inefficient qrn is dominant Reionization is weak qrn (Goebl et al., 2013) Information depth = 1 ML! (without reionization) Oscillation amplitude factor 2: quantification

50 El tubo Zorritos Peru Acknowledgements

51 TOF-LEIS Experiment: ACOLISSA Einzel-Lenses Ion Source Colutron G2 Var. Aperture Target High Energy Beam Beam Chopper Surface Barrier Detector Detector Drift Tube

Institut für Experimentalphysik, Johannes Kepler Universität Linz, A-4040 Linz, Austria.

On the Surface Sensitivity of Angular Scans in LEIS D. Primetzhofer a*, S.N. Markin a, R. Kolarova a, M. Draxler a R. Beikler b, E. Taglauer b and P. Bauer a a Institut für Experimentalphysik, Johannes