Nuclear Reaction Analysis Resonances Gábor Battistig

1. Nuclear Reaction Analysis Resonances Gábor Battistig Research Institute for Technical Physics and Materials Science (MTA - MFA) Budapest, Hungary battisti@mfa.kfki.hu

2. Inelastic nuclear collision with nuclear excitation Nuclear reaction in general A(a,b)B Isotope specific! Projectile energy must be higher than Coulomb barrier

3. Ion-Gamma reaction : 19F(p,g)20Ne Q=12.845 MeV Ion-Ion reaction :19F(p,a)16O Q=8.115 MeV Ion-Neutron reaction : 19F(p,n)19Ne Q=-4.020 MeV Particle Induced Activation Analysis (PAA) :19F(p,n)19Ne b+ 19F Energy levels and cross sections in nuclear reactions

4. Natural abundance of stable sotopes • 1H - 99.985% 2H - 0.015% • 3He - 0.0001%4He - 99.999% • 6Li - 7.56% 7Li - 92.44% • 9Be - 100% • 10B - 19.8% 11B - 80.2% • 12C - 98.89% 13C - 1.11% • 14N - 99.64% 15N - 0.36% • 16O - 99.76% 17O - 0.04% 18O - 0.20% • 19F - 100% • 23Na - 100% • 24Mg - 78.99% 25Mg - 10.0 % 26Mg - 11.01% • 27Al - 100% • 28Si - 92.23% 29Si - 4.67% 30Si - 3.10% • 31P – 100% • 50Cr – 4.35% 52Cr – 83.79% 53Cr – 9.5% 54Cr – 2.36%

5. Most used particle induced nuclear reactions of light elements

6. Principle 4He+, 3He+, 2H+, 1H+, etc Sample Absorber foil Detector 16O(d,p1)17O 16O(d,p0)17O 12C(d,p0)13O Counts Energy

7. Vacuum chamber LN2 trap Sample Vacuum chamber Surface barrier detector LN2 trap Ion beam g detector Filter foil Ion beam Sample Anular surface barrier detector g detector Filter foil Experimental setup

8. Experimental results600 nm SiO2 layer; 900 keV, Deuteron beam Yield: Well known reference sample is needed for quantification !!!

9. Experimental results170 nm AlxN layer, 1.7 MeV d beam Many reactions, many, sometimes overlapping peaks. Total amount of the given isotope can be determined.

10. Thin sample : interferences 900 keV 2H+ on TiOxNy film Numerous overlapping peaks from 14N(d,p0-7) and 14N(d,a0,1) reactions. Reaction Q-values are known In principle, interferences can be accounted for. In practice we avoid having to.

11. Reference samples Anodic isotopic Ta2O5 thin films for 16O and 18O Certified 16O and 18O films available from different sources. For thin targets, the cross section ratios of 12C(d,p)13C, D(3He,p)4He, 14N(d,a)12C, 14N(d,p)15N, 15N(d,a0)13C and 15N(p,a0)12C to that of 16O(d,p1)17O have been obtained by using stoichiometric frozen gas targets of CO2, NO and D2O. This enables the reliable and robust Ta2O5 reference targets to be used as a reference for NRA determinations of D, 12C, 14N and 15N. Davies, J. A., T. E. Jackman, et al. (1983). "Absolute calibration of 14N(d,a) and 14N(d,p) reactions for surface adsorption studies." Nucl. Instr. and Meth. 218: 141-146. Sawicki, J. A., J. A. Davies, et al. (1986). "Absolute cross sections of the 15N(d,a0)13C and 15N(p, a0)12C reaction cross sections." Nucl. Instr. and Meth. B15: 530-534.

12. x d x E inc q C(x) s (x) E Area A dE Depth Profiling : Principle • A channel of width dEc at energy Ec in the spectrum corresponds to a slice of width dx at depth x in the sample, with Ec and dEc being inversely related to x and dx through a linear combination of the stopping powers for the incident and outgoing particle • The number of particles accumulated into that histogram bin is proportional to C(x), dx, and s(Ex), where Ex is the energy of the incident beam when it gets to depth x;

13. Depth profiling Cross section Depth profiling nitrogen in titanium via 14N(d,a1)12C Spectra Concentration profile

14. Ion implantation of SiC RBS + channeling = lattice disorder

15. RBS + NRA = More information W. Jiang et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 501

16. dE/dx not needed Shape of s(E,q) much more important than absolute value. Precision standards are used rather than precision cross sections (Standardless NRA?) Approximate relative cross sections are needed to help in experimental design (isotopes …) Reaction Q values are needed - these are easily accessible and well known. Thin sample : summary

17. Resonances18O(p,a)15N cross section Cross section in the resonance: Breit-Wigner (Lorenz) function Differential cross section [mb/sr] Proton Energy [keV]

18. Most used Narrow Resonances in Depth Profiling • Reaction Resonance energy Resonance width • 18O(p,a)15N 152 keV 100 eV • 29Si(p,g)30P 413.9 keV • 15N(p,a)12C 429 keV 120 eV • 30Si(p,g)31P 620.4 keV 68 eV • 18O(p,a)15N 629 keV 2000 eV • 27Al(p,g)28Si 632.23 keV 6.7 eV • 23Na(p,g)24Mg 676.7 keV <70 eV • 27Al(p,g) 28Si 991.86 keV 70 eV • 52Cr(p,g)53Mn 1005 keV 50 eV • 13C(p,g)13N 1748 keV 135 eV

19. Depth Profiling by Resonance The resonance is scanned through the target depth by scanning the incident beam energy. Resonance samples the given isotope at depth

20. Principle of depth profiling with narrow resonances

21. An excitation curve Concentration profile C(x) Corresponding excitation curve N(E) Yield Beam Energy [keV] G(E) beam + Doppler energy spread G(E) rersonance lineshape T(E) beam energy straggling S<C(x)> „straggling” of C(x)

22. Excitation curve G(E) beam -Gaussian, + Doppler energy spread due to the thermal vibration of the target atoms G(E) rersonance lineshape - Lorantzian

23. f(u) Energy loss u DE T(E) beam energy straggling The charged particles lose their energy in independent collisions with electrons. 152 keV protons in CH2 f(u;x) tends towards a Gaussian for large x Energy loss [keV]

24. „Straggling” S<C(x)> ‘ straggling ’ of C(x) On average, m energy-loss events per unit length * * * * f(u) f(u) f(u) f(u) For thickness xmx events on average f(u)*f(u) g(u;x) 0 u

25. Ta218O5 /Ta sample, anodically oxidised, 20 mC /point Experimental excitation curves Si18O2 /Si sample, thermally grown, 20 mC /point Beam energy spred + Doppler broadening: 100 eV Resonance width: 100 eV

26. Tilting the sample – increases the virtual thickness of the layer 

27. Depth resolution • + • Narrow resonance width • Large dE/dx (~ 100 keV) • „negligible cross section outside the resonance – Background-free • - • Straggling – beam broadens by depth • Multiple Scattering at tilted sample Yield Proton Energy [keV] Depth resolution vs Depth : tilt angle line: straggling circles: MS crosses: overall

28. As for thin samples, plus need for accurate S(E) low energy – large stopping – high depth resolution Stronger requirement for shape accurate s(E,q) for accurate depth profiling Straggling and Multiple scattering gradually decreases resolution Depth Profiling by Resonance - Summary

29. Typical experimental results

30. exchange growth 800 600 400 200 0 150 155 160 165 170 175 180 Isotopic tracing study of the microscopic mechanisms of oxygen transport in the oxide growing during dry oxidation of silicon. 18O depth profile Experimental excitation curve Energy [keV] Interpretation of the spectra in terms of 18O depth profile, demonstrating surface exchange and that the growth takes place at the SiO2/Si interface through interstitial oxygen movement: direct confirmation of the Deal and Grove model for growth x > 10 nm. • No isotopic exchange in the matrix (natural abundance, 0.2%) except near the surface.

31. 18O depth profile Yield Energy [keV] Sequential oxidations in 100 mb 16O2 (40 h) at 1100°C, yielding 1600 Å Si16O2 then in 18O2 (5 h, 10 h and 24 h: additional 100, 285 and 405 Å). Excitation curve registration with target tilted to 60°. I. Trimaille et al. GPS, Paris

32. Isotopic tracing by sequential oxydation of SiC 16O2 (40 h) then 18O2 (5 h, 10 h and 24 h) 6H-SiC C terminated surface 6H-SiC Si terminated surface Yield Yield Thickness [Å] Energy [keV] Energy [keV] Sequential 16O2/18O2 oxidations, same conditions as for Si. SiC is a polar crystal: silica grows on both faces, similarly to the Si case, but the Si and C faces produce slow and fast growth. Isotopic tracing measurements of this type allow one to investigate with great sensitivity the near surface and interface properties of the silica produced by oxidation of SiC.

33. Hydrogen profiling with a nuclear resonance Hydrogen implantation profile in silicon (1016 cm-2, 40 keV) from W.A. Lanford, NIMB66(1992),68 1H(15N,ag)12C

34. Study of thin hafnium oxides depositedby atomiclayer deposition J.-J. Ganem, NIM B 219–220 (2004) 856 Excitation curves measured using the 151 keV 18O(p;ac)15N resonance on 3.5 nm (a) and 7.5 nm (b) HfO2 samples oxidized in18O2 atmosphere at 425 C just after: deposition (black circles), post-deposition N2 anneal at 425 C (open circles) and post-depositionN2 anneal at 800 C (open squares). After deposition the films present chlorinecontamination and a lack of oxygen. They areunstable toward thermal oxidation since a highoxygen transport and exchange mechanisms occurduring the process. Oxygen diffusion can be significantlyreduced after a thermal anneal in N2atmosphere.

35. Ultrathin siliconoxynitride film formation Experimental excitation curves of the 18O(p,a)15N reactionfor samples with (a) deferent 15N areal densities, sequentiallyoxidized in 16O2 (60 min) and in 18O2 (90 min). Thearrows indicate the energy position of the surface (dashed) andof the SiO2/Si interface (solid) in each sample; (b) no N prior tooxidation, oxidized under the same conditions as samples in (a). N amounts as low as1/30 of a monolayer at the surface of Si wafershamper the oxidation of Si, and the higher the Nconcentration, the thinner the oxynitride films; (ii)during the film growth, N and O are responsiblefor the atomic transport, while Si remains immobile; (iii) N, which is initially present at the surfaceof the Si wafer, migrates during oxidation, remainingat the near-surface and at the near-interfaceregions of the film.

36. Silicon isotopic tracing with the 29Si(p,c) narrow resonancenear415 keV 29Si(p, c)30P excitation curves from an enriched siliconsingle crystal before and after thermal oxidation, showing lossof silicon during the oxidation process. I.C. Vickridge et al, NIM B 161±163 (2000) 441

37. Annealing of ZrAlxOyUltrathin Films on Si in a Vacuumor in O2 E. B. O. da Rosa et al., Journal of The Electrochemical Society, 148 G695-G703 (2001) ZrAlxOy films were deposited at arate of 0.3 nm/min by reactive sputtering using a Zr80-Al20 atomic composition target in an oxygen-containing plasma directly on Si(001) substrates. Postdeposition annealingswere performed ex situ at 600°C for 10 min, either in highvacuum (p10-5 Pa) or in 710-3 Pa of dry 98.5%18O2. Areal densities of Al and Si were estimated from the areas of theexcitation curves of the 27Al(p,g)28Si and 29Si(p,g)30P nuclear reactionsaround the resonance energies at 404.9 and 414 keV. Theas-deposited film has anapproximate composition Zr4AlO9. Normalized excitation curves of the 18O(p,a)15Nnuclear reactionaround the resonance at 151 keV before and afterthermal annealingsand the used experimental geometry.(b) Normalized 18O concentration vs.normalized depth for the as deposited and 18O2-annealed samples. Solid lines represent the as-deposited sample, empty circles and trianglescorrespond to vacuum and 18O-annealed samples, respectively.

38. (a) Excitation curves of the 27Al(p,g)28Si nuclearreaction aroundthe resonance at 404.9 keVbefore and after thermal annealings and the usedexperimental geometry. (b) Normalized 27Al concentration vs. normalizeddepth for the as-deposited and vacuum-annealed samples.

39. (a) Excitation curves of the 29Si(p,g)30P nuclear reaction aroundthe resonance at 414 keV beforeand after thermal annealings. (b) Normalized 29Si concentrationvs. normalized depth for the as-deposited, 18O2- and vacuum-annealedsamples.

40. Summary • Isotope specific – unique tool for studying transport processes • Absolute concentration by well-known reference samples (no need of exact knowledge of cross section) • Narrow resonances: almost atomic depth resolution at the surface