Nuclear Reaction Analysis Resonances

Transcription

1 SMR SCHOOL ON ION BEAM ANALYSIS AND ACCELERATOR APPLICATIONS March 2006 Nuclear Reaction Analysis - Concept and application Gabor BATTISTIG Research Institute for Technical Physics and Materials Science Budapest, Hungary

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

3 2 ] 3 [ 1 2 MeV A Z Z R e Z Z E A a A a A c = Endoterm Q Exoterm Q c M M M M Q Q E E E E Z Z Z Z A A A A B b A a B b A a B b a A B b a A < > + = + + = + + = + + = ) ( 2 Inelastic nuclear collision with nuclear excitation Inelastic nuclear collision with nuclear excitation Projectile energy must be higher than Coulomb barrier Nuclear reaction in general A(a,b)B Isotope specific! a b B b B th M M M M M Q E + + =

4 Ion-Gamma reaction : 19 F(p,γ) 20 Ne Q= MeV Ion-Ion reaction : 19 F(p,α) 16 O Q=8.115 MeV Ion-Neutron reaction : 19 F(p,n) 19 Ne Q= MeV Particle Induced Activation Analysis (PAA) : 19 F(p,n) 19 Ne β + 19 F Energy levels and cross sections in nuclear reactions 3

5 Natural abundance of stable sotopes 1 H % 2 H % 3 He % 6 Li % 9 Be - 100% 4 He % 7 Li % 10 B-19.8% 11 B % 12 C % 13 C % 14 N % 15 N % 16 O % 17 O % 18 O % 19 F-100% 23 Na - 100% 24 Mg Mg % 25Mg % 26Mg % 27 Al - 100% 28 Si 25 Mg 26 Mg Si % 29Si % 30Si % 31 P 100% 50 Cr 29 Si 30 Si Cr 4.35% 52Cr 83.79% 53Cr 9.5% 54Cr 2.36% 52 Cr 53 Cr 54 Cr 4

6 Most used particle induced nuclear reactions of light elements Proton induced reactions Q [MeV] Deuteron induced reactions Q [MeV] 3 He induced reactions Q [MeV] 4 He induced reactions Q [MeV] 6 Li(p,α) 3 He H(d,p) 3 He H( 3 He,p) 4 He B(α,p) 13 C Li(p,α) 4 He He(d,α) 1 H Li( 3 He,p) 8 Be B(α,p) 14 C Be(p,α) 6 Li C(d,p) 13 C Be ( 3 He,p) 11 B N(α,p) 17 O B(p,α) 7 Be C(d,p) 14 C Be( 3 He,α) 8 Be F(α,p) 22 Ne B(p,α) 8 Be N(d,p) 15 N C( 3 He,p) 14 N P(α,p) 34 S N(p,αγ) 12 C N(d,α) 12 C C( 3 He,α) 11 C O(p,αγ) 15 N O(d,p) 17 O O( 3 He,p) 20 F F(p,αγ) 16 O O(d,α) 14 N O( 3 He,d) 19 F Na(p,αγ) 24 Mg F(d,α) 17 O O( 3 He,α) 19 O Al(p,γ) 28 Si Si(p,αγ) 30 P Cr(p,αγ) 53 Mn

7 Principle C(d,p 0 ) 13 C Sample 4 He +, 3 He +, 2 H +, 1 H +, etc Absorber foil σ(mb sr -1 ) lab Q=2.77 MeV 16 O(d,p 1 ) 17 O Detector Energie (kev) 16 O(d,p 0 ) 17 O Counts 12 C(d,p 0 ) 13 O σ (mb sr -1 ) O(d,p 1 ) 17 O 150 lab Q=1.05 MeV Energy Energie (kev) 6

8 Experimental setup Vacuum chamber Vacuum chamber LN 2 trap Surface barrier detector LN 2 trap Sample Filter foil Ion beam Sample Ion beam γ detector γ detector Anular surface barrier detector Filter foil 7

9 Experimental results 600 nm SiO 2 layer; 900 kev, Deuteron beam Yield: N A = Nb dσ (φ) N dω a Ω Well known reference sample is needed for quantification!!! 8

10 Experimental results 170 nm Al x N layer, 1.7 MeV d beam Many reactions, many, sometimes overlapping peaks. Total amount of the given isotope can be determined. 9

11 Thin sample : interferences 900 kev 2 H + on TiO x N y film Numerous overlapping peaks from 14 N(d,p 0-7 ) and 14 N(d,α 0,1 ) reactions Counts O reference film containing 16 O and 14 N Reaction Q-values are known In principle, interferences can be accounted for. In practice we avoid having to Channels 10

12 Reference samples N U = Y Y U R N R Anodic isotopic Ta 2 O 5 thin films for 16 O and 18 O Certified 16 Oand 18 O films available from different sources. For thin targets, the cross section ratios of 12 C(d,p) 13 C, D( 3 He,p) 4 He, 14 N(d,α) 12 C, 14 N(d,p) 15 N, 15 N(d,α 0 ) 13 C and 15 N(p,α 0 ) 12 C to that of 16 O(d,p 1 ) 17 O have been obtained by using stoichiometric frozen gas targets of CO 2, NO and D 2 O. This enables the reliable and robust Ta 2 O 5 reference targets to be used as a reference for NRA determinations of D, 12 C, 14 N and 15 N. Davies, J. A., T. E. Jackman, et al. (1983). "Absolute calibration of 14 N(d,α) and 14 N(d,p) reactions for surface adsorption studies." Nucl. Instr. and Meth. 218: Sawicki, J. A., J. A. Davies, et al. (1986). "Absolute cross sections of the 15 N(d,α 0 ) 13 C and 15 N(p, α 0 ) 12 C reaction cross sections." Nucl. Instr. and Meth. B15:

13 Depth Profiling : Principle C(x) σ(x) δx x θ E inc A channel of width de c at energy E c in the spectrum corresponds to a slice of width dx at depth x in the sample, with E c and de c 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 σ(e x ), where E x is the energy of the incident beam when it gets to depth x; Area A δε E Nσ ( E) C( x) σ ( E) dx Y = C xσ ( E) = = x 12

14 Depth profiling Depth profiling nitrogen in titanium via 14 N(d,α 1 ) 12 C dσ/dω (mb. sr-1) Cross section Energy (kev) Spectra Concentration profile b) d) 13

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

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

17 Thin sample : summary de/dx not needed Shape of σ(e,θ) 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. 16

18 Resonances 18 O(p,α) 15 N cross section Differential Differenciális Hatáskeresztmetszet cross section [µb/sr] [µb/sr] kev 334 kev 216 kev 629 kev 18 O(p,α) 15 N Q = MeV φ = Proton Energy Energia [kev] Cross section in the resonance: Breit-Wigner (Lorenz) function 2 Γ σ R ( E) = K 2 2 Γ ( E ER )

19 Most used Narrow Resonances in Depth Profiling Reaction Resonance energy Resonance width 18 O(p,α) 15 N 152 kev 100 ev 29 Si(p,γ) 30 P kev 15 N(p,α) 12 C 429 kev 120 ev 30 Si(p,γ) 31 P kev 68 ev 18 O(p,α) 15 N 629 kev 2000 ev 27 Al(p,γ) 28 Si kev 6.7 ev 23 Na(p,γ) 24 Mg kev <70 ev 27 Al(p,γ) 28 Si kev 70 ev 52 Cr(p,γ) 53 Mn 1005 kev 50 ev 13 C(p,γ) 13 N 1748 kev 135 ev 18

20 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 x E = 0 E de dx R 19

21 Principle of depth profiling with narrow resonances 18 O(p,α) 15 N resonance at 152 kev Beam energy E R + E energy loss E in the sample The resonance occurs at depth x in the sample Sample The resonance samples the 18 O at depth x = E/dE/dx FWHM ~ 100 ev average beam energy in sample [kev] 20

22 An excitation curve Concentration profile C(x) Yield Corresponding excitation curve N(E) N( E) = Beam Energy [kev] G( E)* Γ( E)* T( E)* S C( x) S C( x) = n= n= 0 k n f * n ( u) G(E) Γ(E) T(E) S<C(x)> beam + Doppler energy spread rersonance lineshape beam energy straggling straggling of C(x) 21

23 Excitation curve G(E) beam - Gaussian, + Doppler energy spread due to the thermal vibration of the target atoms 2 σ ( E ) D = 2MA E kt M a Γ(E) rersonance lineshape - Lorantzian 2 Γ σ R ( E ) = K 2 2 Γ ( E ER )

24 T(E) beam energy straggling E Energy loss u f(u) The charged particles lose their energy in independent collisions with electrons. 0.2 µg/cm µg/cm kevkev-os protons protonok in CH 2 stragglingje CH 2 -ben f(u;x) tends towards a Gaussian for large x σ ( f ( u )) = S 2Z A x 0.4 µg/cm µg/cm µg/cm 2 1 µg/cm Energia Energyveszteség loss [kev] [kev] 23

25 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 x mx events on average f(u)*f(u) g(u;x) 0 u g P n ( u ; x ) ( mx ) n = = Pn ( mx ) f n = 0 = e mx ( mx ) n! n * n ( u ) 24

26 Experimental excitation curves Si 18 O 2 /Si sample, thermally grown, 20 µc /point Beam energy spred + Doppler broadening: 100 ev Resonance width: 100 ev Ta 2 18 O 5 /Ta sample, anodically oxidised, 20 µc /point 25

27 Tilting the sample increases the virtual thickness of the layer ϕ 1 x ' = x cosϕ 26

28 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 Beütés Yield SiO 2 x Proton Proton Energy Energia [kev] [kev] 10 A 20 A 30 A 40 A Depth resolution vs Depth ψ: tilt angle line: straggling circles: MS crosses: overall 27

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

30 Typical experimental results 29

31 Isotopic tracing study of the microscopic mechanisms of oxygen transport in the oxide growing during dry oxidation of silicon. SiO 2 Silicon 16 O 2 then 18 O 2 18 O depth profile Experimental excitation curve exchange growth Energy [kev] Interpretation of the spectra in terms of 18 O depth profile, demonstrating surface exchange and that the growth takes place at the SiO 2 /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. 30

32 18 O depth profile Coups (u.a) Yield Energie (kev) Energy [kev] Sequential oxidations in 100 mb 16 O 2 (40 h) at 1100 C, yielding 1600 Å Si 16 O 2 then in 18 O 2 (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 31

33 Isotopic tracing by sequential oxydation of SiC 16 O 2 (40 h) then 18 O 2 (5 h, 10 h and 24 h) Coups (u.a) Yield 6H-SiC Si terminated surface Coups Yield (u.a) 6H-SiC C terminated surface [ 18 O](%) 1,0 0,8 0,6 0,4 0, Energy Energie (kev) [kev] Energy Energie (kev) [kev] 0, Thickness Epaisseur (Å) [Å] Sequential 16 O 2 / 18 O 2 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. 32

34 Hydrogen profiling with a nuclear resonance 1 H( 15 N,αγ) 12 C Hydrogen implantation profile in silicon (10 16 cm -2, 40 kev) from W.A. Lanford, NIMB66(1992),68 33

35 Study of thin hafnium oxides deposited by atomic layer deposition J.-J. Ganem, NIM B (2004) 856 Excitation curves measured using the 151 kev 18 O(p;αχ) 15 N resonance on 3.5 nm (a) and 7.5 nm (b) HfO 2 samples oxidized in 18 O 2 atmosphere at 425 C just after: deposition (black circles), post-deposition N 2 anneal at 425 C (open circles) and post-deposition N 2 anneal at 800 C (open squares). After deposition the films present chlorine contamination and a lack of oxygen. They are unstable toward thermal oxidation since a high oxygen transport and exchange mechanisms occur during the process. Oxygen diffusion can be significantly reduced after a thermal anneal in N 2 atmosphere. 34

36 Ultrathin silicon oxynitride film formation Experimental excitation curves of the 18 O(p,α) 15 N reaction for samples with (a) deferent 15 N areal densities, sequentially oxidized in 16 O 2 (60 min) and in 18 O 2 (90 min). The arrows indicate the energy position of the surface (dashed) and of the SiO 2 /Si interface (solid) in each sample; (b) no N prior to oxidation, oxidized under the same conditions as samples in (a). (i) N amounts as low as 1/30 of a monolayer at the surface of Si wafers hamper the oxidation of Si, and the higher the N concentration, the thinner the oxynitride films; (ii) (ii) during the film growth, N and O are responsible for the atomic transport, while Si remains immobile; (iii) N, which is initially present at the surface of the Si wafer, migrates during oxidation, remaining at the near-surface and at the near-interface regions of the film. 35

37 Silicon isotopic tracing with the 29 Si(p,χ) ) narrow resonance near 415 kev 29 Si(p, χ) 30 P excitation curves from an enriched silicon single crystal before and after thermal oxidation, showing loss of silicon during the oxidation process. I.C. Vickridge et al, NIM B 161±163 (2000)

38 Annealing of ZrAl x O y Ultrathin Films on Si in a Vacuum or in O 2 E. B. O. da Rosa et al., Journal of The Electrochemical Society, 148 G695-G703 (2001) ZrAl x O y films were deposited at a rate of 0.3 nm/min by reactive sputtering using a Zr 80 -Al 20 atomic composition target in an oxygen-containing plasma directly on Si(001) substrates. Postdeposition annealings were performed ex situ at 600 C for 10 min, either in high vacuum (p 10-5 Pa) or in Pa of dry 98.5% 18 O 2. Areal densities of Al and Si were estimated from the areas of the excitation curves of the 27 Al(p,γ) 28 Si and 29 Si(p,γ) 30 P nuclear reactions around the resonance energies at and 414 kev. The as-deposited film has an approximate composition Zr 4 AlO 9. Normalized excitation curves of the 18 O(p,α) 15 N nuclear reaction around the resonance at 151 kev before and after thermal annealings and the used experimental geometry.(b) Normalized 18 O concentration vs. normalized depth for the as deposited and 18 O 2 -annealed samples. Solid lines represent the as-deposited sample, empty circles and triangles correspond to vacuum and 18 O- annealed samples, respectively. 37

39 (a) Excitation curves of the 27 Al(p,γ) 28 Si nuclear reaction around the resonance at kev before and after thermal annealings and the used experimental geometry. (b) Normalized 27 Al concentration vs. normalized depth for the as-deposited and vacuumannealed samples. 38

40 (a) Excitation curves of the 29 Si(p,γ)30P nuclear reaction around the resonance at 414 kev before and after thermal annealings. (b) Normalized 29 Si concentration vs. normalized depth for the as-deposited, 18 O 2 -and vacuumannealed samples. 39

41 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 40