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  • 2015-4

    Joint ICTP/IAEA Workshop on Advanced Simulation and Modellingfor Ion Beam Analysis

    F. Schiettekatte

    23 - 27 February 2009

    Universite de MontrealCanada

    IBA intro II

    RBS, EBS, ERD & NRA

  • Universitede Montreal

    Measure the atomic concentration and distribution in a targetDepth information comes from electronic energy loss (dE/dx)Types of interactions:- Elastic collisions

    Energy/particles conserved in collision Rutherford / non-Rutherford cross sections

    - Nuclear Reactions Energy not conserved, possible creation of new particle

    - Ray emission: X, y Element identification

    backscattering Nuclear reactions

    elastic recoils

    ICTP/IAEA Workshop on IBA simulation, Trieste, February 2009

  • Outline

    Kinematics and stopping powerRutherford Backscattering Spectrometry (RBS)Elastic Backscattering Spectrometry (EBS)Elastic Recoil Detection (ERD)Nuclear Reaction Analysis (NRA)

    ICTP/IAEA Workshop on IBA simulation, Trieste, February 2009

    In the ideal case:

    - Strait-line, schematic trajectories

    point source, point detector

    uniform energy loss:energy-to-depth correspondence

    - Classical kinematics

    Usually applies, even for NRA

    - Probability of collision

    Rutherford cross section- Assumed in RBS, ERD

    - If minimal approach distancenot too small or too large

    - Known at all angles and energies

    > quantitative n = qNt\ AQ[da

    ICTP/IAEA Workshop on IBA simulation, Trieste, February 2009

  • Depth profiling

    Beam (, Scattered ion / recoil / reaction product (E2)

    ICTP/IAEA Workshop on IBA simulation, Trieste, February 2009

    -

    2 MeV He ->50 nm Au /200 nm SiO2 / Si

    Ion backscattered

    - Same ion in & out- most of the time 1 -2 MeV He

    Advantages

    - easy to set up Simple detectors & electronics

    can achieve

  • \lnP InGaAs -RBS examples:

    - InGaAs/lnP/QW/lnP InGaAs: 150 nm analysed using 3 MeV He+

    good sensitivity to heavy atoms good separation of In vs Ga,As Ga, As indistinguishable light atoms (P) barely visible composition must be extracted

    by comparison to a simulationHeavier ions help to separate masses

    - here: 5 MeV O3+ on GaAsN Ga, As separable despite

    horrible energy resolution- or use PIXE

    ICTP/IAEA Workshop on IBA simulation, Trieste, February 2009

    Still elastic collisions

    - energy conserved- cross-section (probability of detection)

    affected by nuclear interactions 104

    Advantages

    - higher cross-sections here, normalized to Rutherford 10

    resonances g more sensitivity, especially to light ~B 0

    elements 10

    - Solves an inconvenient of RBS Examples: _2

    - x25 for a-> O 3.04 MeV 1 0

    Inconvenience - x125 for a-> C 4.26 MeV

    - unobvious energy & angular dependence have to be measured reliably

    2 4 6Energy (MsV)

    - but many useful cross-sections available theoretically- e.g. SigmaCalc

    - good only for thin layers: resonance width usually small

    - still piled-up spectrum extraction of depth profile by comparison to a (sophisticated) simulation

    ICTP/IAEA Workshop on IBA simulation, Trieste, February 2009

  • Channel

    RBS: InZnO/glass

    - He 4.5 MeV @ 170 (

    i ?i Zn

    examples:- Ge1.xCx/Ge with x=1 %

    (plus surface contamination) problem:

    - ZcZGe.:aGe(Ec)~50ac(Ec)- [C]~1%:YGe~500Yc

    solution:- 4.26 MeV a resonance- a12C~125aR

    - similar solution for 16O a0 ~ 25 aR with a 3.04 MeV

    - not so good for 14N 9.2 MeV resonance with a 3.2 MeV protons

    - Many people around in yourspectrum

    - Can't stay next to your target!

    Still: we haven't detected H, He,ICTP/IAEA Workshop on IBA simulation, Trieste, February 2009

    Elastic Recoil Detection> Detection of collision partner+ Energy conserved

    Simple expression for K+ Rutherford cross-section

    Simple expression for a (small exception: MeV H e ^

    - Need to filter out beam oridentify recoiled atoms

    - Limitations in experimentalgeometry

    grazing incidence (roughness) limited depth of probe can't easily do channeling

    H) _

    Nuclear reactions> Detection of reaction product

    + No particular geometry

    + If narrow resonance: best depth

    resolution achievable (e.g. 15N -> 1H)

    Non-Rutherford cross-sections

    Usually relatively small

    Known only at certain angles,otherwise must be measured

    - Theoretical models for some of them Unobvious shape & amplitude:

    uncertainty

    a IICTP/IAEA Workshop on IBA simulation, Trieste, February 2009

  • Elastic Recoil Detection

    incident ions4M,M2

    Elastic: energy conserved (ziZ2 /E)Direct detection of the atoms from which the target consistsBut mass separation/identification required at detectionTwo possibilities- Filter: only let the ion of interest reach the detector (e.g. H)

    Absorber: dE/dx of heavier ions much higher than for light ones- Requires thick foil: energy straggling badly affects depth resolution

    Electrostatic filter Kinematics: for beam scattering, 9 < arcsin M,/M2

    - Identification: measure M or Z of each detected ion Time of Flight (TOF) E-AE: Energy loss in different zones of a gas or solid-state detector

    depending on dE/dx > Z

    11 ICTP/IAEA Workshop on IBA simulation, Trieste, February 2009

    + Cheap ExB filter

    also only E or B

    surface barrier detector (SBD)

    + Small accelerators here 400 kV, near dE/dx max

    + Pretty good energy resolutionfor H^SBD

    2-3 nm depth resolution for H

    - Small detector solid angle- Charge fraction- Depth of probe- Scattering on electrodes

    ICTP/IAEA Workshop on IBA simulation, Trieste, February 2009

  • c

    .will be discussed further tomorrow

    ICTP/IAEA Workshop on IBA simulation, Trieste, February 2009

    " p, d, 3He, a, y

    notation: 15N(p,a)12C

    Four types of nuclear reactionsp,d, He, a, IN... _ R e s o n a n c e s j n cross-section (EBS)

    - Broad reactions producing a "new" particle Exothermic:

    - energy increase- one of the reaction products often has a

    higher energy than the backscattered ions:easily distinguishable

    - High energy = small dE/dx :bad depth resolution

    Endothermic:- less energy than scattering, lost in

    background- but "new" particle produced, distinguishable

    - Reactions producing a "new" particle andfeaturing a sharp resonance

    - Nuclear excitations or reactions producinga photon (e.g. p,y)

    14 ICTP/IAEA Workshop on IBA simulation, Trieste, February 2009

  • Useful exothermic NRA reactions

    IS.3K 0-MISSSI MOJ3.'S: O.1C4.OJ

    2.L514S6

    1B9MI9JBP5

    17 SIS1-5S?

    I.[?1,11

    Table from Guy Demortier,J. Electr. Spectr. 129 (2003) 243

    15 ICTP/IAEA Workshop on IBA simulation, Trieste, February 2009

    M. B. Huang, L. J. Huang, I. V.Mitchell, W. N. Lennard, W. M. Lau,J. P. Noel, Nucl. Instr. and Meth.B100 (1995) 149

    - 660 keV p -> 5-doped Si

    - 8.3 urn Al foil to stop H broad peak bad energy resolution

    - but high cross-section: precise total amount

    - excellent depth resolutionachieved by using beveledsamples or successiveetching

    here, 0.7 nm

    Fig. 2. Chjrged particle spflctnjm observed for The "ifCp.unuclear reaction.

    i: I

    i16

    Fig. J . E rn>n depth profile for H fi-dopcc) Tuyer in Si: # th i* w

    ICTP/IAEA Workshop on IBA simulation, Trieste, February 2009

  • Endothermic reaction: 14N(a,p)17o

    absorberexample:

    - GaAs.,.xNx/GaAs, x=0.01 - 0.03

    - problem: light atom, low concentration in

    heavy substrate 14N: no non-Rutherford cross-

    section available Can't use 2H beam in the lab

    - solution: endothermic 14N(a,p)17O reaction

    - 3.7 MeV He- -1 MeV p

    advantage over ERD:- no geometric constraint

    channeling- better depth of probe

    foil to stop a (i resolution)

    17 ICTP/IAEA Workshop on IBA simulation, Trieste, February 2009

    1a in'

    r1i

    F =F

    Extremely narrow>104 contrastAs the beam energy increase, the ion entersdeeper into the material before reactingDepth resolution:- 5-7 nm at normal incidence- 2-3 nm at gazing angle !

    - Doppler broadening

    W. A. Lanford, H. P. Trautvetter,J. F. Ziegler, and J. KellerAppl. Phys. Lett. 28, 566 (1976)

    18 ICTP/IAEA Workshop on IBA simulation, Trieste, February 2009

  • Conclusion

    Ion beam analysis give- quantitative depth profiles if you carefully

    know/measure all the parameters

    - depth resolutions -10 nm, better if you work a bit

    RBS: ~ppm sensitivity for heavy atoms- Spectrum components usually not separated:

    need to compare to a simulation

    ERD/EBS/NRA for lighter atoms- Not as simple as RBS

    Next sessions will introduce all the complications when welook behind the schematic principles

    19 ICTP/IAEA Workshop on IBA simulation, Trieste, February 2009

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