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2015-4 Joint ICTP/IAEA Workshop on Advanced Simulation and Modelling for Ion Beam Analysis F. Schiettekatte 23 - 27 February 2009 Universite de Montreal Canada IBA intro II RBS, EBS, ERD & NRA

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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 <10 nm depth resolution- -0.5-2 nm depth resolution

• with sophisticated detectors (e.g. TEA)• at grazing incidence• with beam energy near maximum dE/dx

- ppm sensitivity to heavy elements is lightmatrices

Inconvenience

- not very sensitive to light element in heaviermatrix

- bad mass resolution for elements muchheavier than the beam

- single spectrum, no elemental separationother than the kinematics

• Simulation

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

\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:

- Zc«ZGe.: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 r̂n>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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