Electron Rutherford Backscattering, a versatile tool for the study …people.physics.anu.edu.au/~vos107/ecasia.pdf · 2015. 9. 24. · For electrons scattering from atoms the same

Electron Rutherford Backscattering, a versatile tool for the study of thin films

Maarten Vos

Research School of Physics and EngineeringAustralian National University

Canberra Australia

Acknowledgements:Many collaborators over the last yearsIn particular: M. Went, R. Elliman P.L. Grande, G. Marmitt, R. Moreh, A. Winkelmann

Ubiquitous question in nano/surface scienceWhat is my sample composition 'near' the surface

Two widely used tools:

XPS: an electron based X-ray spectroscopy-very surface sensitive, requires UHV-often requires surface preparation - fully quantitative analysis requires the full understanding of many aspects(cross sections, line shapes, mean free path, analyser transmission) One of the main topics of ECASIA




RBS: an ion based scattering experiment- measures deeper in the sample, but can also be used in a surface sensitive mode- interpretation simple (Rutherford cross section, stopping power)- routinely done fully quantitative (better than 10% accuracy)- in general does not require UHV- larger equipment




RBS: an ion based scattering experiment- measures deeper in the sample, but can also be used in a surface sensitive mode- interpretation simple (Rutherford cross section, stopping power)- routinely done fully quantitative (better than 10% accuracy)- in general does not require UHV- larger equipment

ERBS is RBS with electrons. It is a scattering experiment (like RBS) and has depth sensitivity based on the electron inelastic mean free path (just like XPS). Here I want to demonstrate that it can be used to do quantitative surface analysis. The aim of this talk is to give you some insight in (im)possibilities of this technique

Definitions:

-Elastic scattering: sum of kinetic energies of particles involved is constant

- Inelastic scattering: kinetic energy of particles decreases due to creation of electronic excitations

Definitions:

-Elastic scattering: sum of kinetic energies of particles involved is constant

- Inelastic scattering: kinetic energy of particles decreases due to creation of electronic excitations

after collision:

Stationary atomk0

q = k1 - k0

-q

before collision:

Atom (Mass M) acquires q2/2M kinetic energy, projectile kinetic energy is reduced by this amount

k1

e-

For ion scattering the energy loss of the projectile (mass M1) scattering from an atom with mass M2 is usually expressed as the kinematic factor k:

The kinematic factor is considerable smaller than 1 (e.g. 0.5 for He scattering from Siover large angles) as M1 has of the same order of magnitude as M2.

For ion scattering the energy loss of the projectile (mass M1) scattering from an atom with mass M2 is usually expressed as the kinematic factor k:

The kinematic factor is considerable smaller than 1 (e.g. 0.5 for He scattering from Siover large angles) as M1 has of the same order of magnitude as M2.

For electrons scattering from atoms the same formula applies. However now M1 isabout 3 orders of magnitude smaller than M2 and k is very close to 1.(remember the electron to proton mass ratio is 1:1836)

k= 0.998 for scattering from H (θ=135o)k = 0.999990 for scattering from Au

Good energy resolution is essential for ERBS

Example:40 keV electrons scattering over 135° from SiO2 on which about 1 Å ofAu was evaporated.

Example:40 keV electrons scattering over 135° from SiO2 on which about 1 Á ofAu was evaporated.

-Clean Gaussian peaks! A fitters delight. The area ratio of the Au, Si and Opeaks can be determined with great precision



Separation of peaks within a few % of calculated ones (except when samplecharges)



Separation of peaks within a few % of calculated ones (except when samplecharges)

Signal from heavy elements very strong.Rutherford cross section scales as Z2

Large difference in width of the peaksWhy?

after collision:

moving Atom, momentum pKinetic energy p2/2M

k0

q = k1 - k0

-q

before collision:

Atom (Mass M): change in kinetic energy: (p - q)2/2M – p2/2M=q2/2M-p• q/M

k1

p - q

Why different width? Atoms are vibrating, even at 0K (zero point motion)Hence energy transfer is Doppler broadened.

e-

Our main electron spectrometer

slit lens

Hemisphericalanalyser 0.3 eV resolution

We measure a series of angles simultaneously

This spectrometer was not designed for ERBS. It could be simplified in a big way.

History of 'ERBS'

- First observation of shifts and broadening of the elastic peak in electron scatteringH. Boersch et al

-From 2000 onwards somewhat more activity in this field.Varga, Tokesi, in Debrecen (Surf. Interface Anal. 2001; 31: 1019)Vos, Canberra (Phys. Rev. A 2002; 65: 012703 )

-Detection of hydrogen:Yubero Sevilla Cooper, Hitchcock (McMaster Canada) gas phase Avanasev, Moscowand others

H2O

Energy Loss (eV)

Example of measurement H (and D)by ERBS for the case of ice

H signal is weak, as cross sectionscales as Z2

D2O at 3 keV resembles H2O at 1.5 keV, but D peak is sharper

O H

O H

O H

O D

D2O

-2 0 2 4 6 8

How well can one determine the sample composition?

4 samples of well-established composition,all containing oxygen, some low Z elementsand high Z elements:

Li2CO3

CaCO3

TiO2

HfO2

When possible evaporate Au on the surface, to check spectrometer performance and zero point of the energy scale

Experiment reproducesnominal sample compositionWithin 10%, better for the main peaks.

Monte Carlo simulations suggest that for most cases multiple scattering effects are small and do not affect the interpre-tation (except when cross section of elements differ hugely)

For isotropic systems:

peak width:

with Ekin the mean kinetic energy of the atom

and Erec the mean recoil energy (q2/2M)

reckin EE 34)( =σ

For isotropic systems:

peak width:

with Ekin the mean kinetic energy of the atom

and Erec the mean recoil energy (q2/2M)

reckin EE 34)( =σ

A comparison of measured and calculated meankinetic energies for some compounds.

(J. Chem. Phys. 143, 104203 (2015))

Kinetic energy marked by * corrected for multiple scattering based on Monte Carlo simulations. The width is more sensitive to multiple scattering effects than the composition.

Example: Study of surface modification by sputtering

TiO2 film sputtered with Xe ions

Now 3 peaks visible: Xe, Ti and O

Measurement in 2 geometries:

Xe Ti O

Example: Study of surface modification by sputtering

TiO2 film sputtered with Xe ions

Now 3 peaks visible: Xe, Ti and O

Measurement in 2 geometries:

Bulk sensitive surface

sensitiveSurface sensitive spectrum is not consistentwith bulk stoichiometry.Needs at least two layers to fit the spectrum-one with Xe and O deficient-one pure TiO2

Thickness of surface layer,obtained from fit, is 14 nm, somewhat larger than implantation range of 3 keV Xe

Xe Ti O

400

75°

With the same spectrometer we can take energy loss data over a larger energy loss range,Using a lower incoming energy (5 keV, rather than 40 keV) the O peak is not resolved from the Ti peak and one can study the band gap region.

Additional information from electronic excitations


After sputtering the sample is O deficient. Not all Ti atoms are in the 4+ state and their remaining d-electrons can be easily excited causing an additional hump near 2 eV.



After sputtering the sample is O deficient. Not all Ti atoms are in the 4+ state and their remaining d-electrons can be easily excited causing an additional hump near 2 eV.

The possibility to study both composition and electronic structure with the same spectrometer adds a lot of versatility.


ERBS has isotope selectivity, a rare capability in electron spectroscopy

Here the signal of TiO2 layers grown using 18O or 16O

Using special designed samples ERBS was used to study diffusion of O atoms in TiO2

This material is of technological interest as these materials are candidates for RERAM devices.

As grown

Annealing

Remove Si3N4 cap

Measure by ERBS

Samples used for diffusion study

Si3N4 layer prevents O exchangeDuring annealing stage and is removed before the measurement.

ERBS spectra of annealed sandwich structure

Amount of Diffusion occurred as a function of annealing temperature

ERBS in single crystals

Crystal lattice has a big influence!

Ti O

TiO2 (rutile)The variation in intensity ratio is a consequence of presence of Kikuchi lines that affect Ti and O in different ways.

By comparing with calculations one cantest in this way the dynamical theory of diffraction.

However, there is no magic here,just looking at the crystal planes willtell you a lot.

Calculated influence of the outgoing direction on the measured intensity

Some planes very similar e.g. (001) plane (type a)

Some planes are bright for Ti but dark for O e.g. (011) plane (type b)

Some planes visible for O but not for Ti e.g. (111) planes (type c)

Some planes strong for Ti but weak for O e.g. (121) plane (type d)

[110]

[111]

[112]

[113]

[001]

[100] [110][110][010]

Ti O

[101]

(001)

(110

)

(110

)

(001)

[011]

(011)(011)

(011)(011)

(031)

(111)(111)

(100)(010

)

(101)(101)

(101)(101)

[111][111]

[110][110][110] [210](1

21)

(121

)

(121)(121)

[113][113]

[112][112]

Our analyser uses slit lenses and the detector tells us where the electron went through the slit

Thus we can measure the intensity along a line.By rotating the crystal we can choose where we measure.

Experiment (solid line)and theory (dynamical theory diffraction, Winkelmann) (dashed) show good agreement

Red: Ti atomsBlue O atoms

D type Easiest to understand using time reversal,Consider an electron coming from the analyser Impinging on the crystal

Due to diffraction standing wavesform when Bragg conditions is fulfilled(basically along the crystal planes)

Maximum intensity if atoms near maxima of standing waves.

[110]

[111]

[112]

[113]

[001]

[100] [110][110][010]

Ti O

[101]

(001)

(110

)

(110

)

(001)

[011]

(011)(011)

(011)(011)

(031)

(111)(111)

(100)(010

)

(101)(101)

(101)(101)

[111][111]

[110][110][110] [210](1

21)

(121

)

(121)(121)

[113][113]

[112][112]

Metal interface formation

Combining the measurement of atomic and electronic structure in the same experiment

-Mo and Pt foils were used

-Al was deposited on these foils.

Al concentration at surface can be monitored around 2 eV energy loss (Al elastic peak).

At larger energy loss we see the`plasmon' spectra as in (Reflection) electron energy loss spectroscopy (REELS).

Al concentration at surface can be monitored around 2 eV energy loss (Al elastic peak).

At larger energy loss we see the`plasmon' spectra as in (Reflection) electron energy loss spectroscopy (REELS).

The change in the evolution is very different for Mo and Pt.

For deposition on Mo the Al plasmon at 15 eV is visible straight away, but for deposition on Pt no characteristic Al plasmon appears initially.

Interpretation: for Mo an Al film is formed straight away after evaporation, but for Pt the Al reacts with the Pt and forms an alloy.

For much thicker evaporated layerspure Al is formed as well on Pt.

Note that the plasmon appears now split.Are there two different plasmons?

For much thicker evaporated layerspure Al is formed on Pt.

Note that the plasmon appears now split.Are there two different plasmons?

No, remember all detected electrons have scattered elastically as well.

Thus detected electrons have plasmon + recoil loss

The electrons that scattered from Pt and created a plasmon have a smaller total energy loss than those scattered from Al.

Quick note on the analysing REELS measurements

Normally REELS measurements are analysed by either first extracting an effective loss function from a single measurement (Tougaard-Chorkendorff procedure) or alternatively by extracting a bulk and surface loss function from two spectra (Werner)

As an alternative approach we can also fit directly a set of REELS spectra based on a model dielectric function and the corresponding bulk loss function (DIIMFP) and surface loss function (DSEP)

Procedure uses partial intensities as input (obtained from Monte Carlo simulations), but is surprisingly insensitive to the precise values used.

Here demonstrated for copper

Calculate partial intensities from Monte Carlo

Calculate normalised DIIMFP and DSEP at each energy assuming a model dielectric function (set of extended Drude oscillators)

Spectra constructed from sum ofn-fold convolution of the DIIMFP weighted by partial intensity

Surface excitation parameter is left free, to be justified afterwards but appears to be reasonable

The same dielectric function used for all measurement, and optimised by simultaneous fitting of all 6 spectra.

Copper

}

}REELS}

Optical

Copper

The dielectric function obtained in this wayis very consistent with the optical datadown to 2.5 eV

Summary of ERBS

-The goodSimple peak shapes, well understood cross sectionsGood for quantitative analysis (but be careful with single-crystals)

Multiple scattering can often be ignored, at least in first approximationSimple interpretation

Larger probing depth compared to XPS, surface preparation often not required

Information on electronic structure as well as elemental composition

Sensitive to crystal structure

New physics to be explored

Laboratory based technique, requires less expensive equipment than RBS, When made commercially, should be comparable in cost to XPS

Summary of ERBS

-The goodSimple peak shapes, well understood cross sections Good for quantitative analysis (but be careful with single-crystals)







-The badIf the sample consists of heavy elements only, it will be hard to get useful information

Summary of ERBS

-The goodSimple peak shapes, well understood cross sections Good for quantitative analysis (but be careful with single-crystals)







-The badIf the sample consists of heavy elements only, it will be hard to get useful information

-Not explored so farIntegration in electron microscopySmall spot size implies large large dose/cm2, not suitable for organic materials

If you have a sample for which ERBS could give help addressing any outstanding questions, come and see me.

If you have a sample for which ERBS could give help addressing any outstanding questions, come and see me.

Thanks for your attention!

Documents

Electron Rutherford Backscattering, a versatile tool for the study …people.physics.anu.edu.au/~vos107/ecasia.pdf · 2015. 9. 24. · For electrons scattering from atoms the same