Electron Energy-loss Spectroscopy and Energy Dispersive X-Ray Analysis

Nano-Analysis

Techniques

• Inner-shell ionization – an emission of characteristic x-ray quanta or Auger electrons

• Electron energy-loss spectroscopy

– The electronic structure

– The elemental composition

– Efficient for low-Z elements

The volume irradiated by the electron

The volume from which the stimulated signal arises

Nanodimension

Low energy, 100-1000 eV

Resolution

Microanalysis in EM Technique Input

Signal Output Signal

Depth Sampled (nm)

Lateral Resolution (nm)

Elemental Range

Sample Detection Limit (at%)

Wavelength dispersive X-ray spectroscopy (WDX, WDS)

electron Photon (X-Ray)

~ 1000 nm ~ 1000 nm Z4 Flat polished surface

0.005% (50 ppm)

Energy dispersive X-ray spectroscopy in SEM (EDX, EDS)

Electron Photon (X-ray)

~ 1000 nm ~ 1000 nm Z5

Flat polished surface

0.1%

Energy dispersive X-ray spectroscopy in TEM (EDX, EDS)

Electron Photon (X-ray)

~ 10 -100 nm (Sample thickness)

~ 2 nm Z5 TEM foil 0.1%

Electron energy loss spectroscopy in TEM (EDX, EDS)

Electron electron ~ 10 -100 nm (Sample thickness)

~ 1 nm Z3 TEM foil 0.1%

Auger electron spectroscopy (AES)

Electron electron 0.5-2 nm 400 nm Z2

Surface 0.1%

Accuracy: 5%

Inelastic scattering of electrons

Through smaller angles than for elastic scattering The cross section varies linearly with atomic number

< 1 eV; ~ 10o, ~ 1um, heat 50eV – keV: EDX, EELS

5-30eV, ~ 100 nm, dominant

Interaction volume and information volume

Depends on materials and beam energy, Kanaya-Okayama range (empirical)

20 kV, Cu

Generation - escape

Limited range

889.0

67.1

00276.0

Z

AER

X-Ray Generation

• Bremsstarhlung: electrons being decelerated by the nucleus and electrons in the atoms in the materials - not useful, background

E

EEiZI

0

Beam current

• Characteristic X-rays: The energies/wavelengths of the emitted X-rays

Maximum energy is the beam energy

Low energy X-rays absorbed by the

specimen

Characteristic X-rays

Remove an electron from an inner shell: vacancy

An outer shell electron falling into the vacancy (~ 10-12 s)

The allowed transitions: n 1; l=1; j= 1 or 0

Angular momentum

K

Transition to

Transition is from..

• Sharp and element-dependent: Useful for analysis

2CZ

B

eB

CZhcE

2

• X-Ray wavelength:

• X-Ray Energy:

B, C = constants depending on the X-ray line

Moseley’s Relation:

EDX and WDX

WDX: •Use crystal monochromators to disperse the emitted X-ray spectrum in terms of Bragg diffraction angle and hence wavelength • Slow to acquire (30 min) • Increased spectral resolution: 10 eV • Sensitivity to all elements (especially light elements) • Confined to dedicated analytical SEMS (Electron Probe Microanalysers, EPMAs) • Detection limit: ~ 0.005%

EDX: • Detector: pn junction – e-h generation ~ the energy of the X-ray photon • Fast electronics: separate the pulses •Detectors collect X-rays in a near-parallel fashion (1 min) • Spectral resolution: 100-150 eV • Common attachment

EDX: poor energy resolution • Peak overlap • no chemical-state information can be extractive (< 10 eV)

Florescent yield

X-ray emission is very inefficient for low Z (and absorption), EEELs is more efficient technique for

analysing low Z elements

EDX

Detector: • reversed p-i-n (p-type, intrinsic, n-type) • e-h generation

Charge is not completely detected Collect e-h pair

Cooled by liquid nitrogen to • Reduce thermal e-h pair • Prevent the Li atoms from diffusing, • Reduce noise in the FET preamplifier

3.8 eV is required to form an e-h (not all energy creats e-h holes) A Cu Ka, 8040/3.8=2300 e-h holes ~ 10-16C – small signal – preamplifier (FET)

•Be(7-12um) > Na •Ultrathin (<100nm polymer) > C •Windowless (in vacuum)> Boron

Interpretation of X-ray Spectra Experiment • Beam energy > 2 X highest peak energy • Reproducible spectrum from the same area • Counts: enough to recognize peaks; not too high (<3000 counts/sec; dead time< 50%) to minimize sum and escape peaks

Analysis • Prior knowledge of the sample: likely elements • Be aware of stray irradiation peaks: Fe Cr Ni Cu Zn Al Pt Mo • Confirm elements by looking for other peaks from that elements (KLM) • Work from high energy to low energy identifying peaks (at high energy, few peaks and better resolution of neighbouring peaks) • Peak shapes and energies (0-20 kV)

•K series – B (Z=4) to Ru (Z=44) • Z > 16(S) look for K;

• L series – Cl(Z-17) up • Z> 42(Mo) look for L

• M series - ~ Ag(Z=47) up • Peak overlaps:

• S K 2.31keV Mo L 2.29 keV Pb M 2.35 keV • N K 0.39 keV Ti L 0.45 keV

A Si K X-ray escape from the Si detector

Two X-rays arrive at the same time

EELS

• Compositional analysis: efficient for light elements

• Chemical analysis: shapes of energy loss edges depend on local bonding and oxidation state

EELS Spectrometer

Inelastic scattered electrons: Small

scattered angle < 1o

Typical energy loss: < 1 kV, a spectrometer can separate this.

Bends the electrons through 90o

with a magnetic prism.

Curved faces of the magnet: focus the electrons

Additional quadrupole and sextupole lenses: fine tune the focus

Parallel detection: a diode array with 1024 or more diodes to collect the

whole spectrum in parallel

Typical Energy Loss spectra YBa2Cu3O7 Log Scale

Zero Loss: Unscattered, elasticlly, phonon-scattered electrons

Plasmon Losses: low-loss (5-50 eV)

Core Losses

100 kV 99 kV Electron Energy

• Width of Zero peak: energy spread of the microscope: 0.3 eV (FEG) • Phonon: thermal diffuse scattering: < 1eV; ~ 50 mrad

• Plasmon: •oscillation of conduction band electrons •Characteristic plasmon energy • A few mrad • Thick sample: multiples of the plasmon energy

p

p

tII

exp0

Single electron excitation: background

~ 30 eV up - SEs

•Excitation of a core level electrons • Edge: excited to the lowest empty state or higher energies • Each shell has its own edge •It can be obtained from a much smaller volume than EDX

Quantifying EELS

• Thin specimen – to avoid multiple scattering which obscures the edges – the Plasmon peak < 1/10 the zero loss

• Edge: the intensity is spread over large range of loss rather than in a sharp peak… the edge area is difficult to measure… fit the background above the edge – Fit the background using I = AE-r above the edge and extrapolate this

background under the edge – Subtract the background from the edge – Calculate the edge area within a window: C – Calculate the ionisation cross-section and integrate over same window

(collection angle-the objective aperture, microscope voltage) : not accurate ~ 10%

– Repeat for other edges and the compositions

A

B

A

A

B

A

C

C

X

X

EDX and EELS

Techniques Signal to noise Low Z Detection limit Spatial resolution

Artefacts Compositional accuracy

EDX high Boron (5) 0.1 at% 2 nm Escape, stray, sum

Better < 1%

EELS low Better- Li and He (2)

0.05 at% (for some elements)

Better (1 nm) No stray etc. Poor 10%

ELNES and EXELFS

• ELNES: Energy loss near edge structure – The probability of the electron

ending up at a particular energy level in the conduction band depends on the conduction band density of states (DOS)

– Due to the selection rule, the near edge structure doesn’t show the full DOS

– DOS depends on the bonding state of the atom

• EXELFS: Extended energy loss fine structure – 50-200 eV beyond the edge – The structure depends on local

arrangement of atoms around the excited atom

– To get nearest neighbour distances and radial distribution functions

– Low noise spectra are needed

Al K, from Ni3Al, background subtracted

EFTEM • Energy-filtered Microscopy

– Collect an image from a given energy loss – Two ways:

• STEM: focused probe; collect an energy loss spectrum through a slit at the desired energy loss; scan to create an image

– collecting spectrum at each image point – Useful for the detection of low concentrations of

an element

• TEM: Use extra lenses after the energy selecting slit to allow the original image to be reformed (GIF, Gatan imaging filter)

– collecting an image at each energy loss

– Applications • Zero loss filtering

– Allow thick regions to be examined

• Thickness determination: t/ map – Thickness map: a zero loss image (I0) and an

unfiltered image (Iunf) or a loss image (Iloss), : total inelastic mean free path …. Non-uniform composition?!

• Core-loss mapping: Elemental map – Jump-ratio image: dividing the iamge beyond the

edge by an image below the edge – thickness variations are removed (at least approximately)

– 3 Window method (can be quantified): 2 pre-edge and one post-edge, the two pre-edge images are used to estimate the background under the post-edge image and the background can be subtracted … the thickness variations!

– Image spectroscopy: a series of images is collected both before and after the edge of interest – to get a better background (longer time larger dose)

Si 110 CBED at 100 kV

00

1lnlnI

I

I

It lossunf

Normal EFTEM: slice // xy plane,

low energy resolution

STEM Spectrum Imaging: sample drift

and distortion

Appendix

Inelastic Scattering

• Electron-Specimen Interactions with Energy Loss

• Differential Cross Section for Single-Electron Excitation

• Bethe Surface and Compton Scattering

• Approximation for the Total Inelastic Cross Section

Electron-Specimen Interactions with Energy Loss • Elastic preserves: Kinetic energy and momentum • Inelastic conserves total energy and momentum

– Energy conversion: atom-electron excitation – Energy loss: the primary beam

Excitation Mechnisms: • Oscillation in molecules and phonon excitations in solids:

• E ~ 20meV-1 eV • Monochromator • Low beam intensity – low spatial resolution • Infrared

• Intra- and interband excitation of the outer atomic electrons/ plasmons • Broad maxima: E ~ 3-25 eV • Concentration of C/V band electrons; chemical bonds, band structure • Visible and ultraviolet

• Ionization of core electrons • Edge E • Spectrum ~ eV beyond E

• low loss – less localized ( excite atom from ~ 10 nm – small angle) • For low atomic number: total inelastic cross section > total elastic cross section • Thick specimens: chromatic aberration due to energy loss • Energy - heat

Differential Cross Section for Single-electron Excitation

• Energy Transferred – E – Ei→Ef – Selection rules: l=1 – Scattering vector:

if kkq '

• Conservation of momentum, energy cos'2' 0

22

0

2 qkqkkn

cos'

cos'22

'cos'222

0

2

0

22

0

222

0

2

m

qkqk

mqqk

mkk

mE n

222

0

22

0

2 cos'' Ekqkq

2

0

2

2

0

2

2

0

2

2

0

2

0

2

2

2

cos'cos'

E

E

p

Em

k

Em

k

q

k

qE

Inelastic cross section: 2

4

2

if

i

ffirV

k

km

d

d

iijisi rkira exp)( ifjfsf rkira

exp)(

2

33*2

exp)(,)(exp jiiijisjijfsifif rdrdrkirarrVrarkirV

22

22

23*

23*2

'')(...'1)()('exp)()'( ifisjfsjjisjjfsjjisjjfs xqaruaqrdrarqirardrarqiraq

4

2

2

0

2

'

)'(

2 q

qme

d

d fi

2

22

2

0

2

4

222

2

0

2

4

22

2

0

2

'2'

'

2'

)'(

2 q

xme

q

xqme

q

qme

d

d ififfi

22

22

22

2

2

2

22

22

0

222

0

22

2

0

2

2

22

2

0

2 1

2

22

42

'2 E

if

HE

if

HE

if

HE

ififfix

a

x

a

x

kak

xme

q

xme

d

d

Differential Cross Section for Single-electron Excitation

• Generalized Oscillator strength (GOS)

2

22

2

2

2

'

)'(2' ifif x

Em

q

qEmqf

222

0

4

2222

4

2222

22

22

2

2

22

22'

)4(

'

2

4'

22

'

E

if

E

if

HE

if

HE

if

HE

if

H

fi qf

EE

eqf

Emap

qf

EmaEm

qf

a

x

ad

d

• Generalized Oscillator strength (GOS) per unit energy loss

Ed

Eqdf

EE

e

Edd

d if

E

fi

,'1

)4( 222

0

4

• Bethe Surface

Ed

Eqdfif

,'

• Bethe ridge – Compton scattering