TEM Techniques

IMAGING DIFFRACTION SPECTROSCOPY

Amplitude

contrast (diffracion

contrast)

Phase

contrast (high-

resolution

imaging)

Selected

area

diffraction

Energy

dispersive

X-ray

spectroscopy

Electron

energy loss

spectroscopy

Electron

holography

Z-contrast

imaging

Convergent

beam

diffraction

Micro-/

nano-

diffraction

Energy-filtered

TEM (EFTEM)

X-ray

mapping

TEM/STEM

Tomography

Phase contrast imaging

(high resolution TEM: HRTEM)

Phase shift due to the inner potential of

specimen

Phase shift:

zyxVdzdz

d ,,'

22

Ewith (interaction constant)

Total phase shift:

yxVdzzyxVd t ,,,

d

Electron beam

z

! phase change depends on potential V

which electrons see, as they pass

through sample

point

resolution

information

limit

u, [nm-1]

χ(u)uE sinuE

HRTEM: contrast transfer function

! opposite sign of T(u) - oposite contribution to

contrast

u < point resolution:

images are directly

interpretable

u > point resolution: no

direct interpretation is

possible

T(u)

No simple correspondence between the image

intensity and the atom column positions!

Additional calculations are necessary!

f - defocus

- wave length

Cs - spherical aberration

u - spatial frequency

432

2

1uCufu s

Example: HRTEM simulation for GaAs

projected

potential

by courtesy of Prof. Kerstin Volz

same thickness,

only defocus

change

HRTEM of an isolated ZnTe nanowire

- visualization of crystal structure

- analysis of defects

{110}

{211}

{111}

HRTEM of an isolated ZnTe nanowire

Twin formation

{110}

{211}

{111}

HRTEM of an isolated ZnTe nanowire

TEM Techniques

IMAGING DIFFRACTION SPECTROSCOPY

Amplitude

contrast (diffracion

contrast)

Phase

contrast (high-

resolution

imaging)

Selected

area

diffraction

Energy

dispersive

X-ray

spectroscopy

Electron

energy loss

spectroscopy

Electron

holography

Z-contrast

imaging

Convergent

beam

diffraction

Micro-/

nano-

diffraction

Energy-filtered

TEM (EFTEM)

X-ray

mapping

TEM/STEM

Tomography

High Angle Annular Dark Field

Scanning Transmission Electron

Microscopy

(HAADF STEM) –

Z contrast imaging

Parallel incidence of converegent electron probe

Williams & Carter

The electron beam

must scan parallel

to the optic axis at

all times !

Magnifications is

controlled by scan dimensions

on the specimen, not the imaging

lenses of the TEM!

thin crystalline specimen

Electron Energy Loss Spectrometer

primary electrons

diffracted beam

X-rays

direct beam

Energy-Dispersive X-ray Spectrometer

elastically and inelastically scattered electrons

High-Angle Annular Dark-Field Detector

HAADF STEM – High Angle Annular Dark Field

Scanning Transmission Electron Microscopy

Z contrast technique

HAADF detector

I ~ Z3/2

Electron probe

Sample

Z contrast

image

position y

intensity

y

x

r

Z

HAADF detector

I ~ Z3/2

Electron probe

Sample

Z contrast

image

position y

intensity

y

x

r

Z

ZnTe

Au

Z contrast technique

cS-corrected HAADF STEM of (In,Ga)As and

Ga(Sb,As) layers embedded in GaAs

Humboldt-Universität zu Berlin, Institut für Physik, AG Kristallographie

Technische Universität Berlin, Institut für Physik

Element Atomic number

Z

Ga 31

As 33

Sb 51

Material Mean Atomic

number <Z>

GaAs ½ (31 + 33) = 32

GaAs0.5Sb0.5 ½ {31+½(33+51)}

=½ {31+42} =36.5

cS-corrected HAADF STEM of (In,Ga)As and

Ga(Sb,As) layers embedded in GaAs

Humboldt-Universität zu Berlin, Institut für Physik, AG Kristallographie

Technische Universität Berlin, Institut für Physik

TEM Techniques

IMAGING DIFFRACTION SPECTROSCOPY

Amplitude

contrast (diffracion

contrast)

Phase

contrast (high-

resolution

imaging)

Selected

area

diffraction

Energy

dispersive

X-ray

spectroscopy

Electron

energy loss

spectroscopy

Electron

holography

Z-contrast

imaging

Convergent

beam

diffraction

Micro-/

nano-

diffraction

Energy-filtered

TEM (EFTEM)

X-ray

mapping

TEM/STEM

Tomography

Selected area diffraction

image plane (Gaussian

image plane)

object plane (specimen)

objective lens

intermediate lens

viewing

screen

back focal plane (Brennebene)

Beam path in image mode

d1

d2

f

amorphous polycrystalline single crystalline

Selected area electron diffraction (SAD)

using a parallel beam Crystal Structure:

Epitaxial Orientation Relations:

(100)[001]LiAlO2 || (0001)[11.0]GaN

020

1-100

selected area

aperture:

dmin = 500…100 nm

Bragg`s law

constructive interference:

n: reflection order (integer number)

: diffraction angle (Bragg angle)

d: interplanar spacing

: wave length

n d sin 2

At the Bragg angle the electron waves

interfere constructively

Bragg 1913 description

of diffraction by reflection

Camera length

22tanL

r22tan

L

r

22tanL

r

d: distance of (hkl) reflecting planes

r: distance of diffraction spots

L: camera length

2sin2d

22tanL

r

dL

r

[001]Si +

a polycrystalline unknown phase

Camera length L needs to be calibrated

using a known material!

dhkl = n L/r

400 Si

d100(Si) = a = 0.5431 nm

n = 4

Lcalibr = rSi d100(Si) / n

dphase = Lcalibr /rphase

rSi

rphase

is compared to the d-values of possible phases

TEM Techniques

IMAGING DIFFRACTION SPECTROSCOPY

Amplitude

contrast (diffracion

contrast)

Phase

contrast (high-

resolution

imaging)

Selected

area

diffraction

Energy

dispersive

X-ray

spectroscopy

Electron

energy loss

spectroscopy

Electron

holography

Z-contrast

imaging

Convergent

beam

diffraction

Micro-/

nano-

diffraction

Energy-filtered

TEM (EFTEM)

X-ray

mapping

TEM/STEM

Tomography

Energy-dispersive X-ray spectroscopy

(EDXS)

thin crystalline specimen

Electron Energy Loss Spectrometer

primary electrons

diffracted beam

X-rays

direct beam

Energy-Dispersive X-ray Spectrometer

elastically and inelastically scattered electrons

High-Angle Annular Dark-Field Detector

EDXS – Energy Dispersive X-ray Spectroscopy

energy loss electron

K

L1

L2

L3

conduction band

valence band

EF

E

EVac free electron

primary electron

Auger electron

characteristic X-rays

electron excited into an unoccupied state

Fundamental interaction processes

EDXS – Instrumentation: Silicon Drift Detectors

(SDD)

Set-up and working principle

of a state-of-the-art EDX detector

Parameters:

• Energy resolution: 129 eV (MnK)

• Semiconductor-based drift technology

• Peltier cooling (-25°C, no need of LN2) © BRUKER AXS

Full width at

half maximum

Scattering volume in thin specimens

1 nm

12 nm

200 nm

50 nm

TEM specimen

lateral

resolution

Monte-Carlo Simulation of the

paths of electrons through bulk

silicon as used for scanning

electron microscopy (SEM);

acceleration voltage: 100 kV

material: Si

50 µm

electron beam Bulk material

EDXS – Quantitative analysis I

EDXS spectrum of GaAs

Preparation of spectrum for analysis:

• Removal of Escape peak

which is due to detector material

• Modelling and subtraction of background

• Deconvolution of peaks basing on

Gauss distribution functions

• Quantification of chemical composition

t → ∞

bulk specimen (SEM) -

infinite specimen thickness

ZAF-Method: takes into

account Absorption (A),

Fluorescence (F),

atomic number (Z)

B

A

B

A

B

A

I

I

FAZ

FAZ

C

C

)(

)(

thin specimen (TEM)

t < tmax

thin foil approximation

BI

AI

Bk

Ak

BC

AC

Cliff-Lorimer factor (CLF)

CLF have to be calibrated

for each element

(especially light one!)

at the same specimen

thickness

tmax < t << ∞

- Specimen thickness t

- Geometry of object

- High primary electron

beam energy

Modified ZAF-Method

tmax = f (mass

absorption coefficient,

detector angle, mean

sample density)

EDXS – Quantitative analysis II

BA

ABAB

AQwa

AQwak

)(

)(

EDXS – Experimental modes

1. Point analysis

spectrum

2. Line scan

composition profile

3. Elemental map

2d elemental distr.

A

B C

1 3

2

energy

inte

nsity

A

C B

position p

ositio

n

position

co

mp

ositio

n

A B

Example A: III-V-based overgrown structures

STEM HAADF image : Z-contrast Structure I

AlGaAs

InGaP

GaAs

InGaP

AlGaAs

line scan

20 nm

AlGaAs

GaAs

3 nm

Question: segregation of P?

Inte

nsity (

a.u

.)

Example 3: III-V-based overgrown structures

(Al,Ga)As

(In,Ga)P

GaAs (Al,Ga)As

position (nm) 20 40 60 10 30 50 70 80 90

dark region

P enrichment

In enrichment

Structure I STEM probe size: 0.7 nm, spot distance: 0.5 nm

As depletion

HAADF

Elemental

map:

probe size

0.7nm

In

P

As

Al Ga

III: Ga-In-Al

V: P-As

Example 3: III-V-based overgrown structures

Structure I

GaAs

InGaP

AlGaAs

InGaP HAADF

STEM HAADF image : Z-contrast

Question: segregation of In?

Structure II

GaAsP

InGaP

AlGaAs

InGaP

3 nm

~4 nm

InGaP

AlGaAs

GaAsP

InGaP

100 nm

Example B: III-V-based overgrown structures

Inte

nsity (

a.u

.)

InGaP AlGaAs

bright region

position (nm) 20 40 60 10 30 50 70

P depletion

In depletion

As enrichment

Structure II

Ga(In)As(P)

STEM probe size: 0.7 nm, spot distance: 0.5 nm

HAADF

2: III-V-based overgrown structures

EDXS- Energy-Dispersive X-ray Spectroscopy

Difficulties in TEM:

• small exitation volume

• small detector collection angle

• specimen drift at high magnifications

• calibration is necessary for quantitative analysis

low peak intensity

reduced accusition time

Advantages:

• all elements are visible at once

• fast simple qualitative analysis

• elements down to Be

• probe sizes used:

down to 0.7- 0.2 nm

EDXS mapping on the subnanometer scale

M.-W. Chu et al.,

National Taiwan University

Phys. Rev. Lett. 104 (2010) 196101 JEOL-2100FS with a probe Cs-corrector

3 ms per pixel, totally 13 s

probe size of ~ 0.1 nm

probe current of ~ 33 pA

1.47 Å

HAADF

(Z-contrast): EDXS map:

Electron energy loss spectroscopy

(EELS)

+

Eenergy Filtered Transmission Electron

Microscopy

(EFTEM)

energy-loss electron

K

L1

L2

L3

conduction band

valence band

EF

E

EVac free electron

primary electron

Auger electron

characteristic X-rays

electron excited into an unoccupied state

Fundamental interaction processes

Magnetic

Prism

In-column Filter (e.g., LEO EM 922 Omega

and JEOL JEM 2200 FS)

Post-column Filter (GATAN Imaging Filter)

for any TEM

Energy

selecting slit

Energy

dispersive

plane with

slit

Experimental setup for EELS and EFTEM

Williams & Carter

Magnetic prism: a spectrometer and a lens

Si-L23 edge

C-K edge

inte

nsi

ty i

n c

ou

nts

x 1

03

8

2

4

6

zero-loss

peak

plasmon

excitation

Si

C

neighboring atoms

energy levels of inner shells

valence band

EF

unoccupied states

K

L3 L2 L1

283 eV

99 eV

x 100

energy loss in eV 0 100 300 200

Electron energy loss spectroscopy (EELS)

Humboldt-Universität zu Berlin, Institut für Physik, AG Kristallographie

Hahn-Meitner-Institut Berlin

Imaging of the element distribution in the interface region / phase by PEELS:

A) series of spectra, B) STEM-BF image, C) concentration profiles.

Series of single energy-filtered images (above),

procedure of background extrapolation and subtraction (below)

Cr-L23 map Post-edge image Pre-edge 2 image Pre-edge 1 image

200 nm

‘ phase

phase

Energy loss in eV Energy loss in eV

Cr-L23 edge Cr-L23 edge

Net signal

Post edge 1 2

Energy-filtered TEM - Three-window technique

Humboldt-Universität zu Berlin, Institut für Physik, AG Kristallographie

Hahn-Meitner-Institut Berlin

Ti-L23 Cr-L23 Al-L23

TEM-BF RGB image

[100]

[010]

200 nm

phase

‘ phase

Energy-filtered TEM imaging of the element distribution

in SC16 after creep ( = 0.5 %) at 950 °C

La-Mn-containing film on SrTiO3

Data courtesy: D. Muller et al. Cornell University

(La0.7Sr0.3MnO3)

From Lit.:

P. Hawkes, new book: Advances in Imaging and Electron Physics

Atom column EEL - Spectrum Imaging:

Example: EELS mapping on

subnanometer scale

Si-L23 edge

C-K edge

inte

nsi

ty i

n c

ou

nts

x 1

03

8

2

4

6

zero-loss

peak

plasmon

excitation

Si

C

neighboring atoms

energy levels of inner shells

valence band

EF

unoccupied states

K

L3 L2 L1

283 eV

99 eV

x 100

energy loss in eV 0 100 300 200

Fine structures of the ionisation edges

ELNES – Electron

Loss Near Edge

Structure

(bonding information)

EXELFS – Extended

Energy Loss Fine

Structure

(information on short-

range order)

Electron Energy Loss Spectrometry (EELS) &

Energy Loss Near-Edge fine Structure (ELNES)

Carbon: Diamond structure

Carbon: Graphite structure

ELNES fingerprints of carbon

Energy resolution of EDXS/EELS

EDXS EELS

Energy resolution 110 – 130 eV down to 0.3 eV

Comparison between EDXS and EELS

EDXS EELS

Energy scale up to 40 keV up to 3 keV

Energy resolution 110 – 130 eV down to 0.3 eV

Lateral resolution down to 1 nm down to 1 nm

Element mapping line profile,

elemental map

series of EEL spectra,

EFTEM

Detectable elements Z > 4 (Be) 2 < Z < 40

Detection limit 1 at% 1 at%

Quantitative analysis

of chemical comp. yes yes

Analysis of chemical

bonding -

by ELNES and chemical

shift of edges

Analysis of structure - EXELFS

TEM Techniques

IMAGING DIFFRACTION SPECTROSCOPY

Amplitude

contrast (diffracion

contrast)

Phase

contrast (high-

resolution

imaging)

Selected

area

diffraction

Energy

dispersive

X-ray

spectroscopy

Electron

energy loss

spectroscopy

Electron

holography

Z-contrast

imaging

Convergent

beam

diffraction

Micro-/

nano-

diffraction

Energy-filtered

TEM (EFTEM)

X-ray

mapping

TEM/STEM

Tomography

Electron tomography –

reconstruction of 3D structure

Electron tomography: from 2D to 3D imaging

Electron tomography: from 2D to 3D imaging

Please note that in TEM you would also see

the rabbit’s internal features

(organs, bones, etc.)

Electron tomography: from 2D to 3D imaging

Tomography: reconstruction of the interior of an object from its projections

Tilt angles of 90° are required to cover the whole range!

- conventional TEM specimen holder: 20 -30 tilt

- special tomography holder: 75 tilt

Figure from J. Frank, Electron Tomography. Methods for Three-Dimensional Visualization

of Structures in the Cell, Springer Verlag

x-ray tomography

in medicine

electron tomography

in science

Electron tomography

Resolution, sources of artifacts

reduction of „missing edge“

(from wedge to pyramid)

for a dual axis tilt series

Figure from: Jenna Tong et al., IMC16, Sapporo 2006

Sources of arrows:

„missing edge“ – tilting angle is limited by

shadowing of the specimen by holder edge

and limited space between the objective lens

pole pieces

signal-to noise ratio of original

projection images

original resolution of images

misalignment of the tilt axis

|| to the tilt axis:

dx is original resolution of projections

| to the tilt axis (if the images are equaly

distributed over ±90°) :

N

Ddd

zy

for a 100 nm object – 140 images to get

a 2.2 nm resolution N - number of images

D – object size

eyz – elongation factor

In practice:

yzyzedd

depends on maximum tilt angle

Resolution:

High resolution

EM

Analytical

EM

in-situ

EM

Diffraction

in EM

Electron microscopy

(EM)

in material science

•crystallography

•crystalline structure

•atomic arrangement

•defect structure

•strain analysis

•chemical composition

•bonding

•magnetic properties

•strain dependent

•temperature dependent

•current dependent

properties

Conventional

EM

•sample structure

•defect structure

pdf-Dateien der Vorlesungen unter:

http://crysta.physik.hu-

berlin.de/~kirmse/

Teaching

„Inorganic Materials"

Vorlesungen zur

Elektronenmikroskopie:

Teil 1, Teil 2