1.2 Phase contrast imaging - hu-berlin.decrysta.physik.hu-berlin.de/~kirmse/pdf/2012_VL... ·...
Transcript of 1.2 Phase contrast imaging - hu-berlin.decrysta.physik.hu-berlin.de/~kirmse/pdf/2012_VL... ·...
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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
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Phase contrast imaging
(high resolution TEM: HRTEM)
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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
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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
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Example: HRTEM simulation for GaAs
projected
potential
by courtesy of Prof. Kerstin Volz
same thickness,
only defocus
change
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HRTEM of an isolated ZnTe nanowire
- visualization of crystal structure
- analysis of defects
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{110}
{211}
{111}
HRTEM of an isolated ZnTe nanowire
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Twin formation
{110}
{211}
{111}
HRTEM of an isolated ZnTe nanowire
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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
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High Angle Annular Dark Field
Scanning Transmission Electron
Microscopy
(HAADF STEM) –
Z contrast imaging
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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!
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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
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Z contrast technique
HAADF detector
I ~ Z3/2
Electron probe
Sample
Z contrast
image
position y
intensity
y
x
r
Z
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HAADF detector
I ~ Z3/2
Electron probe
Sample
Z contrast
image
position y
intensity
y
x
r
Z
ZnTe
Au
Z contrast technique
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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
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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
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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
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Selected area diffraction
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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
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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
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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
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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
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[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
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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
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Energy-dispersive X-ray spectroscopy
(EDXS)
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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
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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
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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
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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
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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
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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
)(
)(
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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
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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?
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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
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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
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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
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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
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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
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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:
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Electron energy loss spectroscopy
(EELS)
+
Eenergy Filtered Transmission Electron
Microscopy
(EFTEM)
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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
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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
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Williams & Carter
Magnetic prism: a spectrometer and a lens
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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)
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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.
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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
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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
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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
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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)
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Electron Energy Loss Spectrometry (EELS) &
Energy Loss Near-Edge fine Structure (ELNES)
Carbon: Diamond structure
Carbon: Graphite structure
ELNES fingerprints of carbon
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Energy resolution of EDXS/EELS
EDXS EELS
Energy resolution 110 – 130 eV down to 0.3 eV
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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
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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
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Electron tomography –
reconstruction of 3D structure
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Electron tomography: from 2D to 3D imaging
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Electron tomography: from 2D to 3D imaging
Please note that in TEM you would also see
the rabbit’s internal features
(organs, bones, etc.)
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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
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x-ray tomography
in medicine
electron tomography
in science
Electron tomography
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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:
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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
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pdf-Dateien der Vorlesungen unter:
http://crysta.physik.hu-
berlin.de/~kirmse/
Teaching
„Inorganic Materials"
Vorlesungen zur
Elektronenmikroskopie:
Teil 1, Teil 2