Chemical/Compositional Analysis
• Incident electron undergoes inelastic scattering
causing ejection of core (inner shell) electron
• Energy loss of incident electron, or energy of
ejected electron or x-ray, is characteristic of the
target element
• Can determine compositional & chemical
bonding information
incident
electron
ejected orbital
electron

Electron
Relaxation
Auger
electron
emitted
(AES)
scattered primary
electron (EELS)
x-ray
photon
emitted (EDX)
Chemical/Compositional Analysis
• Incident electrons undergo inelastic scattering
with sample (X-rays, Auger electrons, secondary
electrons)
Electron Beam
Secondary
electrons
(1-10 nm)
X-rays
(0.2 – 2 mm)
Auger
electrons (1 nm)
Backscattered
electrons
(0.1 – 1 mm)
Electron Spectroscopy
Electron, x-ray
Spectroscopy
EDX
AES
Compositional
Information
EELS
PES
Compositional &
Chemical (Bonding)
Information
EDX
• EDX: Energy dispersive x-ray
analysis
• EDAX: Energy dispersive analysis
of x-rays
• EPM or EPMA: Electron probe
micro-analysis
• EDS: energy dispersive
spectrometry
• EMP: electron microprobe
EDX
• Incident electron has sufficient
energy (critical ionization energy) to
eject a core shell electron
From Ohring, Fig. 6-14(a) & (b), p. 278
EDX
• Requires electron energy ~ keV
Critical Ionization Energy for Pt
Shell
K
Critical Ionization
Energy (eV)
78.39
LI
13.88
LII
13.27
LIII
11.56
MI
3.296
MII
3.026
MIII
2.645
MIV
2.202
MV
2.122
EDX
• Outer shell electron fills the inner
shell vacancy producing an x-ray
photon (x-ray fluorescence)
From Ohring,
Fig. 6-14(c),
p. 278
EDX
• X-ray nomenclature:
Ka : L → K
Kb : M → K
La : M →L
Energy transition
Terminating
energy level
• Letters denote
principal
quantum
numbers (K: n =
1, L: n=2, etc.)
Kb
La
Ka
adapted from Loretto, Fig. 2.3, p. 30
EDX
• Energy of emitted x-ray is
determined by difference in electron
energy levels :
hn = E(Ka) = EK - EL
Kb
La
Ka
adapted from Loretto, Fig. 2.3, p. 30
EDX
• X-ray energies are characteristic of
the element
From Ohring, Fig. 6-16, p. 281
EDX
• Can identify the element from the
x-ray lines emitted
(Ti)
From Ohring, Fig. 6-15(b), p. 280
EDX
• X-ray detectors
• EDS
• Si p-i-n junction diode &
MCA
• Resolution ~ 150 eV
• Based on energy of x-rays
(EDS)
From Williams, Fig. 1.16, p. 10
EDX
• X-ray detectors
• WDS
• Uses Bragg reflection from
crystal with known interplanar
spacings to select l
• Resolution ~ 5 eV
• Based on l of x-rays
from Schroder, Fig. 10.15, p. 669
EDX
from Schroder, Table 10.1, p. 671
from Schroder, Fig. 10.16, p. 672
EDX
• Can perform EDX using SEM or STEM
(AEM)
• Can produce elemental maps
From Schroder,
Fig. 10.4,
p. 655
EDX
• Can form compositional maps
EDX
EDX-SEM :
• Lateral resolution ~ 1 mm
• Depth resolution ~ 1 mm
EDX-AEM :
• Lateral resolution ~ beam diameter (~0.5-2 nm)
• Depth resolution ~ sample thickness (~2000 Å)
Electron Beam
Secondary
electrons
(1-10 nm)
X-rays
(0.2 – 2 mm)
Auger
electrons (1 nm)
Backscattered
electrons
(0.1 – 1 mm)
EDX Quantification
• Can determine amount of element
present (to within ~ 0.1 at %) by
measuring x-ray line intensity
Method 1: Calculation
• Intensity of x-rays from a depth d is :
I = Ie(d)cswxe-md/cosq e dW/4p
Ie(d) = intensity of e-beam at depth d
c = atomic concentration
s = ionization cross-section
wx = x-ray yield (fluorescence yield)
m= x-ray absorption coefficient
q = detector angle wrt e-beam
e = detector efficiency
dW = detector solid angle
EDX Quantification
Method 2: Comparison with known
standards
• Compare x-ray intensity of sample with
x-ray intensity from standard with known
composition
EDX
• X-ray emission competes with Auger
process
• Fluorescence yield is low below Na
• EDX can detect elements above Na
from Schroder, Fig. 10.14, p. 668
• Can also use incident x-rays instead of
electron beam
• XRF: X-ray Fluorescence
• XRFS: X-ray fluorescence
spectroscopy
AES
Auger electron spectroscopy
• Incident electron (few keV) ejects core
electron from sample
• Energy from electron transition is
transferred to another electron (the Auger
electron) causing it to be ejected
incident
electron
ejected orbital
electron

Electron
Relaxation
Auger
electron
emitted
(AES)
scattered primary
electron (EELS)
x-ray
photon
emitted (EDX)
AES
• Incident electron has sufficient
energy to eject a core shell electron
from Ohring, Fig. 6-14(a) & (b), p. 278
AES
• Outer shell electron fills the inner
shell vacancy causing ejection of
Auger electron
from Ohring, Fig. 6-14(d), p. 278
AES
• Auger process requires 3 electrons
(incident, core, Auger)
• Can detect all elements except H &
He
from Ohring, Fig. 6-14(d), p. 278
AES
• Auger nomenclature:
KLL
Level of
first ejected
electron
Level of electron that
moves from outer to
inner shell to fill
electron vacancy
Initial level
of Auger
electron
AES
• Energy of Auger electron is
determined by difference in electron
energy levels :
Work function
E(KL1L2) = (EK - EL1) – (EL2 + f)
Energy
released
Energy required
for Auger electron
to escape surface
AES
• Auger electron energies (~ 30 –
3000 eV) are characteristic of the
element
• Can detect all elements except H &
He
from Ohring, Fig. 6-17, p. 281
AES
• Usually Auger signals, N(E), are
differentiated, dN(E)/dE, to accentuate
them from the background
direct Auger
spectra
differentiated
Auger
spectra
From Schroder, Fig. 10.10, p. 663
AES
• Can identify the element from the
AES spectrum
Ti
From Ohring, Fig. 6-15(c), p. 280
AES
• Depth resolution determined by
escape depth of electrons, < 20 Å
• AES is a surface-sensitive
technique; requires UHV
• Depth profiling achieved using
sputter gun
from Yu & Cardona, Fig. 8.5, p. 420
AES
• Lateral resolution ~ 10 - 50 nm (field
emission source, scanning Auger) to 100 mm
(non-scanning)
• Cylindrical mirror analyzer (CMA)
• Spectrometer resolution ~ 4 – 10 eV
• Sensitivity ~ 0.1 – 1 at%
• Quantification (10 % accuracy) achieved
using calibrated standards
from Ohring, Fig. 6-18, p. 284
EELS
Electron energy loss spectroscopy
• Can examine energy loss of
incident electron
incident
electron
ejected orbital
electron

Electron
Relaxation
Auger
electron
emitted
(AES)
scattered primary
electron (EELS)
x-ray
photon
emitted (EDX)
EELS
Typical EELS Spectrum
• Energy loss peak is characteristic
of the elements
DE = EK = binding energy
of ejected electron
Energy loss
zero-loss
peak (used
as energy
reference)
energy
loss due to
inner shell
ionization
EELS
• EELS detector
From Williams, Fig. 1.15, p. 9
EELS
• EELS usually employed in TEM
(AEM)
From Williams,
Fig. 1.2, p. 2
EELS
• Fine structure is present in EELS spectrum
• ELNES (energy loss near edge structure)
• EXELS (extended energy loss spectroscopy)
• Gives local atomic structure
 chemical bonding information
 atomic bond lengths (e.g., VCA)
(EXELS)
(ELNES)
EELS
• EELS is complementary to EDX
• More sensitive to low Z elements
than EDX
• Fine structure gives local atomic
structure information
X-ray Absorption Spectroscopy
• Can also observe fine structure in x-ray
absorption
• Measure transmission or fluorescence from
sample as a function of incident x-ray photon
energy
• Core level excitations produce peaks in
absorption or fluorescence
• Fine structure in absorption edge gives
chemical bonding information
 XANES (x-ray absorption near-edge
structure) or NEXAFS (near edge x-ray
absorption fine structure
 EXAFS (extended x-ray absorption
fine structure)
from C. Lamberti, Surf. Sci. Rep. 53 (2004) 1-197
X-ray Absorption Spectroscopy
m(E)x = ln [Ii(E) / It(E) ]
k = (2p/h) √ 2mo(hn – Eo) ; Eo = photoelectron binding energy
X-ray Absorption Spectroscopy
• Synchrotron radiation is linearly polarized
• Polarization-dependent EXAFS
 gives information on bond orientation
 can measure Da and Da in strained
layers
X-ray Absorption Spectroscopy
• Can achieve surface sensitivity (~ 10 Å) by
using grazing incidence geometry
• Or detect Auger or photoelectrons (lower
escape depth compared to fluorescence)
 surface EXAFS (SEXAFS)
Photoemission Spectroscopy (PES)
X-ray photoelectron spectroscopy (XPS)
• EDX and AES use incident electrons
• XPS uses incident x-rays (few keV) to
cause ionization (photoelectric effect)
• Measure energy of ejected electron
• = electron spectroscopy for chemical
analysis (ESCA)
Ultraviolet photoelectron spectroscopy
(UPS)
• Uses uv photons
From Schroder,
Fig. 10.34, p. 702
PES
K.E. of ejected electron is characteristic of
the element :
Ephotoelectron = hn - EB - f
incident
photon
energy
binding
energy
of ejected
electron
(ionization
energy)
• Can detect elements above Li
• Requires very good spectrometer for H,
He
PES
from Ibach and Luth, Fig. V.1, p. 125
PES
• Typical XPS spectrum
From Ohring, Fig. 6-15(d), p. 280
PES
XPS Advantages:
• X-rays less prone to damage
surfaces than electrons (e.g.,
electrons can reduce hydrocarbons
on surface to carbon)
• EB is sensitive to chemical
surroundings (e.g., Si versus SiO2)
• Typical applications are
determination of electronic states
(e.g., oxides, heterojunction band
alignments)
ARXPS
Angle-Resolved XPS:
• At grazing angles of detection only
electrons from top surface region can
escape
• Surface-sensitive technique
XPS detector
path length of
electrons is too
large for
escape
PEEM
PEEM:
• Photoelectron emission microscopy
= photoelectron spectromicroscopy
• Provides laterally resolved PES
Summary
From Ohring, Fig. 6-15, p. 280
Synchrotron Sources
• Many techniques require a strong
photon source over a wide energy
range  synchrotron radiation
• EXAFS, SEXAFS, XSW, X-ray
reflectivity, GIXS
• Canadian Light Source (CLS)
National Synchrotron Facility
http://www.cls.usask.ca/
Spatial Resolution
• Resolution determined by interaction
volume
Electron Beam
Secondary
electrons
(1-10 nm)
X-rays
(0.2 – 2 mm)
Auger
electrons (1 nm)
Backscattered
electrons
(0.1 – 1 mm)
Summary
Ohring, Fig. 6-3, p. 276