X ray photoelectron spectroscopy

X-ray photoelectron
spectroscopy
Electron Spectroscopy for Chemical
Analysis (ESCA)
Kai M. Siegbahn
Obtained Nobel Prize
For his work on XPS

Introduction:
X-ray photoelectron spectroscopy (XPS) is a surface-
sensitive spectroscopic technique widely used to
investigate the chemical composition of surfaces.
XPS technique is based on Einstein’s idea about the
photoelectric effect, developed around 1905
The concept of photons was used to describe the
ejection of electrons from a surface when photons were
impinged upon it

L2,L3
L1
K
Incident X-ray
Ejected Photoelectron
1s
2s
2p
The Photoelectric Process
XPS spectral lines are
identified by the shell
from which electrons
were emitted (1s, 2s, 2p
etc.)
The kinetic energy of the
ejected photoelectron
KE=hv-BE-F

L2,L3
L1
K
Emitted Auger Electron
1s
2s
2p
Auger Relation of Core Hole
L electron falls to fill
core level vacancy
(step1).
KLL Auger electron
emitted to conserve
energy released in
step1.

X-Rays
Irradiate the sample surface, hitting the core electrons (e-) of the atoms.
The X-Rays penetrate the sample to a depth on the order of a
micrometer.
Useful e- signal is obtained only from a depth of around 10 to 100 Å on
the surface.
The X-Ray source produces photons with certain energies:
MgK photon with an energy of 1253.6 eV
AlK photon with an energy of 1486.6 eV
Normally, the sample will be radiated with photons of a single energy
(MgK or AlK). This is known as a monoenergetic X-Ray beam.

Why the Core Electrons?
An electron near the Fermi level is far from the nucleus,
moving in different directions all over the place, and will
not carry information about any single atom.
Fermi level is the highest energy level occupied by an electron in
a neutral solid at absolute 0 temperature.
The core e-s are local close to the nucleus and have
binding energies characteristic of their particular element.
Core e-
Valence e-
Atom

How Does XPS Technology Work?
A monoenergetic x-ray beam emits photoelectrons from the from the
surface of the sample.
The x-ray photons The penetration about a micrometer of the
sample
The XPS spectrum contains information only about the top 10 - 100
Ǻ of the sample.
Ultrahigh vacuum environment to eliminate excessive surface
contamination.
Cylindrical Mirror Analyzer (CMA) measures the KE of emitted e-s.
The spectrum plotted by the computer from the analyzer signal.
The binding energies can be determined from the peak positions
and the elements present in the sample identified.

COMPONENTS OF XPS:
A source of X-rays
An ultra high vacuum (UHV)
An electron energy analyzer
 magnetic field shielding
An electron detector system
A set of stage manipulators

XPS Instrument
X-Ray Source
Ion Source
CMA
Sample introduction
Chamber

WHY WE USE UHV?
 Remove adsorbed gases from the
sample.
 Eliminate adsorption of contaminants on
the sample.
 Prevent arcing and high voltage
breakdown.
 Increase the mean free path for electrons,
ions and photons.

Cylindrical Mirror Analyzer (CMA)
Slit
Detector
Electron Pathway through the CMA
0 V
+V
0 V 0 V
0 V
+V
+V
+V
X-Rays
Source
Sample
Holder

KE versus BE
E E E
KE can be plotted depending
on BE
Each peak represents the
amount of e-s at a certain
energy that is characteristic
of some element.
1000 eV 0 eV
BE increase from right to left
KE increase from left to right
Binding energy
#ofelectrons
(eV)

Interpreting XPS Spectrum:
Background
The X-Ray will hit the e-s in
the bulk (inner e- layers) of
the sample
e- will collide with other e-
from top layers, decreasing
its energy to contribute to
the noise, at lower kinetic
energy than the peak .
The background noise
increases with BE because
the SUM of all noise is taken
from the beginning of the
analysis.
Binding energy
#ofelectrons
N1
N2
N3
N4
Ntot= N1 + N2 + N3 + N4
N = noise

XPS peak identification
 Electronic Effect:
 Auger lines
 Chemical shifts
 X-ray satellites
 X-ray “Ghost”
 Energy loss lines

XPS Study of Changes in the Chemical Composition of
Langasite Crystal Thin Surface Layers during Vacuum
Annealing
The aim of this work was to study the chemical composition
of LGS La3Ga5SiO14 langasite crystal wafer surface after
thermal vacuum annealing at 650°C and at elevated
temperatures (1000°C). To analyze the surface and
subsurface layer chemical composition we used X-ray
photoelectron spectroscopy (XPS).
The 1050°C, 30 min, and 1000 °C, 5 h annealing
experiments showed that in both cases the crystals lost color,
and the chemical composition of the wafer surface changed:
the gallium concentration decreased abruptly

Figure 1 shows the photoelectron spectra of the crystal
surfaces before and after annealing from which the gallium
line intensity can be seen to decrease by an order of
magnitude.

Advantages and Disadvantages
 Advantages
 Non-destructive technique.
 Surface Sensitive (10-100
Å).
 Quantitative
measurements are
obtained.
 Provides information about
chemical bonding.
 Elemental mapping.
 Limitations
 Very expensive
technique.
 High vacuum is required.
 Slow processing (1/2 to
8 hours/sample).
 Large area analysis is
required.
 H and He can not be
identified.
 Data collection is slow 5
to 10 min.

 XPS is used to measure:
 Elemental composition of
the surface (top 1–12 nm
usually).
 Chemical or electronic
state of each element in
the surface.
 Uniformity of
composition across the
top surface (line
profiling).
 Applications in the
industry:
 Failure analysis
 Polymer surface
 Corrosion
 Adhesion
 Semiconductors
 Thin film coatings
Uses

References:
 Siegbahn, K.; Edvarson, K. I. Al (1956). "β-Ray spectroscopy in the
precision range of 1 : 1e6". Nuclear Physics
 Turner, D. W.; Jobory, M. I. Al (1962). "Determination of Ionization
Potentials by Photoelectron Energy Measurement".
 journals.tubitak.gov.tr
 nanohub.org
 srdata.nist.gov
 www.eaglabs.com
www.files.chem.vt.edu
 Bio interface.org
 www.spectroscopynow.com
 www.surfaceanalysis.org
 www.csma.ltd.uk

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Editor's Notes