X-Ray Photoelectron Spectroscopy

(XPS)

www.nano4me.org © 2018 The Pennsylvania State University XPS 1

 Outline
 Basic principles
 Instrumentation
 Peak characteristics
 Quantitative analysis
 Depth profiling

www.nano4me.org © 2018 The Pennsylvania State University XPS 2

 X-Ray Photoelectron Spectroscopy (XPS)
 X-ray Photoelectron Spectroscopy (XPS), also known
as Electron Spectroscopy for Chemical Analysis
(ESCA), is used to determine quantitative atomic
composition and chemistry.
 A sample is irradiated with monochromatic x-rays,
resulting in the emission of photoelectrons whose
energies are characteristic of the elements within the
sampling volume.
 An XPS spectra is created by plotting the number of
electrons verses their binding energy.

www.nano4me.org © 2018 The Pennsylvania State University XPS 3

 Historical Notes on XPS
 Kai Siegbahn (b.1918): inventor of
modern-day XPS was working at
Uppsala University in Sweden in the
1950s and 1960s.
 Siegbahn was a physicist, and chose
to call his method electron
spectroscopy for chemical analysis
(ESCA), because he understood the
importance of the chemical information 1918-2007
that XPS or ESCA can provide.
 Siegbahn shared the Nobel Prize for
Physics (1981) for his work.

www.nano4me.org © 2018 The Pennsylvania State University XPS 4

 X-ray Photoelectron Spectroscopy
 Based on Einstein’s photoelectric effect.
 Many materials emit electrons when light shines upon them

 Core electrons with kinetic energy Ek are ejected by

incident X-rays.

EK = hν – Eb – φsp EVac
hν Eb e-
EK = kinetic energy
Eb = binding energy
h = Planck’s constant
ν = frequency of X-rays
φsp = spectrometer work function

www.nano4me.org © 2018 The Pennsylvania State University XPS 5

 X-Ray Photoelectron Spectroscopy (XPS)

 An x-ray beam usually comprised

of k-alpha x-rays is focused on x-ray
the sample.
 The absorption of incident x-rays
results in the ejection of
electrons.
 The energy of the ejected
electrons is measured by the
detector.

www.nano4me.org © 2018 The Pennsylvania State University XPS 6

 X-Ray Photoelectron Spectroscopy

Released into the public domain by its author, Bvcrist at the English Wikipedia project.

www.nano4me.org © 2018 The Pennsylvania State University XPS 7

 Energy of the ejected electrons

Released into the public domain by its author, Bvcrist at the English Wikipedia project.

www.nano4me.org © 2018 The Pennsylvania State University XPS 8

 X-Ray Photoelectron Spectroscopy (XPS)
 Each atom has a unique XPS spectra.
 XPS can determine elemental composition,
stoichiometry, electrical/chemical states and examine
surface contamination.
 XPS is an elemental analysis technique that is unique in
providing chemical state information of the detected
elements, such as distinguishing between sulfate and
sulfide forms of the element sulfur.

www.nano4me.org © 2018 The Pennsylvania State University XPS 9

 Inelastic mean free path (λM)
 When an electron with kinetic
energy E moves through a
solid matrix M, it has a 1000

λ, monolayers
probability of traveling a
100
certain distance before losing
all or part of its energy due to
10
an inelastic collision.
 The average distance traveled 1

before such a collision is 1 10 100 1000

Energy, eV
known as the inelastic mean
free path λM(E).

www.nano4me.org © 2018 The Pennsylvania State University XPS 10

 Inelastic mean free path (λM)
 Since the energy ranges used
in XPS analysis are typically 1000

λ, monolayers
50–1200 eV, the values of λ are
very small, corresponding to 100

only a few monolayers.

10
 Photoelectrons must originate
from atomic layers very close to 1

surface to be detected. 1 10 100 1000

Energy, eV
 Therefore, the XPS technique is
very surface-specific.

www.nano4me.org © 2018 The Pennsylvania State University XPS 11

 XPS surface analysis
What is a surface?
 Surfaces
(3 atomic layers) using XPS and
angle resolved XPS (ARXPS) Ultra-thin films: 1-10 nm

Surface: ~1 nm Thin film: 10 nm to 2 µm

 Ultra-thin films
(3-30 atomic layers) using XPS
and angle resolved XPS Bulk

 Thin films
(3-600 atomic layers) using XPS
in combination with sputter
etching for profiling

www.nano4me.org © 2018 The Pennsylvania State University XPS 12

 X-Ray Photoelectron Spectroscopy (XPS)
XPS is used to measure:
 elemental composition of the surface (top 1–10 nm usually)
 empirical formula of pure materials
 elements that contaminate a surface
 chemical or electronic state of each element in the surface
 uniformity of elemental composition across the top surface (or line
profiling or mapping)
 uniformity of elemental composition as a function of ion beam
etching (or depth profiling)

Electron energy analyzer

EK = hν – Eb – φsp X-ray

Photoelectron

Sample

www.nano4me.org © 2018 The Pennsylvania State University XPS 13

 Outline
 Basic principles
 Instrumentation
 Peak characteristics
 Quantitative analysis
 Depth profiling

www.nano4me.org © 2018 The Pennsylvania State University XPS 14

 XPS instrumentation

 UHV System
 X-ray source
 Electron analyzer Hemispherical energy analyzer
Magnetic Shield
 Ion gun

Lenses for
energy Detector
X-ray adjustment
source Lenses for
analysis
area definition
Analyzer control &
Sample computer system

www.nano4me.org © 2018 The Pennsylvania State University XPS 15

 XPS instrumentation
Ultra-High Vacuum System
 Allows longer photoelectron path length
 Ultra-high vacuum keeps surfaces clean,
preventing the contaminations to produce
X-ray signal
 Pressure < 10-8 Torr Pressure (Torr)
 Vacuum pumps 102
Low Vacuum
 Roughing Pump 10-1
Medium Vacuum
 Turbo Pump 10-4
High Vacuum
 Ion Pump 10-8
Ultra-High Vacuum
10-11

www.nano4me.org © 2018 The Pennsylvania State University XPS 16

 XPS instrumentation
Al Window
X-ray source
X-ray
 Dual Anode X-ray source
Filament Filament
 Mg Kα radiation: hν = 1253.6 eV Mg Al
Silver
Shield
 Al Kα radiation: hν = 1486.6 eV substrate
Anode
 Monochromated using quartz crystal 15 kV

Quartz crystal
Electron
source

X-ray
Electron beam
beam

X-ray Source XR 50
http://www.specs.de/cms/front_content.php?idcat=118
Al anode
Sample

www.nano4me.org © 2018 The Pennsylvania State University XPS 17

 XPS instrumentation
Electron analyzer
 Lens system to collect photoelectrons
 Analyzer to filter electron energies
 Detector to count electrons

Hemispherical energy analyzer

Magnetic Shield

Lenses for
energy Detector
X-ray adjustment
source Lenses for
analysis
area definition
Analyzer control &
Sample computer system

www.nano4me.org © 2018 The Pennsylvania State University XPS 18

 XPS instrumentation
Ion gun
 Sample cleaning
 Depth profiling
 Ar+ is the most widely used ion

Electron analyzer
X-ray Ar+ gun
source

e-
e- e-
e-

Sample

www.nano4me.org © 2018 The Pennsylvania State University XPS 19

 Outline
 Basic principles
 Instrumentation
 Peak characteristics
 Quantitative analysis
 Depth profiling

www.nano4me.org © 2018 The Pennsylvania State University XPS 20

 XPS spectrum
 Photoelectron peaks reflect discrete binding energies
of the electrons present in the solid
 MoO3 film excited by Al Kα (1486.6 eV)

www.nano4me.org © 2018 The Pennsylvania State University XPS 21

 XPS Peak: spin orbit coupling
 XPS peak is originated from the photoelectrons in the core-levels
and valence band.
 In the quantum mechanics, the nomenclature for a core level is nlj:
 n : principle quantum number
 l : orbit angular momentum quantum number
 j : total angular momentum quantum number; j = l + s
 s : spin angular momentum quantum number, s=±1/2

If l=0, single XPS peak

if l>0, a doublet peak-- spin orbit (l-s) coupling

0 s
n l=2 1 p
2 d
2d3/2 3 f

j=l-s
j=l+s

www.nano4me.org © 2018 The Pennsylvania State University XPS 22

 XPS Peak: spin orbit coupling
 Since s can be ±1/2, each level with l>0 is split into two sublevels
with an energy difference known as the spin-orbit splitting.
 The degeneracy of each of these levels is 2j+1

Δ = 3.13 eV

Orbital l J Degeneracy Electron level

Mo 3d5/2 1s 0 1/2 1 1s

Mo 3d3/2 2s 0 1/2 1 2s
2p 1 1/2 2 2p1/2
2p 1 3/2 4 2p3/2
3d 2 3/2 4 3d3/2
3d 2 5/2 6 3d5/2
4f 3 5/2 6 4f5/2
4f 3 7/2 8 4f7/2
MoO3 film

www.nano4me.org © 2018 The Pennsylvania State University XPS 23

 XPS Peak intensities

s orbital p orbital d orbital f orbital

1 1:2 2:3 3:4

1s 2p1/2 2p3/2 3d3/2 3d5/2 4f5/2 4f7/2

Binding Energy (eV)

www.nano4me.org © 2018 The Pennsylvania State University XPS 24

 XPS energy levels
 The photoelectron’s binding energy will be based on
the element’s final-state configuration.

Initial State Final State

Free
electron
Conduction band Conduction band
Fermi
level
Valence band Valence band
2p
2s

1s

www.nano4me.org © 2018 The Pennsylvania State University XPS 25

 Energy lines

 Photon energies,
in eV, of principle
K- and L- shell
emission lines.

X-ray data booklet, Lawrence Berkeley

National Laboratory, 3rd edition, 2009

www.nano4me.org © 2018 The Pennsylvania State University XPS 26

 XPS energy: chemical shifts
 When an atom makes a bond with another atom, the
valence electron density changes resulting in an
adjustment of the electrostatic potential affecting the
core electrons.
 Change in the binding energies of the core electrons
cause shifts in the corresponding photoelectron peaks
EK = hν – Eb – φsp

 Atom loses valence charge, BE increases

 (Mo -> MoO2)
 Atom gains valence charge, BE decreases
 (MoO3 -> MoO2)

www.nano4me.org © 2018 The Pennsylvania State University XPS 27

 XPS energy: chemical shifts
 Atoms of higher positive oxidation state show a higher
binding energy
 Extra coulombic interaction between the photo-
emitted electron and the ion core
Element level Compound Binding Energy
(eV)
Mo 3d5/2 Mo2C 227.8
Mo 3d5/2 Mo 228.0
Mo 3d5/2 MoO2 229.4
Mo 3d5/2 MoS2 229.4
Mo 3d5/2 MoCl3 230.0
Mo 3d5/2 MoCl4 231.0
Mo 3d5/2 MoCl5 232.2
Mo 3d5/2 MoO3 232.6

www.nano4me.org © 2018 The Pennsylvania State University XPS 28

 Outline
 Basic principles
 Instrumentation
 Peak characteristics
 Quantitative analysis
 Depth profiling

www.nano4me.org © 2018 The Pennsylvania State University XPS 29

 Quantitative analysis
 Atomic concentration of elements can be calculated:

 We define sensitivity factors:

 σij : Photoionization cross-section of peak j of element i
 λ(KE): Inelastic mean free path length

 Therefore, ni is dependent on the peak area (Iij) and sensitivity factor

Peak area
Intensity

Kinetic energy

www.nano4me.org © 2018 The Pennsylvania State University XPS 30

 Quantitative analysis
 Atomic sensitivity factors for X-ray sources at 54.7o

C.D. Wagner, et al, Surf. Interface Anal. 3, (1981) 211.

www.nano4me.org © 2018 The Pennsylvania State University XPS 31

 Outline
 Basic principles
 Instrumentation
 Peak characteristics
 Quantitative analysis
 Depth profiling

www.nano4me.org © 2018 The Pennsylvania State University XPS 32

 Depth profiling
 To remove surface contaminations or find elemental
distribution throughout the film thickness
 Ar+ energy: 1-3 KV
 Sample rotation

Start XPS spectrum Ar+ sputtering

Electron analyzer
X-ray Ar+ gun
source

Sample

www.nano4me.org © 2018 The Pennsylvania State University XPS 33

 Depth profiling
 Cu(In,Ga)Se2 film deposited on ITO

Elemental distribution Ga 3d analysis

 Ga accumulation at the interface  Formation of Ga

 Cu diffusion into the ITO layer oxide at the interface

www.nano4me.org © 2018 The Pennsylvania State University XPS 34

 Summary
Vacuum Range
 Requires ultra high vacuum
 Sample compatibility with UHV environment may be an
issue with biological samples
Sensitivity
 Measures the elemental composition of the top 10 nm
 Can detect all elements except H and He
 Detection limits typically ~ 0.1 atomic percent
 Typically the smallest analytical area ~10 µm
Material analysis
 Can analyze metals, inorganic, polymers
 Sample compatibility with UHV environment

www.nano4me.org © 2018 The Pennsylvania State University XPS 35

 Online sources
• PNNL EMSL: www.emsl.gov
• AVS Science & Technology Society: www.avs.org
• AVS Surface Science Spectra: www.avs.org/literature.sss.aspx
• Evans Analytical Group: www.cea.com
• NIST X-ray Photoelectron Spectroscopy Database: www.srdata.nist.gov/sps/
• NIST Electron Inelastic-Mean-Free-Path Database: www.nist.gov/srd/nist71.htm
• QUASES-IMFP-TPP2M QUASES-Tougaard Inc.: www.quases.com
• Surfaces & Interfaces Section, National Physical Lab. www.npl.co.uk/npl/cmmt/sis
• XPS MultiQuant www.chemres.hu/aki/XMQpages/XMQhome.htm
• ASTM International: www.astm.org

www.nano4me.org © 2018 The Pennsylvania State University XPS 36