Transcript 2XPS_김정원

광전자 분광법
Photoelectron Spectroscopy (XPS, UPS)
김정원
(E-mail: [email protected])
미래융합기술본부 소재게놈측정센터
1
표면분석I 2016
Introduction
- 광전자분광법(PES)란 무엇인가 ?
- PES로 무엇을 할 수 있는가 ?
- PES의 간략한 역사
- Principles of XPS
Chemical shifts
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Photoelectron Emission
energy, polarization, angle
Vacuum UV
x-ray
Low energy electrons
UPS
energy, angle, spin
valence
core
XPS
High energy electrons
XPS: X-ray Photoelectron Spectroscopy (hv = 50~2000 eV)
UPS: Ultraviolet Photoelectron Spectroscopy (hv = 6~80 eV)
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Schematic Diagram of PES Process
Electronic structure
Solid
UPS
XPS
Atomic composition
Chemical structure
Energy conservation
EB: element & chemistry specific
in case of core levels
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Typical XPS spectra of Ag
3d5/2
MNV
150
Al Kα
Mg Kα
MNV
kCounts [a.u.]
3d3/2
100
3p3/1
3p3/2
3s
50
4s 4p 4d (valence)
1200
1000
800
600
Binding Energy [eV]
400
200
0
EB: constant irrespective of excitation source
AES transition: varies with excitation energy (constant kinetic energy)
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약어들
Abbreviations
XPS: X-ray Photoelectron Spectroscopy
ESCA(XPS): Electron Spectroscopy for Chemical Analysis
UPS: Ultraviolet Photoelectron Spectroscopy
PES: Photoelectron (Photoemission) Spectroscopy
KE: Kinetic energy
BE (EB): Binding energy
ARXPS, UPS, PES: Angle-Resolved XPS, UPS, PES
XPD: X-ray Photoelectron Diffraction
PED: Photoelectron Diffraction
IP(E)S: Inverse Photoemission Spectroscopy
EELS: Electron Energy Loss Spectroscopy
Electron Spectroscopy: AES, EELS, PES
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PES로 무엇을 할 수 있는가 ?
-Non-destructive Elemental Identification except H and He
(다른 방법? X-ray fluorescence, SIMS, AES)
- Quantification (원소농도분석): ~ %정확도
- Chemical State Identification (e.g. Si, SiO2)
- Surface/Adsorbate Structure
- Electronic Structure
valence band level positions
band structure mapping
work function
many body effect, etc.
- Microscopy with Chemical Sensitivity
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Available? Where?
XPS (ESCA):
서울? 대부분의 종합대 및 공대, 일부 실험실 (not open), KIST, 생기연
지방? 대부분의 국립 종합대, 일부 종합 사립대
기초과학지원연구원 (대전, 부산), 화학연
대부분의 대기업 분석실 (not open), 포항방사광
UPS (Ultraviolet Photoelectron Spectroscopy):
기초과학지원연구원 (대전, 부산), KAIST, KIST
많은 개별 실험실 (not open)
vs. Evans Analytical Group (USA), Toray Research Center (Japan)
Pros: cheap and relatively fast
Cons: instrument-dependent, weak analysis
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A Brief History of PES
- 1887, H. Hertz: 광전효과(photoelectric effect) 발견
- 1899, J. J. Thompson: 전자 발견
- 1900, M. Planck: Quantum theory (quantized energy)
- 1905, A. Einstein: Quantum theory로 광전효과 설명
- 1958, W. E. Spicer: UPS spectra와 DOS 관련주장
- 1967, K. Siegbahn: XPS (ESCA) 확립
- 1969, HP - Commercial XPS
- 1970s: Synchrotron Radiation
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History: Discovery of Electrons
Photoelectric effect
Metal plate in a vacuum, irradiated by ultraviolet light, emits
charged particles (Hertz 1887), which were subsequently
shown to be electrons by J.J. Thomson (1899).
Classical expectations
Light, frequency ν
Vacuum
Electric field E of light exerts force
chamber
F=-eE on electrons. As intensity of
Collecting
Metal
light increases, force increases, so KE
plate
plate
of ejected electrons should increase.
Electrons should be emitted whatever
the frequency ν of the light, so long as
E is sufficiently large
I
Ammeter
Potentiostat
Lecture note: Fisher (Univ. College London )
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For very low intensities, expect a time
lag between light exposure and emission,
while electrons absorb enough energy to
escape from material
History: Photons and Electrons
Photoelectric effect
Einstein
Actual results:
Maximum KE of ejected electrons is
independent of intensity, but
dependent on ν
For ν<ν0 (i.e. for frequencies
below a cut-off frequency) no
electrons are emitted
Einstein’s
interpretation (1905):
light is emitted and
absorbed in packets
(quanta) of energy
E  h (1.1)
Millikan
An electron absorbs a
single quantum in
order to leave the
material
There is no time lag. However,
rate of ejection of electrons
depends on light intensity.
The maximum KE of an emitted electron is then predicted to be:
K max  h  W (1.2)
Planck constant:
universal constant of
nature
h  6.63 1034 Js
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Work function: minimum
energy needed for electron to
escape from metal (depends on
material, but usually 2-5eV)
Verified in detail
through
subsequent
experiments by
Millikan
광과 시료원자와의 상호작용
표면탈출: XPS
이차전자 증폭 발생: SEM
탄성 충돌: XRD, LEED, RHEED
비탄성 충돌: EELS
X-선, UV
(EDX/WDX)
XES
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Theory of Photoemission (binding energy)
frozen orbital approximation (Koopman’s theorem)
+ e (KE)
initial state
A(N)
final state
A+(N-1)
A( N )    A ( N  1)  e
Considering energy conservation
Ei ( N )    E f ( N  1)  KE
BE    KE  E f ( N  1)  Ei ( N )
  kHF   relax   correl   rel
kth orbital energy by Hatree-Fock calculation
first approximation
correlation energy
Relaxation energy
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relativistic energy
Element-specific Core Levels
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Chemical Shifts (Initial state effect?)
valence shell
(charge qi)
core shell
electrons
rij
Surroundings
(charge qj)
Charge potential model in ionic bonds
All core levels undergo same chemical shift
(approximation)
similar to Madelung potential
EB ↑ as q ↑ (note sign)
(Fig. 3.3)
for neutral atom
Coulomb interaction between core electron and
nucleus screened by valence electron charge
Dependent on chemical environment such as
oxidation state
electronegativity of neighboring atoms
number of surrounding atoms
J.C. Vickerman and I.S. Gilmore, Surface Analysis, The principal Techniques, 2nd ed. (Wiley, 2008)
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Chemical Shifts of S 1s and S 2p
Formal oxidation state is good indication of EB
It is only valid in ionic bonds
If there is covalent/ionic bond character mixing,
Charge density on an atom is the best criterion
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A Example of Chemical Shifts
Not Always Possible
Tabulated Values Vary Widely
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Final state consideration
eEi

hot electron
e-
electron
ejection
Δt
hole
valence shell
initial state
Instant excitation
hole
Ef
final state
Relaxation
(dynamic screening,
solvation…)
Ei(N) +  = Ef(N-1) + KE (e-)
BE =  - KE (e-) = Ef(N-1) – Ei(N) ≈ orbital energy
But, final-state relaxation is much dependent on its atomic environment
Relaxation energy –> binding energy (time-dependent)
Relaxation time –> peak width
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Spin-orbit splitting
J=L+S
Unpaired electron
after ionization
3d5/2
Ag 3d
J=L-S
3d3/2
Ag 3d: (3d)10 +  → (3d)9 + eL=2, S=1/2, J=L+S,,, L−S = 5/2, 3/2
2D , g = 2×(5/2)+1 = 6
5/2
J
2D , g = 2×(3/2)+1 = 4
3/2
J
Branching ratio = 6:4 = 3:2
380
378
376
374
372
370
368
Binding energy [eV]
p, d, f core levels split into two doublet peaks
BE (J=L−S) > BE (J=L+S) considering final state energy
Splitting ↑ as Z ↑
What about p and f levels?
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364
Final State Effects
Initial state effect: Koopman’s theorem
Final state effect: : 1~10 eV
-the created core hole after photoionization affects the energy
distribution of the emitted electrons in different ways.
Relaxation effect
Multiplet splitting
Multielectron excitations
-shake up and shake off satellites
-electron-hole excitation (continuous satellite): asymmetric line
shape
Plasmon loss peaks
Vibrational effects
Ref. Electron spectroscopy, theory, techniques, and applications Vol.2 (Academic Press. 1978)
Chap. 1, C.S. Fadley, Basic concepts of x-ray photoelectron spectroscopy
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Relaxation effect
Atomic relaxation (gas phase)
by Franck-Condon principle
BE  E f ( N  1)  Ei ( N )
As Ef ↓ , BE ↓
Instantaneous electronic transition (A)
but relaxation afterward (F)
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Extra-atomic relaxation
by surroundings (sold phase)
Typical shake-up in C 1s
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Instrumentation
- Vacuum System
- Photon Source
- Electron Energy Analyzer
- Resolution
- Binding Energy Referencing
- Charging compenstation
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Universal Curve
Inelastic electron mean free path (IMFP) ~ Intensity attenuation length
I ( x)  I 0 e  x 
 ( E )  nm
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IMFP
Other
consideration (Escape
Other
consideration
(Escapedepth)
depth)
Comparison of AES and EDX analysis volume
Hard X-ray PES (HAXPES)
HAXPES
Conventional VUV
SX-PES
Probing depth (3λ) up to > 10 nm
Accessing bulk or buried interface
Depth profile of thin film
Lecture note: R. Claessen (Univ. Wϋrzburg)
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Photon Source
 Intensity,
Focus, Monochromatic, energy selection
 dual anode, monochromatic, discharge lamp, SR
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Characteristic X-ray Lines
Line
Y Mζ
Zr Mζ
Cr Lα
Cu Lα
Mg Kα
Energy (eV)
132.3
151.4
572.8
929.7
1253.6
Width (eV)
0.47
0.77
3.0
3.8
0.7
Al Kα
Si Kα
Cu Kα
1486.6
1739.5
8048.0
0.85
1.0
2.6
Line
Separation (eV)
Relative Intensity
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Satellites of
Mg Kα
α12
α3
α4
α5
α6
β
0.0
100
8.4
8.0
10.2
4.1
17.5
0.55
20
0.45
48.5
0.5
Al K Monochromatic X-ray Source의 구조
PHI Quantera
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C 1s: Effect of monochromatization
10 ~ 40 times intensity reduction
 Focusing at the sample (up to 10 ㎛ recently)
- Intense beam
- Imaging

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Resonance Lines of Rare Gas Discharge
UV sources : each monochromator available
Resonance Line
He I
He II
Ne I
Ar I
Kr I
Xe I
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Energy (eV)
21.2175
23.0865
40.8136
16.6704
Intensity (%)
100
2
16.8474
11.6233
11.8278
10.0321
100
100
50
10.6434
8.4363
9.5695
15
Synchrotron Radiation
- High Intensity and resolution, energy tunability, focused Beam,
polarization, pulsed beam
- Big Facility means Big Money ! – Shared facility
Pohang Acceleration Lab.
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Hemispherical Energy Analyzer
Concentric Hemispherical Analyzer : CHA
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Electron Detection
Channeltron (CEM)
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Microchannel Plate (MCP)
Position-sensitive Energy Analyzer
Channeltron array
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MCP + CCD (or DLD)
Energy Resolution of Analyzer
Small Epass  Large Eretard
High resolution
Low throughput (low count rates)
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Large Epass  Small Eretard
Low resolution
High throughput (high count rates)
Total Energy Resolution
ΔETotal =
√(ΔEanal.2 + ΔEphoton2 + ΔEthermal broad2 + ΔEinhomogen.2 + ..)
ΔEanal.= Epass(d/2Ro + α2/4) for HEA type
d= slit width
Ro=mean radius
α=a half acceptance angle
ΔEthermal broad ~ 3/2kT
<Practical measurement
of total resolution>
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Differential surface charging
Peak Broadening or shift
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Charging Compensation
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Charging Compensation
With a Magnetic lens system (Cratos)
Nearly complete charging compensation !
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Modern ESCA
Features
1.
2.
3.
4.
5.
Monochromatic x-ray (1486 eV)
Angle-resolved analyzer
UV discharge lamp (optional)
Automatic sample transfer
Charge compensation
가격 $1M?
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Data Processing
- Smoothing
- Background and X-ray Satellite 제거
- Peak Fitting and Deconvolution
- Practical Programs
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Smoothening
- NOT recommended, but
- 너무 오래 걸리거나, 변하는 시료, 또는 미분하기 전에
- Savitsky-Golay method
- Adjacent Averaging
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Background Removal
 Background
–
–
–
–
–
–
Inelastic Energy Loss Mechanism
Complicated Process
Sample Dependent
Geometry Dependent
Instrument Dependent
Must be Removed for Quantification
 Removal
Methods
– Linear, Shirley, Tougaard
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Linear Background Removal
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Shirley Method
integrated background
Key point: 어떤 점 x에서 BG 는 x 보다 높은 K. E.를 가진
전자들의 total intensity에 비례한다
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Shirley BG Removal in Action
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Peak Fitting and Deconvolution
Lorentzian
Gaussian
Voigt
L( x) 

2A
 4( x  xc ) 2   2

( x  xc ) 2 
G ( x) 
exp  2

2

  2


A
V ( x)  A
2 ln 2  L

3 2


t 2 
 e 


2
G

ln 2
L
G
2









4 ln 2
x  xc  t 


G

Lorentzian width: core hole life time: ΔE·τp≥ ħ
Gaussian width: instrumental resolution, system inhomogeneity
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2
1

 dt


Least Square Peak Fitting
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Peak Fitting in action
J.F. Watts and J. Wolstenholme, An Introduction to Surface Analysis by XPS and AES (Wiley, 2003)
C 1s
urea formaldehyde/epoxy coating
Any principles?
1)
2)
3)
4)
5)
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Proper background subtraction should be made.
Initial guess is important.
Any peak should have physical meaning.
Fitting is just fitting (no result from nothing)
Peak splitting within an system resolution
Fitting Programs
CasaXPS http://www.casaxps.com/
Unifit http://www.uni-leipzig.de/~unifit/
NIST Database for the Simulation of Electron Spectra for Surface Analysis (SESSA)
http://www.nist.gov/srd/nist100.htm
Non-commercial programs
Fitt-win http://escalab.snu.ac.kr/
XPSPEAK 4.1 http://www.uksaf.org/software.html
FitXPS
Compro http://www.sasj.jp/COMPRO/index.html
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Applications
- Elemental Identification
- Quantitative Analysis
- Chemical Shifts
- Work function Measurement
- Angle-resolved Techniques
- Depth profile
- Microscopy
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Elemental Identification
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정량분석: Basic Concept
XA
IA
  
IA
I A I A

 Ii Ii
i
I
XA
IA
시료 중 원소 A의 농도
I A
완전히 원소 A로만 이루어진 시료에서 측정된
원소 A의 XPS 피크 세기 (Atom Sensitivity Factor)
시료에서 측정된 원소 A의 XPS 피크 세기

값은 알기 어려우나,
A
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I

A
I

는 비교적 알기 쉬움
i
정량분석: RASF 이용
다른 조건들이 동일할 때 원소들 피크의 상대적 세기
Analyzer, Source, Geometry dependent parameter
X Sr 
6443.2
1.843

6443.2
1.843
6009.1
5399.7
2.001
0.771

.6
 1080
0.296
 20.4%
Element
RASF
Measured Intensity
Sr 3d
1.843
6443.2
Ti 2p
O 1s
2.001
6009.1
0.771
5339.7
C 1s
0.296
1080.6
Concentration
20.4
17.5
40.8
21.3
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RASF
Relative elemental sensitivity
12
10
Relative Sensitivity
3d
8
4f
2p
6
4
4d
2
1s
0
Li B N F Na Al P Cl K Sc V M Co Cu G As Br Rb Y Nb Tc Rh Ag In Sb I Cs La Pr P Eu Tb Ho T Lu Ta Re Ir Au Tl Bi
Be C O Ne M Si S Ar Ca Ti Cr Fe Ni Zn G Se Kr Sr Zr M Ru Pd Cd Sn Te Xe Ba Ce Nd S G Dy Er Yb Hf W Os Pt Hg Pb
Elemental Symbol
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정량분석: complicated problem
http://www.nist.gov/srd/surface.cfm
i  i (E )
e
K. E.  EA
i ( E A )
Material-dependent inelastic
mean free path (IMFP)
e
K. E.  EB
i ( EB )
Probing different volumes in mixed (A and B) materials
Correction must be made !!
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TPP-2M equation for IMFP
By Tanimura, Powell, Penn
Surf. Interface Anal. 21, 165 (1994)
E
 2
E p [  ln( E )  (C / E )  ( D / E 2 )]
  0.1 
  0.191
0.994
E p2  E g2
 0.069  0.1
density (g/cm3)
 0.5
C  1.97  0.91U
D  53.4  20.8U
(Å)
Band gap (eV)
atomic or molecular weight
U  N / M  E p2 / 829.4
E p  28.8( N  / M )1/ 2 free electron plasmon energy (eV)
number of valence electrons
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Valence band spectra
3.0
HeI (21.21 eV)
XPS (1253.84 eV)
HeII (40.81 eV)
2.5
Zn 3d
XPS
6
2.0
x10
final state
UPS
1.5
EF
1.0
initial state
0.5
core levels
0.0
14
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12
10
8
6
4
Binding Energy (eV)
2
0
-2
Photoionization intensity (cross section)
http://ulisse.elettra.trieste.it/services/elements/WebElements.html
Lecture note: S. Kim (KAIST)
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Atomic Cross-section
UPS
XPS
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UPS의 상대적인 특징
1. Peak is broad
Why? Due to electronic band structure or orbital hybridization
2. Feels more surface-sensitive than with XPS
Why? Low electron attenuation length & high O atomic photoioinization
cross-section at low photon energy
3. Very difficult to quantify
Why? This is not atomic character but bonding character. It means that
you need more clear and physical idea of your specimen in
simplified way
4. Resolution is better
Why? Gas discharge source provides a sharp fluorescent light
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Example of XPS analysis-1
Pigment from Mummy Artwork
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Example of XPS analysis-2
Valence electron state of Pt in PtPc
4+
2+
표면분석I 2016 김정원
Metal-semiconductor Contact
Energy Level Diagram
표면분석I 2016 김정원
Organic-Metal contact in OLED
Device performance depends on the balanced carrier injection
and transport
To maximize device efficiency : Enhance charge injection
표면분석I 2016 김정원
Energy levels and UPS spectra
interface dipole
ionization potential
UV
(X-ray)
work function
HOMO level
  h  EF  Ecutoff
S. Braun et al., Adv. Mat.(2009)
표면분석I 2016 김정원
Angle-Resolved PES
Better Surface Sensitivity
SiO2
Si
표면분석I 2016 김정원
SiO2
Si
XPS에 의한 SiO2 박막 두께 측정
7k
XPS Intensity (counts)
SiO2
Si
6k
SiO2
5k
4k
3k
Si
2k
Analyzer
1k
108
106
104
102
100
98
96
Binding Energy (eV)
exp

ISiO

I
SiO2 [1  exp( t ox /L cos q )]
2
ISiexp  ISi exp(  t ox /L cos q )
x-ray
(hν)
e
q
L cos q
t OX  Lcosθ ln(R exp /R 0  1)

I SiO
2
R0 

Si
Rexp 
exp
I SiO
2
I
I Siexp
L = Attenuation length of Si 2p
correct measurement of SiO2 thickness from R0 and L
표면분석I 2016 김정원
XPS에 의한 SiO2 박막 두께 측정 결과
SiO2
t OX  Lcosθ ln(R exp /R 0  1)
a-Si
K. J. Kim, Thin Solid Films 500, 356 (2006)
K. J. Kim, Surf. Interface Anal. 39, 512 (2007)
30.0k
0 sec
3 sec
6 sec
9 sec
12 sec
15 sec
20 sec
30 sec
40 sec
50 sec
60 sec
70 sec
80 sec
25.0k
Intensity (cps)
Film Thickness by XPS, ThXPS (nm)
3.0
20.0k
15.0k
10.0k
5.0k
0.0
108
106
104
102
100
98
Binding Energy (eV)
96
94
2.5
m : 1.00 (0.010)
c : - 0.0004 (0.013)
2.0
1.5
1.0
0.5
0.0
0.0
0.5
1.0
1.5
2.0
2.5
Nominal Thickness, Thnom (nm)
0 offset and the same linearity in the sub-nm region
표면분석I 2016 김정원
3.0
Solid in periodic potential
표면분석I 2016 김정원
Angle-Resolved UPS
Band Mapping using Valence Spectra
표면분석I 2016 김정원
Band Structure of Graphite
표면분석I 2016 김정원
ESCA sputter depth profile
단점: ① sample damage
② very slow
표면분석I 2016 김정원
Sputtering conditions
표면분석I 2016 김정원
Photoemission Electron Microscopy
SPEM PEEM
표면분석I 2016 김정원
Photoelectron Spectromicroscopy
Contaminated Coronary Stent
Secondary Electron Image
XPS Spectrum of Contaminated Area
F
F
O
O
C
Cl Si Si F
1000
표면분석I 2016 김정원
800
600
400
Binding Energy (eV)
200
0
Contaminated Coronary Stent
Secondary Electron Image
Carbon Spectrum of Contaminated Area
CF3
CF2O
C-C
CFO
O=C-O
CF
294
표면분석I 2016 김정원
C-O
290
286
Binding Energy (eV)
282
Contaminated Coronary Stent
Fluorocarbon Map
500 x 500um
표면분석I 2016 김정원
Hydrocarbon Map
Summary
XPS Characteristics
Surface sensitive
Accurate quantification
Detection of all elements above He
Provides chemical state information
Can be applied to both inorganic and organic materials
Can be applied to both conductors and non-conductors
UPS Characteristics
More surface sensitive
No quantification
Information on chemical bonds and electronic band structure
표면분석I 2016 김정원
References
1. Electron spectroscopy, theory, techniques, and applications I-IV,
edited by C.R. Brundle (Academic, NY, 1977)
2. Photoemission in Solids I&II, edited by M. Cardona and L. Ley
(Springer-Verlag 1978)
3. G. Ertl and J. Kuppers, Low energy electrons and surface chemistry
(VCH, 1985)
4. D.P. Woodruff and T.A. Delchar, Modern techniques of surface
science (Cambridge, 1986)
5. Practical surface analysis, Vol.I, edited by D. Briggs and M.P.Seah
(Salle & Sauerlander)
6. S. Hufner, Photoelectron Spectroscopy (Springer 1996)
7. J.F. Watts and J. Wolstenholme, An Introduction to Surface Analysis
by XPS and AES (Wiley 2003)
8. J.C. Vickerman and I.S. Gilmore, Surface Analysis, The principal
Techniques, 2nd ed. (Wiley, 2008)
9. Website http://www.xpsdata.com/
표면분석I 2016 김정원