PHOTOELECTRON SPECTROSCOPY (PES)

Law of Photoelectric effectAlbert Einstein, Nobel Prize 1921

Kaiser-Wilhelm-Institut (now Max-Planck-Institut) für Physik Berlin, Germany

INTRODUCTION

High-resolution electron spectroscopyKai Siegbahn, Nobel Prize 1981

Uppsala university, Sweden

Valence band

EB

ωhE

EF

Evac

Detector (Ef)

φ

Ek

Discrete core levels

≈

kinB EE −−= φωh

ωhBE

kinE

φ

Where, = Binding energy referred to the Fermi level= Photon energy= Work function of spectrometer

= Kinetic energy of ejected photoelectron

BASIC CONCEPT

ENERGY REFERENCE

Same level

e-

Fermi level

Spectrometer

Vacuum level

φspec.

.speckinB EE φω −−= h

sampleBkin EE φω −−= h

)( . samplespecsampleBkin EE φφφω −−−−= h

.specBkin EE φω −−= h

Core level

φsample

Fermi level

Vacuum level

EB

Sample

ωh

BASIC REQUIREMENT OF PES SYSTEM

1. A monochromatic photon source.

2. An electron energy analyser (which can disperse the emittedelectrons according to their kinetic energy, and thereby measure the flux of emitted electrons of a particular energy.

3. A high vacuum environment.

LIGHT SOURCES

X-rays

Al K α 1486 eV

Mg Kα 1254 eV

XPS, ESCA

He-lamp 21.2 eV40.8 eV

Ne-lamp 16.8 eV26.9 eV

UV

H-lamp 4 ≤ hv ≤ 12 eV

UPS

Synchrotron radiation PES

SYNCHROTRON RADIATION

•Unique feature of Synchrotron radiation�Ultra-bright light, high polarization�Highly directional, collimated beam�Wide spectrum of wavelength, stretching from IR to X-rays

•Spot size ~1x3 mm2

•High energy resolution

HardX-ray

Ultravioletlight

Visiblelight

Infraredlight

Radiowave

Wavelength (meter)

Atom nucleus

Atom Protein Cell

10 eV4000 eV

BASIC REQUIREMENT OF PES SYSTEM

1. A monochromatic photon source.

2. An electron energy analyser (which can disperse the emittedelectrons according to their kinetic energy, and thereby measure the flux of emitted electrons of a particular energy.

3. A high vacuum environment.

ELECTRON ENERGY ANALYSERS

ELECTRON ENERGY ANALYSER

Ek

Retarding Voltage (Eret.)

Epass

Ek-Eret=Epass

ENERGY RESOLUTION

Schematic view of a spherical deflector analyser. Source and exit slit widths s and w. Sphere radii R1 and R2 and the optical radius Rm = (R1+R2)/2.

Vf

E

N(Ek)

EkinE

IN

N(Ek)

EkinE

∆E

OUT

V f

Epass

ENERGY RESOLUTION

)(2

1

1

2

R

R

R

R

e

EV pass

f −=

2

22α++=∆

mmpass R

w

R

s

E

E

broading a giveAnalyser Q

Energy resolution

Choose:s, w and Epass� small

BASIC REQUIREMENT OF PES SYSTEM

1. A monochromatic photon source.

2. An electron energy analyser (which can disperse the emittedelectrons according to their kinetic energy, and thereby measure the flux of emitted electrons of a particular energy.

3. A high vacuum environment.

WHY ULTRA HIGH VACUUM (UHV) IS NEEDED ?

Consider an ideal gas.The number of gas atoms/molecules striking a unit area of a surface per second is given by;

T

P

R

N

V

NRT

N

NnRTpVBut

M

RTwhere

V

NI

a

a

n

=⇒==

==π

νν 8

4

)(1

105.3)(1

106.28

4 222

2224 torrinp

smMT

px

m

Ninp

smMT

px

MT

pR

R

NI a

n

≈

==π

Ex. Assume an atom of radius of ca. 3 Å. Then there is room for ca. 1015

atom on a cm2.

With p ≈ 10-6 torrM = 28kg/kmol In ≈ 3x1014 (1/cm2s)T = 300 K

If every atom sticks to the surface a monolayer is formed in about 3 sec!.

At p ≈ 10-10 torr a monolayer is formed in about 3x104 seconds≈ 8 hours.Big differences (in sticking probability) between different materials.

(From experience at 10-10 torr Si, Au etc days, Al, Cu etc. hours, Ti, Mo etc. minutes)

AN EXAMPLE

HOW UHV CAN BE OBTAINED?

•Good choice of material selected ex. stainless steel

•Good design of chamber

•Good pumping systems

•Long time baking

On a larger scale, the sample surface is hit with photon flux focused in a small spot. Electrons emitted by the photon are from ca. 10-100Å of the surface layers depending on electron escape depth in the sample investigated.

SAMPLE SIZE?

Electrons : give a high surface sensitivitySinceλ = mean free path is small in solids

λ = average distance an electron travels before it is inelastically scattered, i.e. Suffers energy losses

λ(Ek) increases monotonically with Ekin (for Ekin ≥ 50 eV)

SENSITIVITY

The flux of electrons of a given energy and momentum decaysexponentially with the distance travelled in a material.

I0I

x

θ

θcos

x

θλ cos0

⋅−

=x

eII

λx

eII−

= 0

Small λ gives high surface sensitivity

Electrons that have not beeninelastically scattered have beengenerated close to the surface.

The surface sensitivity can be enhanced by selecting a larger electronemission angle, θ. Normal emission , θ=0o.

ELECTRON’S FLUX

A SIMPLE LAYER ATENUATION MODEL

d d d

Io Io Io

θ

θλ

λ

λλλ

cos

0

0

)3

()2

()(

0

1

1

1

1

.....)1(

dtot

dtot

ddd

tot

e

II

e

II

eeeII

−

−

−−−

−=

−=

=++++=

1cos

cos

0

−=

=

=

−

θλ

θλ

d

bulk

Surf

d

totbulk

Surf

eI

I

eII

IId d d

Io Io Io

θ

d

Surf layer

Normal emission

Angle dependent

z 0d

. 1 ,

overlayer from )1(

overlayer no withi.e.

0

0

dthicknessofoverlayerwitheII

Thus

toponoverlayerwhenebyattenuatedissignalsubstrateThe

ecdzceI

cdzceI

d

Subst

Overl

d

dd

z

Overlayer

zCleanSubstrate

−=

−=∝

=∝

−

−−

∞ −

∫

∫

λ

λ

λλ

λ

λ

λ

Photoelectron Spectroscopy (PES) + Angle resolved photoelectron spectroscopy (ARPES)

Examine core-level hv ≈ 100-4000 eV

•Elementals annalysis

•Quantitative analysis

Thickness

Location

Examine valence levels hv ≈ 10-50 eV

•Surface band structure

•Density of state (DOS)

UNIQUE ADVANTAGE FOR PES TECHNIQUES

KINETIC ENERGY (eV)

ELEMENTALS ANALYSIS

Binding Energy 2s = 117eV Binding Energy 2p = 73 eV

Binding Energy 2s = 149 eV Binding Energy 2p = 99 eV

EFB exhibits chemical shifts (~1-10 eV)

depending on chemical surrounding�Information about the surrounding elements can be obtained.

In solids a specific signal from the atoms in the outermost surface layers can be obtainedsince they feel a different surrounding(potential) than atoms in the bulk. Surface shiftes < 1 eV.

Chemical shifts

Surface shifts

KINETIC ENERGY (eV)

QUANTITATIVE ANALYSIS

4f

5d

5p

5s

PHOTOIONIZATION CROSS SECTIONS

Graphitelike carbon

C 1s SiC

286 284 282Binding Energy (eV)

θe=75o

θe < 75o

Int e

nsit y

(re l

ativ

e un

its)

CHARACTERIZATION

SiO2

SiC

Carbon ?

)(

)(;

)(

)( 2

SiCI

SiOISi

SiCI

gIC ==

Emission angle0 20 40 60

80

Si

C0

2

4

Rat

ios

C/Si x 20

SiO2

C=0.05 Å

25.5 Å

SiC

)(

)(;

)(

)( 2

SiCI

SiOISi

SiCI

gIC ==

)1(

)1(

cos

coscos

2 −⋅=

−⋅⋅=

eSi

ox

eC

g

eC

ox

d

SiSiC

SiSiO

dd

CSiC

Cg

eC

CSi

eeC

CC

θλ

θλθλ

+

⋅⋅⋅⋅= 1ln

AA

BB

B

A

C

C

I

IOxide

σσλ

+

⋅

⋅⋅⋅⋅=

−

1ln1

λ

σσλ

d

CC

BB

B

C eC

C

I

IGraphite

Thickness

Layers attenuation model

A:=oxide, B:= bulk SiC , C:= graphite

TECHNIQUES

ADVANTAGES

•Good energy resolution (chemical shifts & surface shifts)•Semiquantitative analysis, within a few %, fairly easy

DISADVANTAGE

•Poor spatial resolution (in most cases).

PHOTOELECTRON SPECTROSCOPY