photoelectron spectroscopy · 2020. 1. 29. · why xps? •can be easily applied to a broad range...

www.ccmr.cornell.edu

Photoelectron Spectroscopy Info Talk

1

Dr. Muhammad Salim (Mo)

Research Support Specialist, Cornell Center for Materials Research

Clark D21-F, Cornell University

Email: [email protected]

Tel: 607-255-8549


Why XPS?

• Can be easily applied to a broad range of materials with limited to no sample preparation

• Many types of analysis is possible• Elemental, bonding, spatial, depth, & much more

• Many attachments for unique sample characterization or measurements

• A lot of information can be obtained• Quantitative & Qualitative

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www.ccmr.cornell.edu 3

Diagram of Typical XPS

e-

e-

e-Detector

Photoelectronemission

Electron emission

X-ray emission


Typical XPS Spectra

4

“Survey”

“Wide” “Elemental”

“Low resolution”

“High sensitivity”

“Atomic%s”

“Detailed”

“High resolution”

“Chemical bonding”

“Narrow”


X-Ray Photoelectron Spectroscopy

5

Conduction Band

Valence Band

L2,L3

L1

K

Fermi

Level

Free Electron

Level

Incident X-ray

Ejected Photoelectron

1s

2s

2p

Typical X-Ray excitation sources:

Al Kα1 :: hv = 1486.6 eV

Mg Kα1 :: hv = 1253.6 eV

The ejected photoelectron has kinetic energy:

KE=hv-BE-

Spectral lines are identified by the shell from which the

electron was ejected (1s, 2s, 2p, etc.).

Work function,, of the detector is known and constant

Core

Ele

ctro

ns


Ultraviolet Photoelectron Spectroscopy (UPS)

6

Conduction Band

Core Electrons

L2,L3

L1

K

Fermi

Level

Free Electron

Level

Incident UV


2s

2p

UPS spectral lines originate from electrons in the valence

band

BE=hv-KE-

UV photons originate from differentially pumped He

discharge lamp

He I :: 21.2 eV

He II :: 40.8 eV

HeII is more sensitive to p-bonds over He I

Since electrons originate from valence band, spectra

provides high bonding information

Low KE of incident photons increase surface sensitivity


PES techniques and their information of energy levels

7


Elemental XPS Spectrum


Photoionization Cross Section

• Scofield cross-sections

are proportional rate of

emitted photoelectrons

• Typically the C 1s

transition is given a value

of 1, sometimes F 1s

• Peaks are created with

areas proportional to

Scofield cross-sections


Spin-Orbit Splitting

Gold 4f5/2 and

4f7/2 peaks

•Peak doublets can make analysis

trickier due to:

•Making background choice

more difficult

•Greater likelihood of

interference with other peaks

or artifacts


Quantitative Analysis

•Typically use the peak with largest

RSF value in calculations

•Can use survey scan data or peak

scan data to calculate atomic%, if

taken at Resolution4

•If doublet peaks are close together,

use combined RSF values.

•RSF

• Au 4f7/2 = 9.58

• Au 4f5/2 = 7.54

•Au 4f = 17.12 = sum of both Au

4f peaks

High Resolution Au 4f

Survey Spectra


High Resolution and High Sensitivity Spectra

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High Sensitivity Ca 2p

High Resolution Ca 2p


High Sensitivity Spectra

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• Useful for detecting the presence of very low relative% species that may not show up in Survey spectra

C 1s

O 1s

F 1s

Nitrogen?



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• Useful for detecting the presence of very low relative% species that may not show up in Survey spectra

• Same pass energy as Survey can be used• E.g., 150, 200eV

• Smaller step sizes than Survey• Minimum effective step size depends

on instrument parameters

• 0.2eV

• Longer acquisition times

N1s

With data from spectra on previous slide, Relative %N = 0.15%



• Long Acquisition times• Green: 1 Scan• Red: 60 Scans

• Signal-to-noise (S/N) ratio increases with number of scans

• quantitative improvement is proportional to the square root of the number of scans

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N1s – 1scan

N1s – 60 scans


High Resolution vs High Sensitivity Spectrum

• High Sensitivity:• Collected with high pass energy (e.g, 150eV, 200eV)• Useful for detecting presence of low relative% species in

sample that may not be seen in survey spectra• Larger FWHM of peaks• Should not be used to study chemical bonding• Higher counts/s

• High Resolution:• Collected with low pass energy (e.g., 50eV)• Typically used to analyze chemical bonding of species

• De-convolution of spectrum into contributing peaks/bondings

• Smaller FWHM of peaks• Lower counts/s

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Hemispherical Analyzer

•Analyzer resolution is typically 1%

of the pass energy

•In order to get ~0.3eV resolution,

need a pass energy of ~30 eV

____________________________

•KE of electrons in the analyzer

that hit the center of the detector is

the pass energy (PE)

•Between transfer lens aperture

and analyzer entrance slit, the

kinetic energy of electrons changes

by Vslit, from KE to ~PE

ESCA 2SR: D=7%


High Resolution Spectrum

• There is a redistribution of charge of the

outer electrons when a chemical bond is

formed

•This results in a shift in binding energies

of core electrons

•Many chemical shifts listed in the XPS

handbook and NIST databases (free)


Chemical Shifts- Electronegativity Effects

• Chemical shift (eV) is

proportional to the summation of

nearest neighbor interactions

• There are several

electronegativity scales (Pauling,

e.g.)

• Double-bonds have twice

contribution of single bonds


• Sampling depth of PES is usually taken as 3λcos(θ)—the depth at which

95% of the photoelectrons with normal takeoff angle originate

• θ is the photoemission angle

• is the IMFP (inelastic mean free path of e)

• 63.3% is from 1λcos(θ) depth

• 99% of electrons come from ~5

Default in the system 1 is 55°, system 2 is 0°, both systems are capable of

tilting the sample, changing

• Sampling depth is also substrate and photon energy dependent– Will be discussed in detail later, UPS vs XPS

Information Depth

cos(55)= 0.573


Photoemission angle

Photoelectrons to detector

X-rays from source

Sample surface normal

θ

Photoemission angle (θ), is the angle between the sample’s surface normal and the analyzer.

Θ = 30o

Sample

Photoelectrons to detector

Θ = 0o


•Tilting the sample changes the sampling depth of XPS– Larger photoemission angles, smaller sampling depth

•At high angles, elastic scattering of electrons may be significant

More Surface

Sensitive Less Surface

Sensitive,

greater

electron

escape depth

= 0° photoemission angle,

(90° take-off angle)

= 75°

Angle Resolved XPS


Angle resolved example

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Al2O3

Si-N layer

Si-O layer

Each layer is 2nm thick

θ = 0o

θ = 45o

High-Resolution Si2p


Angle Resolved Example continued..

24

θ = 0o θ = 45o

Al2O3

Si-N layer

Si-O layer

Each layer is 2nm thick

Si-N

Si-O


Depth Profiling/Sputtering/Ion Cleaning

•Utilize as a last resort or necessity

•Sputtering a sample surface can remove impurities

•Depth profiling can be very informative and can produce A LOT of data

•Depth profiling can cause

•Sample damage

•Surface roughening due to varying sputtering rates of elements

•Implantation of sputtering gas


Imaging XPS

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• Mapping of distribution of elements across surface of sample

• Lateral resolution:• ~50um size for very defined features


Imaging XPS Acquisition

• Can map where electrons originate on sample in different ways

• Moving stage, deflecting analyzer

• Analyzer focus spot on sample can be moved, measuring area across sample surface.

• Possible area on sample that can be measured:

• 6x6mm in high magnification

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Non-monochromated source

Large X-ray target area, covers entire sample

Can use deflector to scan sample area. Sample stage doesn’t need to move

To analyzer, detector

Deflector, can choose where electrons that reach analyzer are originating from sample surface

Scans area of surface, collecting electrons at each point


Imaging XPS Acquisition

• Can map where electrons originate on sample in different ways

• Moving stage, deflecting analyzer

• Analyzer focus spot on sample can be moved, measuring area across sample surface.

• Possible area on sample that can be measured is only limited by stage movement

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Monochromated source

Small X-ray spot on sample

To analyzer, detector

Analyzer is only focused on one spot

Stage will need to be moved, to scan a large area on sample


Imaging XPS Example..

Brighter the point, the larger the peak detected at the binding energy for Titanium

Camera

i-XPS taken at Aperture 4 (large spot size, poor spatial resolution)


Imaging XPS Example

• Use of dual source, X-rays cover entire area of sample

• Dual source X-rays hit sample at an angle

• Can create shadows on surface, result of areas not hit by X-rays

• Rough surface, structures on surface, etc.

• Can use larger spot size or larger step size to create a general map of surface, then zoom in to a smaller region with a smaller spot size or smaller step size for detail

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Imaging Spectroscopy Example…

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Material doped with Fluorine gradient across thickness

Side with highest Fluorine concentration

Side with lowest Fluorine concentration

Needed to show gradient exists across entire cross-section


Imaging Spectroscopy Continued..

32

High %Fluorine side

Low %Fluorine side

Moving Analysis Area


Brief Introduction to Ultra-Violet Photoelectron Spectroscopy

33

“UPS He I” “UPS He II”


PES techniques and their information of energy levels

34


Ultraviolet Photoelectron Spectroscopy (UPS)

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Conduction Band

Core Electrons

L2,L3

L1

K

Fermi

Level

Free Electron

Level

Incident UV


2s

2p

UPS spectral lines originate from electrons in the valence

band

BE=hv-KE-

UV photons originate from differentially pumped He

discharge lamp

He I :: 21.2 eV

He II :: 40.8 eV

HeII is more sensitive to p-bonds over He I

Since electrons originate from valence band, spectra

provides high bonding information

Low KE of incident photons increase surface sensitivity


UPS• Low sampling depth and high 2p cross-section

• enhancement of signal from the top


Ultra-Violet Photoelectron Spectroscopy of Styrene

• Probes valence electrons

• Extremely sensitive to any surface contamination, requires very clean samples.

• Even exposing sample to air for a few seconds can have large effects on UPS spectra

• Resulting UPS spectra represents the DOS on the sample surface

• Can be used to find work function of sample

37


UPS Sources and information• Source is a differentially pumped gas

discharge lamp• E.g., He, Ne,

• He I (21.2 eV)• He II (40.8 eV)• Ne I (16.6 eV)• Ne II (26.8 eV)

• Always some amount of He I + He II in spectra, unless removed via other means

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Schematic of UPS measurement of work function (ɸ) of a metal

“UPS He I” “UPS He II”


UPS Example

• Inelastically scattered electrons will still escape into vacuum and be detected as secondary electrons to form background

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UPS Example

• HeI of single crystal Au

UPS Can be used to determine WF of metals

• W-width of emitted electrons

• hv-energy of incident UV

• -work function

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UPS Source comparison

He I

• 21.2 eV emission

• Secondary electron peak at ~15eV

• More bonding information in spectra, but more complicated

• No thorough database

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He II

• 40.8 eV emission

• Larger p cross-section

• Secondary electron peak at ~30eV

• Less bonding information, easier to interpret spectra

• No thorough database


Sample and Handling for XPS Analysis

42


Samples for XPS Analysis•Ideal sample:

•UHV compatible, nothing with high vapor pressure

•Very clean, will discuss sample handling

•Conductive, metals or metal thin films on conducting substrate

•Flat, polished surface (deposited on silicon substrate, e.g.)

•1cm x 1cm square is plenty large• System 1 has 1x2mm spot size

• System 2 has ~3x3mm spot size

•Things to consider:

•Can it be broken or modified for mounting?

•Maximum sample sizes:–System 1, ~75 mm diameter and ~50 mm tall

–System 2, 30 mm wide, 90 mm long


Types of Surfaces

Ideal SurfaceDeposited

Thin Film

Surface

Microstructure Contamination layer

Rough/round Surface-

May get shadowing

effects

Laterally

Inhomogeneous-

Emitted intensity

May vary with

Orientation


Overlayer Effects

a) Copper thin film on gold

b) Heterogeneous structure

c) Buried thick copper layer

between gold

d) Copper substrate beneath

gold


Photoemission of electrons leaves

the sample with a net positive charge

The positive charge makes it more

difficult for electrons to escape the

surface

This results in lower kinetic-energy

photoelectrons and shifts peaks to

higher binding energies.

Non-uniform charging of the surface

can lead to peak broadening

Spectra may need Binding Energy

calibration, using a known peak

position and implementing a shift

Insulating Samples: Sample Charging

Incident X-ray- Ejected Photoelectron

+


Insulating Samples: Charge Neutralization

Grid aids in

keeping electric

field uniform


Surface Contact

• Use non-magnetic, ultra-

clean tweezers to handle

the sample

• Try not to touch the

surface to be analyzed

•Any dust generated can

end up on the sample

surface after going into

vacuum


Use of Gloves

• Plastic ziploc bags and

Aluminum foil often have

an oil film on it to prevent

sticking

•If you must handle the

sample directly, use of

silicone-based, powder-

free gloves is

recommended


Aluminum Foil

CasaXPS (T his string can be edited in CasaXPS.DEF/PrintFootNote.txt)

24oct06b_2.dat

Data Set 2 d

Total Acquisition Time 17.067 (mins) (1000.0 (ms) x 1 x 1024)

Source: Al

Name

Ca 2p

C 1s

O 1s

Si 2p

Al 2p

Pos.

345.36

282.64

530.58

99.38

70.96

FWHM

5.754

3.377

3.746

3.170

3.682

Area

3965.5

119061.1

77774.8

3300.3

3699.3

At%

0.497

75.683

16.873

2.568

4.379

Ca

2p

C 1

s

O 1

s

Si

2p

Al

2p

x 103

5

10

15

20

25

30

35

40

CP

S

1000 800 600 400 200 0Binding Energy (eV)

Reynolds Aluminum foil

In Reynolds wrap:

• Aluminum signal is much lower due to a thicker hydrocarbon layer

•Silicon peaks could be due to silicone-based mineral oil

• Background at high BE indicates presence of overlayer

UHV oil-free Aluminum foil


Surface Plasmon Effects•Photoemitted electrons can interact with surface plasmons and generate

resonance at integer multiples of the plasmon frequency

•This interaction reduces the primary peak intensity and is distributed to the

plasmon peaks

•Seen typically in metals or materials with free electrons (e.g., Si)


Sample Damage Due to Irradiation

• Can perform multiple scans

of the sample over time to

check for degradation or

damage

•Can perform scans at the

start and end of

measurement, to check for

sample damage


CO2 Snow Jet for Sample Cleaning

• Compressed CO2 gas is expelled from a nozzle

• Gas condenses into solid particles, which impacts the surface to be cleaned

• Can be used to remove larger visible particles and particles only a few nm in size

• Can remove polymer films, oils, etc.• E.g., Fingerprints

• Cannot removed adventitious carbon layer

53


CO2 Snow for Sample Cleaning

54

CO2 crystals

• Mechanically dislodges contaminant particles

• Upon impact, liquid film may act as a solvent, dissolving contaminants and removing them when the particle rebounds off surface.


Instrumentation for XPS

Surface analysis by XPS requires

irradiating a solid in an Ultra-high Vacuum

(UHV) chamber with monoenergetic soft X-

rays and analyzing the energies of the

emitted electrons.

“System 1”in Clark D-10

• Ion source• Up to 3” diameter puck• Automated scanning• Air-free sample transfer puck• Sample tilting of 2 samples


Scienta Omicron ESCA-2SR• 30 x 90mm stage• Automated scanning• Air-free sample transfer puck• Sample tilting of entire stage

(compucentric)• Ion source

• Cluster Ion source• Magnesium source• UV source (UPS)• Angle-Resolved XPS• In-situ sample heating >800C• XPS Imaging• >10X higher count rates• 3X better signal to noise due to

preamplifier design“System 2” Scienta Omicron ESCA2SR in Clark D10


Analysis Chamber

Dual Source

Control Panel

Analyzer

GCIBIon Gun

UPS

Prep Chamber

Monochromator

Stage Motors


Why UHV for Surface Analysis?

Degree of Vacuum

10

10

10

10

10

2

-1

-4

-8

-11

Low Vacuum

Medium Vacuum

High Vacuum

Ultra-High Vacuum

Pressure

Torr Remove adsorbed gases from

the sample.

Eliminate adsorption of most

contaminants on the sample.

Prevent arcing and high

voltage breakdown.

Increase the mean free path for

electrons, ions and photons.


Anode: X-ray Source

X-rays are produced

by hitting a metal

anode with high-energy

electrons (5-15keV)

>99.9% of this energy

is dissipated as heat,

therefore anode cooling

is critical

Mono vs Non-Mono

Non-mono, higher

background on spectra


Anode: X-ray Source

Mg, Al, and Cu are common

XPS anodes

Mg has a lower x-ray output

than Al

Al K x-rays can probe to

larger BE’s than Mg

E(Al-Mg) = 233 eV


Non-Monochromated vs. Monochromated X-rays Sources

Non-monochromated x-rays

contain Bremsstrahlung radiation

Contains satellite peaks

Peak width ~0.85 eV

X-rays scatter throughout

chamber, creating photoelectrons

on all surfaces. These

photoelectrons help to neutralize

insulating samples

Greater sample heating may

occur

Larger counts/second detected

Monochromators typical cut the

characteristic x-ray line to ~0.3 eV

No satellite peaks (not even from Kα2)

Focus beam onto sample

Insulating samples require an electron

flood gun to neutralize charge build-up


Monochromated and Non-monochromated X-rays

• Energy of the monochromated line is 1486.70eV

• Only Ka1 emission hits sample

• Energy of the primary non-monochromated Ka1,2 doublet is at 1486.57

• Energy difference is 0.13eV

• Other emissions are also observed• Kα3, Kα4, …, Kβ, etc.

• Each emission produces another peak on the spectra

63


Source of Satellites in Non-monochromated X-Rays

64


Generation of non-monochromated X-rays

• The non-monochromated sources produces Ka1,

as well as the satellite Kα2, Kα3, Kα4, and etc...

lines, but at different relative intensities.

• The Kα-1 and Kα-2 lines are ~0.3eV apart

• FWHM is larger for non-mono source

because of Kα1,2 doublet

• ~0.13eV shift of peaks between mono and

non-mono sources

• For every peak you see in a non-monochromated

source XPS spectra, you will see relative satellite

peaks for it

• Casa XPS has ability to easily remove most

satellite peaks from non-mono sources


• XPS of Silver

• Green: non-monochromated X-rays

• Red: monochromated X-rays

• FWHM difference

• ~0.13eV shift

66

Non-monochromated vs Monochromated AlKα Source

Satellite peak

Ag 3d 5/2Ag 3d 3/2


Non-monochromated Mg X-rays

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Ag 3d 5/2

Ag 3d 3/2

Kα3

Kα4

Kα3

Kα4


Non-monochromated Mg X-rays

68

Ag 3d 5/2

Ag 3d 3/2

Kα3

Kα4

Kα3

Kα4

Non-monochromated Mg X-raysSatellite removal in Casa XPS


Utilizing data processing to extract all information

69

Peak deconvolution example• CasaXPS info session examples:

• Basic CasaXPS options

• Elemental analysis

• Chemical bonding analysis

• Spectra calibration

• Basic and Advanced data processing functions

• Data export

• Please bring your own data if you would like me to use as example


XPS Databases/information

• NIST Database for the Simulation of Electron Spectra for Surface Analysis (SESSA)

• https://www.nist.gov/srd/nist-standard-reference-database-100

• NIST Electron Inelastic-Mean-Free-Path Database: Version 1.2• https://www.nist.gov/srd/nist-standard-reference-database-71

• XPS Data for Selected elements (browser)• https://srdata.nist.gov/xps/main_search_menu.aspx

• Handbook of X-ray Photoelectron Spectroscopy: A Reference Book of Standard Spectra for Identification and Interpretation of XPS Data

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https://www.nist.gov/srd/nist-standard-reference-database-100https://www.nist.gov/srd/nist-standard-reference-database-71https://srdata.nist.gov/xps/main_search_menu.aspx


Thank you!

Questions?

71


Focusing sample

72

• Sample must be focused along vertical z-axis

• Focuses X-ray spot from mono source• Maximizes counts (detected signal)

• An incorrectly positioned sample increases the X-ray spot size and greatly counts


Photoelectron Detection

• Both system 1 & system 2 have

128-channel microchannel plate

detectors

photoelectron spectroscopy · 2020. 1. 29. · why xps? •can be easily applied to a broad range...

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