PEEM and Nanoscience
Dr Anton TadichSoft X-ray Beamline
Australian Synchrotron
Email: Anton.tadich@synchrotron.org.au

Source: www.sljus.lu.se/staff/anders/peem_pic.html

1The Australian Synchrotron

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The Soft X-ray Beamline

Vertical Exit Slit

Horiz. Exit Slit

Vertical Focus Mirror• “APPLE II” undulator: variable polarisation: linear (horizontal to vertical), circular (left and right)• Photon energy range 90 – 2500eV; E/DE = 5000 to 10000• Flux: 5 x 1012 photons/sec/200mA; plane grating @1200l/mm (also 250l/mm high flux grating)3

The Soft X-ray Beamline• Current endstation features XPS andNEXAFS capabilities (simultaneous AEY,
TEY, TFY, PEY)

• Phoibos 150 Hemispherical Analyser• Sample cleaving, cooling, heating, LEED,
K-cells, gas introduction/dosing• New user hutch, air conditioned!

• PEEM???

The Growth of Nanotechnology• Nanotechnology represents a rapidly expanding sector of materials technology• There has been a 279% increase in nanotechnology-based products since 2006
• Predicted worldwide turnover of over 1 trillion Euro by 2015.

Source: http://www.nanotechproject.org/inventories/consumer/analysis_draft/

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Characterisation at the Nanoscale• The continual miniaturization of devices and material structures demands novel methods fornanoscale investigations of surfaces, thin films and interfaces• There are a number of tools which excel in either spatial or chemical investigations“Real Space” structure (nm and below)
Scanning and Transmission Electron Microscopy(SEM, TEM), Atomic Force Microscopy (AFM),
Scanning Tunneling Microscopy (STM), …

Morphology, geometric structure, atomic
positions and surface reconstructions“Chemical” information
X-Ray Absorption Spectroscopy (NEXAFS,
EXAFS) Photoelectron Spectroscopy (XPS, UPS)
Infrared (IR), NMR, …

Elemental information, chemical
environment, adsorbate geometry,bonding configuration, oxidation state,magnetic properties etc..

• An understanding of the properties of novel systems benefits greatly from spatially-resolved
spectroscopic measurements with “nanometer” resolution
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Image courtesy: www.theochem.kth.se• NEXAFS and XPS are well established techniques forsurface chemical characterisation• Traditional instruments integrate spectral information overthe photon-illuminated area

• How can we extend these methods to spatiallydiscriminate on the nanoscale?

www.physics.ncsu.edu/stxm/NEXAFS.jpg

http://www.emal.engin.umich.edu/instruments/resources/XPSsurvey.gif

7Soft X-ray Microscopy• Soft X-ray based spectroscopic microscopy using synchrotron radiation is increasingly the main
tool for laterally-resolved spectroscopy on the nanoscale• One can distinguish methods using sequential acquisition (microspectroscopy), and thosefeaturing parallel acquisition (spectromicroscopy)

Microspectroscopy
 Highly focused probe-beam (KB mirror orzone plates) Sequential detection (x-rays, electrons)

Spectromicroscopy “Large” area probe-beam Parallel imaging (electrons)

Images courtesy: www-als.lbl.gov/als/workshops/scidirecthtml/4Magnetic/magnetic.html

8Some examples of microspectroscopy
Scanning X-ray Transmission Microscopy (STXM)•

X-rays focused using zone plate optics.
Sample is rastered at the focus, and thetransmitted intensity measured•

Chemical/elemental information is obtained bydetermining the X-ray absorption by the
sample (NEXAFS)•Can obtain NEXAFS spectra at fixed position,or single image at fixed energy•

Spatial resolution of 30-50nm. Depth
information possible.

•

Samples must be partly transparent at the Xray energies of interest

Experimental geometry of a STXM experimentImage courtesy: srs.dl.ac.uk/XRS/diamond/SpectroMicroscopy

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Scanning Photoemission Microscopy (SPEM)
Scanning photoelectron emission spectroscopy (SPEM)• A SPEM instrument combines zone plate focussing optics and a conventional electron energy
analyzer to perform laterally-resolved photoemission spectroscopy

Image courtesy: http://140.110.203.42/spem.htm

Schematic of U5-SPEM
instrument, NSRRC.

• High energy resolution is achieved due to well-established spectrometer design• Poorer spatial resolution than STXM: combined geometry of illumination and detection yields aresolution of ~ 200nm for modern instruments

10Disadvantages of scanned-mode acquisition•

The probe is a focussed x-ray beam. The spatial resolution is capped at the diffraction limit of
the x-rays•

The focal length of zone plates are a function of x-ray energy: One must displace the sample
as the photon energy is changed•

More time consuming than parallel acquisition; time-resolved studies limited

Parallel imaging with photoelectrons: advantages•

Spatial resolution is only limited by aberrations in electron-optical imaging system. Can be
improved by design•

Rapid measurement: video rate imaging, time resolved studies (e.g. “pump-probe” experiments)

The most popular type of instrument is the Photo Emission Electron Microscope (PEEM)
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Basic Principles of PEEM
• In PEEM, the sample is homogeneously illuminated with soft X-rays or ultraviolet (UV) radiation
with a spot size ~ field of view of the microscope (1 – 100mm)• An objective lens operating at high potential (10-30kV) is used to extract the emitted electrons,
focussing them to an intermediate image at a back focal plane.
• Projector and transfer optics are used to further
magnify the intermediate image onto a 2D detector

Detector

Transfer/Projector

Objective

http://xraysweb.lbl.gov/peem2/webpage/Project/TutorialPEEM.shtml

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The Photoelectron Energy Distribution

Valence Band
Secondary Electron
Tail

Auger

• Primary peaks (Auger and core level)
are unscattered, and are usefulchemical fingerprints (XPS, AES)

Core Level Peaks
Auger

Core Level Electrons• Photoelectrons exhibit a wide range
of energies• Inelastic scattering of electrons leads
to featureless, intense “secondary
electron” tail.

Auger Electrons•

Electrons excited from occupied core levels•

Produced via relaxation of the core hole•

Characterized by their binding energy via:•

Kinetic energy is independent of photon energy.

•

Another elemental fingerprint

Ekin = hn – EB – f•

Binding energies are element specific=> elemental fingerprint.

•

Peak shape yields chemical states as shiftedcomponents etc.

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PEEM2 (ALS)• The spatial resolution of a PEEM instrument is degradedby the energy spread of the transmitted electrons(chromatic aberrations)• PEEMs contain a contrast aperture to restrict theenergy & angular spread of electrons, allowing
tradeoff between resolution and transmission

Image courtesy: J Stöhr et. al, IBM J. Res. Develop, 44(4), (2002), p. 535

PEEM2 microscope (ALS); a resolution of 20nm canbe reached with X-ray energies
Image courtesy: S Anders et al, Rev. Sci. Instr, 70(10) (1999), 3973

• The aperture is typically adjusted enhance
the transmission of secondary electrons

Images14

Commercial PEEM: Elmitec “PEEM III”
Etching pitsin Si(001)
wafer

140mm
Cu on Mo (110)Specifications• Field of view: 2 – 150mm; best resolution <15nm• Sample temperature: RT to 1800K• Electron energy analyser can be added forspectroscopic imaging(Images courtesy www.elmitec-gmbh.com)

5mm

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SPELEEM: Spectroscopic Photoemission and Low Energy Electron Microscope• This instrument combines the structural sensitivity of Low Energy Electron Microscopy(LEEM, resolution <5nm) with the flexible spectroscopy of energy-filtered PEEM.
• Separation between incoming and outgoing electron beams achieved with a magnetic prism• Electron microscopy, X-ray photoemission microscopy and diffraction (angular) mapping
can be performed on the same sampling area => multi-technique approach to studies.

Image courtesy: Locatelli, A et. al, J. Phys. Condens. Mater, 20 (2008), 3

Schematic and photo of SPELEEM

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Elemental and Chemical Contrast
180000

VB

160000hn = 280eV

140000hn = 320eV• Tuning the x-rays through the thresholdbinding energy will lead to creation ofAuger electron (predominant relaxation
process for lower Z elements)

S2p1/2, 3/2

Intensity (a.u)

12000010000080000

Cu 3p1/2, 3/2

Cu Auger
S 2s

60000

Cu 3s

• Auger electron scattering contributes tosecondary electron signal

C Auger

40000
20000

0

Carbon K-Edge NEXAFS (Auger Electron Yield)

1.5
250

200

150

100

50

0

1.4

Binding Energy (eV)XPS survey scans below (280eV) and above (320eV) C1s binding energy,
for a monolayer of Methionine on Cu(421)=>NEXAFS signal

C: 1s -> s*

1.3

Intensity (a.u)• Monitoring the electron yield at the Auger
energy or secondary electron backgroundC: 1s -> p*

1.2
1.1
1
0.9
0.8

0.7
0.6
280

Data courtesy A Tadich, L Thomsen, M Gladys, Australian Synchrotron

285

290

295

300

hn (eV)

305

310

315

320

Elemental and Chemical Contrast• Generation of core holes upon x-ray absorption leads to creation of Auger electrons. Inelastic
scattering of the Auger electron contributes to the secondary electron background• The Auger/secondary electron yield is proportional to the x-ray absorption. At an absorption
edge, secondary electron intensity will increase. This is a large, accessible signal for PEEM.
• Regions of the sample containing that element will appear brighter => element selective
contrast. Tuning through several photon energies allows for NEXAFS analysis => chemistry

Tuning to the absorption edge of anelement will enhance secondary electron
yield for element specific imaging

Fine structure in the absorption edge
(NEXAFS) can be investigated with
PEEM, yielding chemical structure

Images courtesy: J Stöhr et. al, IBM J. Res. Develop, 44(4), (2002), p. 535

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Examples of elemental and chemical imaging• Nickel+photoresist test
pattern for PEEM2 project(ALS).
• With photon energy tuned
to Nickel L3 edge, nickel
areas appear bright• Loss of detail in inner rings
at 30nm due to limit of ebeam lithography patterning

100nm spoke width
http://xraysweb.lbl.gov/peem2/webpage/Project/TutorialContrast.shtml• PEEM image of LaFeO3 thinfilm, generated by tuning
photon energy to the La M5edge (60s exposure time)

20nm

Vertical features are tracks
generated by electrical discharge
on sample surface

Data taken from: Scholl, A. Current Opinions in Solid State and Materials Science, 7 (2003), p59

19• TiSi2 exists in 2 structural phases: low
conductivity C49, and high conductivity C54.
Bonding configuration yields different
NEXAFS spectra for the phases (right)• XPEEM images of a micropatterned TiSi2
sample, taken at 3 photon energies about
the Ti L3 edge

Ti L3 edge
459eV

(445eV)

455eV• NEXAFS-XPEEM results yield location of
C49 phase relative to C54 phase

80mm

Data reproduced from S Singh et. al, Appl. Phys. Lett, 71 (1997), 55

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Magnetic Contrast• PEEM is able to resolve ferromagnetic domains, and determine the size and orientation ofmagnetization vectors, via X-ray Magnetic Circular Dichroism (XMCD)XMCD: The absorption of circularly polarised
light by ferromagnetic domains depends upon
the relative orientation of the domain’s
magnetic moment and photon helicity vector
Image courtesy: J Stöhr et. al, IBM J. Res. Develop, 44(4),(2002), p. 535• Extra contrast in XMCD-PEEM image can obtained bysubtracting two images measured at a fixed photon energy,
but with opposing helicity vectors

hn

XMCD difference-image of ferromagnetic MnAs on GaAs.
Ferromagnetic/paramagnetic striping observed during
growthBlack/white areas are opposing ferromagnetic domains.
Grey areas are paramagnetic, and show no change uponaltering the helicity.

5mm

Data taken from: Engel-Herbert et. al, J. Magn Magn. Mater, 305, p457

Work Function Contrast• Usually performed with UV sources, e.g. Hg lamp (hn = 4.9eV) => “UV-PEEM”• Applied to metals, this technique leads to a narrow energy range
excited from the Fermi Energy, depending on work function• Variations in the work function across the sample surface leads tostrongly varying electron yield.
(100)

(111)

• Due to smaller energy range fromsample, UV-PEEM has higher
resolution (10nm)

UV-PEEM image of polycrystalline Cu.
Individual grains of Cu(100), Cu(110) andCu(111) can be distinguished due to work
function differences between the faces
O Renault et al, Surf. Interface. Anal, 38 (2006), 375

(110)

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Topological Contrast• For rough samples, the electric field around topographical features is
distorted; localised focusing causes contrast in the final image• Shadowing effects also provide an additional contrast mechanism

X-ray PEEM (X-PEEM) of TiO (rutile-phase) nanocrystals
embedded in a TiO (anatase-phase) film. Contrast is due
to nanocrystal topography.
Xiong, G et al . Advanced Functional Materials, 17, pp 2133-2138, 2007

m-NEXAFS of nanoparticles and substrate indicatescontrast is not due to X-ray absorption(elemental) differences
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Case Study: Surface Compositional Gradients of InAs/GaAs quantum dots
G Biasiol et. al, Appl. Phys. Lett, 87 (2005) p.223106• Previous work has obtained QD crosssectional composition using Energy
Selective Imaging (ESI) in a TEM

Cross sectional In concentration (x) for
InxGa1-xAs QD, obtained by ESI(Images courtesy: Walther, T. Phys Rev Lett. 86(11) (2001), p2381• .Understanding of the quantum-confined energy states inself-assembled InxGa(1−x)As/GaAs quantum dots (QD)
require an accurate knowledge of the QD shape andcomposition distribution• Measurements on the top down (lateral) distribution and concentration of In were needed• Energy filtered XPEEM spectra were acquired using SPELEEM at ELETTRA• Localised photoemission spectra and image maps were obtained to quantitatively
determine the lateral In and Ga concentrations
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Results

In4d (76.25eV)

Local Spectra of island and wetting layer,
obtained using 25 x 25nm integration area

• In 4d PEEM image clearlyshows increased concentrationof In at location of QDs• Remnant Ga intensity at QD
locations due to alloying fromsubstrate GaAs => QDs are
actually InxGa1-xAs, as
expected• Quantitative In concentration
map indicates higher than
expected values at QD centres(x = 0.9)

Ga 3d (74.75eV)• Due to surface sensitivity of
PEEM, this is most likely due to
In segregation to the surface of
the QD. This has not been
resolved in ESI measurements
X-PEEM images taken at In4d (upper)
and Ga3d (lower) core levels, for
InAS QDs grown on GaAs(100)In concentration (x) map (x =In/In+Ga)

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Next Generation PEEM: Pure Aberration Correction
All electron-optical systems exhibit image degradation effects:
DiffractionCan be reduced by using high electron energy and large apertures

AstigmatismCaused by mechanical misalignment and tolerances. In a PEEM, corrected using “stigmator”
and “deflector” electrodes.

Spherical aberrations
Electrons entering regions of different field curvature will befocussed to different degrees. By limiting the acceptance angleof electrons (the contrast aperture), thus keeping electrons
near-axis, aberrations are reduced.

Chromatic aberrations
Caused by the energy spread of electrons. Energy fociare spread along principal axis. Small if the energy
range is restricted.
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The radius of the disc of confusion from the objective is given by:2dd  d d2   s   d c2
 2
The contribution of each aberration:

Diffraction

0.61
dd 
sin 

Where:

 = acceptance angle
 = electron wavelength (nm)E0 = Start Energy (eV)Spherical aberrationsd s  Cs sin 3 Chromatic aberrations

d c  Cc

DE = energy spread (eV)

DE
sin 
Eo
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The resolution limit calculated for a typical objective lens:• For  < 20 mrad, resolution is diffraction
limited• For  ≈ 20 - 400mrad, resolution isdominated by chromatic aberrations• For  > 400 mrad, resolution is dominatedby the spherical aberrations• Clearly, improvement is needed for resolving
structures less than 5nm in size!
Notes:

 Rapid increase in transmission as afunction of  One optimal aperture for best
resolution
Figure courtesy of: Th. Schmidt et. al Surface Review and Letters , 9 (2002), no. 1, 223–232.

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• Modern TEM and SEM instruments are now corrected for chromatic and spherical aberrations.
Progress in LEEM/PEEM has been slower.
• Majority of the aberrations are caused by the highelectric field and focussing in the objective lens

• Although the contrast aperture in the back focal
plane can be adjusted (reduce ) to reduce thechromatic/spherical aberrations, it is at theexpense of transmission

Image: J. Feng, Rev. Sci. Instr, 73, no. 3 (2002), 1514

• For PEEM2, 20nm resolution occurs at 1% transmission!
How can we cope with radiation sensitive samples???
• A major effort has been to remove instrinsic aberrations via useof an electron tetrode mirror. Goal is to obtain an order ofmagnitude in resolution for given transmission.
• 2 projects are heading toward completing an aberration
corrected PEEM: the “PEEM3” project at the ALS, and the“SMART” project at BESSY2Image: Wu, Y.K, Nucl Instr. Methods. Phys. Res. A, 519 (2004), 230

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How does an electron mirror correct aberrations?

Some commercial PEEM instruments• A tetrode electron mirror is able to induce aberrations of the opposite sign to those created in
the objective lens of a PEEM

Spherical aberration correction

Chromatic aberration correctione-• Adjustment of the 3 potentials allows correction of a range of aberration coefficients(Cs, Cc ) corresponding to different imaging conditions (E, DE, etc..)

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Incorporating the electron mirror: PEEM3• A magnetic beam separator is used to deflect the electrons into the mirror• The (aberrated) image is formed by the objective lens at the entrance to the beam separator

Design challenge: the separator
needs to designed nearly free ofaberrations in magnetic field

Layout of PEEM3, indicating
magnetic beam separator andelectron tetrode mirror
Image courtesy: J Feng et.al, J. Phys. Condens. Mater, 17, S1339, 2005

• Reflection occurs in the mirror, with the image then diverted to the exit of the beam separator for
subsequent imaging by projection optics
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Simulations of resolution gain using the tetrode mirror
SMART projectImage courtesy: Schmidt, Th et.al., Surf. Rev. Lett, 9(1) (2004) 223

PEEM3

Image courtesy: J. Feng et. al, J. Phys. Condes. Matter, 17 (2005) S1339

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Design vision of PEEM3 at the ALS

Resolution Goals (XPEEM)• 50nm resolution at a transmission of 100%
• 4nm resolution at a transmission of 1-2%

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Progress on PEEM3
An intermediate version of
the instrument is beingcommissioned while theseparator and corrector arecompleted offline

Electron optics

Mu-metal shielding:
3 layers!
Space for corrector

Sample Manipulator
Images courtesy: MacDowell A.A e al, Synchrotron Radiation Instrumentation: 9th International Conference (2007), 1341

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SMART (Spectro Microscope For All Relevant Techniques): BESSY2
W-filter: energy filtering, dispersive energy plane imaging

LEEM optics• The SMART allows for interchange of energy filtered diffraction or real image planes,
permitting angular distribution measurements e.g. for k-space mapping, PED, Fermi Surface

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Elmitec are prototyping an aberration corrected
LEEM/PEEM: The “AC LEEM/PEEM”
Specifications• Imaging modes: PEEM,
LEEM, LEED, MEM….
• Calculated best resolution <3nm
• Up to 8 times the intensity at agiven resolution compared toprevious “PEEMIII” (NOT PEEM3)

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Some useful references
Tonner, B.P et. al, Journal of Electron Spectroscopy and Related Phenomena, 75, 309 (1995)
Anders, S et. al, Review of Scientific Instruments, 70(10), 3973 (1999)

J Stohr et. al, IBM. J. Res. Develop, 44(4), 535 (2000)
Scholl, A et. al, Current Opinion in Solid State and Materials Science, 7, 69 (2003)

Locatelli, A et. al, Journal of Physics C: Condensed Matter, 20, 1 (2008)

Reviews
andbasics of
PEEM

Schmidt, Th et. al, Surface Review and Letters, 9(1), 223 (2008)
Wu, Y.K et. al, Nuclear Instruments and Methods in Physics Research A, 519, 230 (2004)
Wan, W et. al, Nuclear Instruments and Methods in Physics Research A, 519, 222 (2004)
Schmid, P et. al, Review of Scientific Instruments, 76, 023302 (2005)

Aberration
Correction
PEEM3/SMART

Feng, J et. al, Journal of Physics C: Condensed Matter, 17, S1339 (2005)

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