Dr Anton Tadich Australian Synchrotron 2nd AOFSRR Summer School Cheiron School 2008 Australian Synchrotron
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• "APPLE II" undulator: variable polarisation: linear (horizontal to vertical), circular (left and right) • Photon energy range 90 – 2500eV; E/ΔE  = 5000 to 10000 • Current endstation: NEXAFS (TEY, PEY, TFY), XPS (SPECS Phiobos 150 HSA), cleaving facility, sample heating and cooling, preparation chamber, glovebox... 3
• 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/
• The continual miniaturization of devices and material structures demands novel methods for nanoscale 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 5 (a) NEXAFS (b) XPS Near Edge X-ray Absorption Fine Structure (NEXAFS) Spectroscopy • NEXAFS and XPS are well established techniques for surface chemical characterisation • Traditional instruments integrate spectral information over the photon-illuminated area • How can we extend these methods to spatially discriminate on the nanoscale? 6
•  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 those featuring parallel acquisition (spectromicroscopy)
Microspectroscopy
➤ Highly focused probe-beam (KB mirror or zone plates) ▷ Sequential detection (x-rays, electrons)
Spectromicroscopy
>   "Large" area probe-beam A Parallel imaging (electrons) 7
Scanning X-ray Transmission Microscopy (STXM) . X-rays focused using zone plate optics. Sample is rastered at the focus, and the transmitted intensity measured . Chemical/elemental information is obtained by determining 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 X- ray energies of interest Experimental geometry of a STXM experiment 8
A SPEM instrument combines zone plate focussing optics and a conventional electron energy analyzer to perform laterally-resolved photoemission spectroscopy uəds/S♭'EOS'OTT'OやI //:d);y :/sə;unoo əɓɐ •  High energy resolution is achieved due to well-established spectrometer design • Poorer spatial resolution than STXM: combined geometry of illumination and detection yields a resolution of ~ 200nm for modern instruments . 9
. 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) 10
• 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 – 100μm) • 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
http://xraysweb.lbl.gov/peem2/webpage/Project/TutorialPEEM.shtml
• The spatial resolution of a PEEM instrument is degraded be reached with X-ray energies Image courtesy: S Anders et al, Rev. Sci. Instr, 70(10) (1999), 3973
Etching pits in Si(001) wafer 140μm Cu on Mo (110)
Specifications
• Field of view: 2 – 150μm; best resolution <15nm ●  Sample temperature: RT to 1800K • Electron energy analyser can be added for spectroscopic imaging
•  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. Schematic and photo of SPELEEM
. 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 an element will enhance secondary electron yield for element specific imaging Fine structure in the absorption edge (NEXAFS) can be investigated with PEEM, yielding chemical structure
● 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 e- beam lithography patterning http://xraysweb.lbl.gov/peem2/webpage/Project/TutorialContrast.shtml ● PEEM image of LaFeO₂ thin film, generated by tuning photon energy to the La M5 edge (60s exposure time) 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 16
• PEEM is able to resolve ferromagnetic domains, and determine the size and orientation of magnetization vectors, via X-ray Magnetic Circular Dichroism (XMCD) • Extra contrast in XMCD-PEEM image can obtained by subtracting two images measured at a fixed photon energy, but with opposing helicity vectors XMCD difference-image of ferromagnetic MnAs on GaAs. Ferromagnetic/paramagnetic striping observed during growth Black/white areas are opposing ferromagnetic domains. Grey areas are paramagnetic, and show no change upon altering the helicity. 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 Data taken from: Engel-Herbert et. al, J. Magn Magn. Mater, 305 , p457
• Usually performed with UV sources, e.g. Hg lamp (hv = 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 to strongly varying electron yield. • Due to smaller energy range from sample, UV-PEEM has higher resolution (10nm) UV-PEEM image of polycrystalline Cu. Individual grains of  Cu(100), Cu(110) and Cu(111) can be distinguished due to work function differences between the faces
•  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. μ-NEXAFS of nanoparticles and substrate indicates contrast is not due to X-ray absorption (elemental) differences
G Biasiol et. al, Appl. Phys. Lett,  87 (2005) p.223106 • Understanding of the quantum-confined energy states in self-assembled In x Ga(1-x)As/GaAs quantum dots (QD) require an accurate knowledge of the QD shape and composition distribution ● Previous work has obtained QD cross sectional composition using Energy Selective Imaging (ESI) in a TEM Cross sectional In concentration (x) for InçGa 1-x As QD, obtained by ESI • 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 l '(TOOZ) (TT)98 '4197 ләу sлца
Results
●  In 4d PEEM image clearly shows increased concentration of In at location of QDs • Remnant Ga intensity at QD locations due to alloying from substrate GaAs => QDs are actually In çGa 1 çAs, as expected •  Quantitative In concentration map indicates higher than expected values at QD centres (x = 0.9) •  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 0.85 In concentration (x) map (x =In/In+Ga)
All electron-optical systems exhibit image degradation effects: DiffractionんCan be reduced by using high electron energy and large apertures AstigmatismんCaused by mechanical misalignment and tolerances. In a PEEM, corrected using "stigmator" and "deflector" electrodes.
Spherical aberrations
んElectrons entering regions of different field curvature will be focussed to different degrees. By limiting the acceptance angle of electrons, thus keeping electrons near-axis, aberrations are reduced.
Chromatic aberrations
んCaused by the energy spread of electrons. Energy foci are spread along principal axis. Small if the energy range is restricted. The radius of the disc of confusion from the objective is given by: The contribution of each aberration: Diffraction Spherical aberrations Where: α = acceptance angle λ = electron wavelength (nm) Eo = Start Energy (eV) ΔE = energy spread (eV) Chromatic aberrations
For α < 20 mrad, resolution is diffraction . limited For α  ≈ 20 - 400mrad, resolution is . dominated by chromatic  aberrations • For α > 400 mrad, resolution is dominated by the spherical aberrations • Clearly, improvement is needed for resolving structures less than 5nm in size!
Notes:
▷ Rapid increase in transmission as a function of α > One optimal aperture for best resolution
Figure courtesy of: Th. Schmidt et. al Surface Review and Letters , 9 (2002), no. 1, 223–232.
25 . 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 high electric field and focussing in the objective lens • Although the contrast aperture in the back focal plane can be adjusted (reduce α) to reduce the chromatic/spherical aberrations, it is at the expense 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 use of an electron tetrode mirror.  Goal is to obtain an order of magnitude 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 BESSY2
• A tetrode electron mirror is able to induce aberrations of the opposite sign to those created in the objective lens of a PEEM •  Adjustment of the 3 potentials allows correction of a range of aberration coefficients (C s , C c ) corresponding to different imaging conditions (E, ΔE, etc..)
• 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 • Reflection occurs in the mirror, with the image then diverted to the exit of the beam separator for subsequent imaging by projection optics 28
SMART project PEEM3 Image courtesy: Schmidt, Th et.al. , Surf. Rev. Lett, 9(1) (2004) 223 Image courtesy: J. Feng et. al, J. Phys. Condes. Matter, 17 (2005) S1339

Resolution Goals (XPEEM)
50nm resolution at a transmission of 100% . . 4nm resolution at a transmission of 1-2%
An intermediate version of the instrument is being commissioned while the separator and corrector are completed offline Images courtesy: MacDowell A.A e al, Synchrotron Radiation Instrumentation: 9th International Conference (2007), 1341
Ω–filter: energy filtering, dispersive energy plane imaging . 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
Specifications ●  Imaging modes: PEEM, LEEM, LEED, MEM.... • Calculated best resolution <3nm • Up to 8 times the intensity at a given resolution compared to previous "PEEMIII" (NOT PEEM3)
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) 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) Feng, J et. al, Journal of  Physics C: Condensed Matter, 17, S1339 (2005) Reviews and basics of PEEM Aberration Correction PEEM3/SMART