Working principles of LEEM-PEEM Lucia Aballe Michael Foerster - PowerPoint PPT Presentation

1 Working principles of LEEM-PEEM Lucia Aballe Michael Foerster Circe Staff and Support: Virginia Perez (NAPP) Carlos Escudero (NAPP) Jordi Prat (technician) Nahikari Gonzalez (Mechanical engineer) Abel Fontsere, Toni Camps (Electronics engineer) Fulvio Becheri (Controls) Josep Nicolas (Optics, transversal section) Eric Pellegrin (Section Head) Michael Foerster LEEM-PEEM introduction 19/67/2015

BL24 - PEEM 2 … it is connected to the synchrotron … it is a LEEM … it uses high Voltage Michael Foerster LEEM-PEEM introduction 19/67/2015

Low energy electron microscope 3 (LEEM) (Bauer, 1962, 1985) Cathode lense or immersion microscopy: electrons are accelerated by electric field between sample and the first lense (Objective). - 20 kV + STV It combines (full field) electron Sample imaging (i.e. high voltage) with a low „interaction“ voltage. Electron Screen gun Slow electrons - 20 kV Fast electrons Michael Foerster LEEM-PEEM introduction 19/67/2015

Start voltage STV 4  The voltage offset STV between the e-gun and the sample is called Start Voltage. It defines the kinetic energy of the electrons arriving at the sample.  Varying the start voltage between -5 until +100 V, different contrast mechanisms are accesible (work function, quantum confinement).  For negative STV, the electrons do not reach the sample. The sample is acting as electrostatic mirror (mirror electron microscopy (MEM)). - 20 kV + V - 20 kV - V LEEM MEM Michael Foerster LEEM-PEEM introduction 19/67/2015

Quantum confinement IV- LEEM: counting atomic layers in graphene P. Merino & J.A. Martin-Gago (ICMM) Michael Foerster LEEM-PEEM introduction 19/67/2015

Aberrations and resolution 6  The spatial resolution is limited by several effects to typically < 10nm in LEEM. (approaching 1 nm in new aberration corrected type)  Spherical aberration: electron far off center of the optical axis are deviating (Contrast Aperture)  Chromatic aberration (in XPEEM): electrons with different energy are deviating (Energy Analyzer)  Diffraction limit Michael Foerster LEEM-PEEM introduction 19/67/2015

Contrast aperture and dark field 7  To reduce the spherical aberration, the Contrast Aperture (CA) is introduced into a backfocal (diffraction) plane  Darkfield imaging : using objective image diffracted electron beam sample plane lens for the image R. Tromp, IBM backfocal plane L. Martin, M. Monti, J. Marco, J. Figuera (IQFR-CSIC, Instituto de Química Física "Rocasolano") Michael Foerster LEEM-PEEM introduction 19/67/2015

LEED  We can also image a diffraction plane, i.e. - 20 kV + STV LEED Sample (energy defined by the start voltage) Electron Screen gun - 20 kV Illumination aperture Michael Foerster LEEM-PEEM introduction 19/67/2015

Illumination aperture 9  In LEED mode we can define the area of which the diffraction pattern is taken (down to 0.5um) by the Illumination aperture (J.I. Flege, et al University of Bremen) Michael Foerster LEEM-PEEM introduction 19/67/2015

LEEM modes and sensitivity 10 Parameter/ Contrast/ Mode tool sensitivity  LEEM  STV  Topography  Projector  Workfunction  MEM settings  Quantum  LEED  Contrast confinement aperture  Structure  u-LEED  Illumination  DF-LEEM aperture Michael Foerster LEEM-PEEM introduction 19/67/2015

From LEEM to PEEM 11  Technically „easy“: just replace e-gun by photons (UV lamp, laser or Synchrotron)  Photo Emission Electron Microscope (PEEM or XPEEM with X-rays) - 20 kV + STV  Slow photoelectrons are Sample accelerated by the HV of X-ray beam the objective lense Electron gun - 20 kV Screen Michael Foerster LEEM-PEEM introduction 19/67/2015

From LEEM to XPEEM 12 hv- f SE CL x 100 VB E F Kinetic Energy (eV)  Under synchrotron X-ray illumination, all kind of electrons come out of the sample, but mainly low energy secondaries Michael Foerster LEEM-PEEM introduction 19/67/2015

From LEEM to XPEEM 13  Adding an electron analyzer and energy slit: spectroscopic LEEM/PEEM (note that STV is sample offset) - 20 kV + STV Screen Energy Slit X-ray beam Slow electrons Analyzer Fast electrons - 20 kV Michael Foerster LEEM-PEEM introduction 19/67/2015

How to select the electrons we want? 14 hv- f SE CL x 100 VB E F Kinetic Energy (eV) Changing STV moves the electron spectrum through the fixed acceptance window (energy slit): STV = kinetic energy of accepted electrons (XPS) Michael Foerster LEEM-PEEM introduction 19/67/2015

Benefits of X+PEEM  Elemental  scanning h n :  Chemical XAS, EXAFS  Magnetic:  scanning STV (const. h n) : XMCD/XMLD XPS  Directional  diffraction mode: (orbitals) nXLD ARPES  Photon energy  Polarization  Kinetic Energy (STV) Michael Foerster LEEM-PEEM introduction 19/67/2015

Spectromicroscopy “images with spectral contrast” “spectroscopy with spatial resolution” (pixel by pixel) L. Martin, M. Monti, J. Marco, J. Figuera (IQFR-CSIC, Instituto de Química Física "Rocasolano") Michael Foerster LEEM-PEEM introduction 19/67/2015

Dispersive plane  Microscope works best with low kinetic energy, at high STV, transmission is much lower  For XPS, we get better statistics and energy resolution when we image the Dispersive Plane of the analyzer and obtain spatial resolution by an aperture in an image plane (Selected Area) Dispersive image on detector SA in image Michael Foerster LEEM-PEEM introduction 19/67/2015

ARPES with selected area  Microscope in diffraction mode for angle resolved Photoemission spectroscopy (ARPES)  Image k x -k y at constant energy (ΔE ca. 200meV)  Spatial resolution by selected area aperture - 20 kV + STV Screen X-ray beam - 20 kV Michael Foerster LEEM-PEEM introduction 19/67/2015

A more complete picture Th. Schmidt et al, Surf. Rev. and Lett. 5 (1998) Michael Foerster LEEM-PEEM introduction 19/67/2015

Surface sensitive Electron Mean Free Path Electron Mean Free Path secondary 10 10 Au Au electrons Mean Free Path (nm) Mean Free Path (nm) Al Al 5 5 Au Au Ag Ag Au Au Mo Mo Ag Ag Au Au Au Au Ag Ag C C C C Au Au W W photo Au Au Ag Ag Be Be Ag Ag 1 1 electrons Be Be Be Be Ag Ag Be Be Ni Be Ni Be Ag Ag Ag Ag P P C C Mo Mo Fe Fe W W 0.5 0.5 Ag Ag Ag Ag Mo Mo Be Be 0.3 0.3 2 2 5 5 10 50 100 10 50 100 500 1000 2000 500 1000 2000 Electron Kinetic Energy (eV) Electron Kinetic Energy (eV) X ray penetration Electron escape >> depth depth Michael Foerster LEEM-PEEM introduction 19/67/2015

Sample environment 21  Requirements: UHV compatible, reasonable large, flat and not too insulating  Standard options: sputter cleaning, heating (>1500 K), cooling (>150K), low pressure gas exposure, in situ metal evaporation  All electronics connected to the sample must float on HV Michael Foerster LEEM-PEEM introduction 19/67/2015

Customized environment @ALBA 22  In situ magnetic fields: OOP, IP, biaxial IP (small)  In situ electrical poling: OOP or in-plane electrodes Based on design from Based on design from SLS BESSY (F. Kronast) Michael Foerster LEEM-PEEM introduction 19/67/2015

Keep in mind  UHV system with sample transfers, many pumps and valves  High voltage between sample and lense (20 kV in ca. 2mm), risk of discharges, clean, flat samples, sufficient degassing (the day before)  Detector overexposure and damage (there is an automatic protection, but it works only when camera is run in the proper way)  Take normalization image Michael Foerster LEEM-PEEM introduction 19/67/2015

The control panel Focus (Objective) Start voltage Stigmastism e-gun intensity Magnification (FOV), Diffraction mode, Sample movement Dispersive mode Sample rotation Michael Foerster LEEM-PEEM introduction 19/67/2015

Thank you‘s Laura Campos User office Sergi Puso, Sergio Vicente, Gemma Rosas (Systems) Salvador Ferrer The speakers: Alba Staff Juan de la Figuera Florian Kronast You! Michael Foerster LEEM-PEEM introduction 19/67/2015