LEEM BasicsFrank-J. Meyer zu Heringdorf - Universitt Duisburg-Essen - meyerzh@uni-essen.de

PEEM and LEEM History1962 Invention by Ernst BauerGlass-Based Vacuum Apparatus1985 First Operational LEEM Instrument Telieps and Bauer1991 IBM LEEM-I Tromp and ReuterThis Instrument Now in Essen1993 Elmitec LEEMFormer Coworkers of E. Bauer1998 IBM LEEM-IISince Then: LEEM and PEEM has Gained Inreasing Importance: Today More Than 30 Instruments Worldwide (2 in Essen)

OutlinePhotoemission Electron MicroscopyElectron OpticsPhotoemissionExampleLow Energy Electron MicroscopyElectron OpticsDiffraction TechniquesImaging ModesOutlook

Lens Action: Image + Diffraction

Magnetic LensesAs Lenses for OpticsFocal Length

A Few DifferencesImage RotationOnly Convex LensesQuality much Worse !Objective Lens of new S-PEEM in Essen

Basic PEEM OpticsPhotoemissionCreates Electrons Bias VoltageReduces Angle of Photoemitted Electrons, thus increases Numerical ApertureProjector LensesMagnify Image on the ScreenCameraRecords Image to Computer or Video Tapeln-w3.polycnrs-gre.fr/themes/ dichro/PEEM-fr.html

Photoemission of ElectronsSurface IlluminationTypical: HG Lamp (5eV) but also possible Synchrotron Light (ELETTRA, BESSY, CERN) or Laser Light (ESSEN)Electron EmissionAngles depend on Band-Structure of Material and can be quite high. 134.96.28.159/methoden/methoden.html

PEEM Contrast MechanismContrast either by Work Function or DOSPentacene: 5 Benzene RingsContrast Changes During DepositionFoV = 65m1 Layer2 Layers3 LayersF.-J. Meyer zu Heringdorf et al., NATURE 412 (517), 2001

Example for PEEM

Spectroscopic PEEM in EssenPreparation ChamberLEED, Auger ELMITEC PEEM IIIEnergy AnalyzerLN2 Sample Coolinghttp://www.physik.uni-essen.de/SFB616

LEEM: Adding an Electron Gun

LEEM CapabilitiesElectron Energy0 - 100 eV

Field of ViewLEEM: 1 - 20 mPEEM: up to 65 m

ResolutionIn Plane: 5 nmVertical: Atomic Steps Visible

in situ Growth, EtchingRT - 1000C

Low Energy Electron DiffractionElectron Gun VisibleHigh LinearitySpots move with Energyhttp://www.vgsysj.co.jp/sipic.jpgNo Apparent Electron GunSecondary Electrons Spots Dont MoveLEEDLEEM

Microdiffraction, -Spot LEEDLEED on Small AreasPentacene on SiPolycrystalline Layer

The Si(111) 7x7 Surface

Mirror Imaging of Si(111)Electron Energy is 0eVElectrons Return Before they Hit the SampleContrast created by outer PotentialWorkfunctionImage appears Blurred

Bright Field Imaging of Si (111) Different Reflectivity of (1x1) and (7x7) Reflectivity Energy Dependent Contrast Reversal Dependent on Imaging Conditions

Phase Contrast ImagingAnalogue LEEDElectrons from Different Terraces have a Phase Shift at Out-of-Phase ConditionsLocalized Phase ShiftDefocusing of the Image turns Steps into Dark Lines, i.e. Centers of Destructive Interference.

The Si (100) Surface(2x1) and (1x2) DomainsOrientation Changes at each Step

Dark Field ImagingCreator of the LEED Spots Lights up in BrightThe Rest of the Surface Remains Dark

Simultaneous Dark-Field ImagingAu on Si (111)Dominant Low-Coverage Reconstruction: Au-(5x2)Higher Coverage: (5x2) is replaced by sqrt 3 x sqrt 3Nagao et al., Phys. Rev. B. 57, 10100 (1998)5x27x75x25x27x7

Simultaneous Dark-Field ImagingOne Bright-Field Image3 Dark Field ImagesAssign Colors to the DF Images and Overlap Them

Simultaneous Dark-Field Imaging

Conclusions & OutlookLEEM and PEEM Powerful Surface Imaging TechniqueIn Situ Imaging, During Growth PossibleSimilar TEM for Bulk, but for SurfacesMany Different Contrast Mechanisms for Imaging of different Aspects

More Contrast Mechanisms than Mentioned Here:Energy Analyzer and Synchrotron: Spatially Resolved XPS, UPS, Auger, XPEEM

Spinpolarized Gun Allows Magnetic ImagingThanks to: P. Kury, Chr. Seifert, D. Thien, M. Horn-von Hoegen, R. Tromp