Electron Spettroscopies

Transcription

1 Electron Spettroscopies Spettroscopy allows to characterize a material from the point of view of: chemical composition, electronic states and magnetism, electronic, roto-vibrational and magnetic excitations. Determination of the chemical composition: ) Secondary electrons: AES (Auger Electron Spectroscopy) ) Photoemission XPS (X-rays Photoemission Spectroscopy) or ESCA (Electron Spectroscopy for Chemical Analysis), UPS (Ultraviolet Photoelectron Spectroscopy), Inverse Photo-emission or Bremsstrahlung Isocromat Spectroscopy ) Enegry loss spectroscopy: EELS (Electron energy loss Spectroscopy) Photoemission and Auger electrons: infos mainly about the surface of the material. Volume composition may be obtained by removing the outer layers by ion bombardment (depth profiles). EELS: infos about electronic excitations. HREELS (High Resolution EELS): infos about vibrational excitations. More qualitative than AES and photoemission, the penetration depth depends on electron energy

2 AES Auger Electron Spectroscopy: E kin E vac E F VB M 2,3 3p M 1 3s e - e - L 2,3 2p L 1 2s K 1s Ground State One-Hole Initial State De-Excitation Auger Process Two-Hole Final State

3 Electron In: A primary electron beam hits the sample Electron Out: 1) Elastically reflected Electrons 2) Inelastically reflected Electrons 3) Secondary electrons generated by creation of electron-hole pairs 4) Auger secondary electrons AES Differentiating the spectrum the background due to secondary electrons is removed

4 In the de-excitation process Auger emission competes with fluorescence Auger dominates at low atomic numbers, fluorescence at high atomic numbers Auger Process Auger Electron X-Ray Fluorescence Fermi Level 3d M 4,5 3p M 2,3 3s M 1 2p L 3 2p L 2 2s L 1 1s K Fermi Level 3d M 4,5 3p M 2,3 3s M 1 2p L 3 2p L 2 2s L 1 1s K Photon

5 Computing the kinetic energy of Auger electrons: XYZ Process, One-Particle Scheme, Energy Conservation E kin E b XYZ E X E Y E b b b X Binding Energy of the X-Level Binding Energy of the Y-Level E b Y Z E b E kin XYZ Kinetic Energy of the Auger Electron Binding Energy of the Z-Level Work Function Z Usually additional terms must be included accounting for the twohole final state correlation interaction and the relaxation effects E kin XYZ E b X E b Y E b Z F R F R Two-Hole Final State Correlation Energy Relaxation Energy

6 Principal Auger Lines while Spanning the Periodic Table of the Elements KL 1 L 2 L 1 M 1 M 2 Auger Processes L 1 L 2 M 1 Coster-Kronig Process (the initial hole is filled by an electron of the same shell) CCC Transition CCV Transition Core-Core-Core Core-Core-Valence CVV Transition Core-Valence-Valence

7 Relative Energies Origin of the multiplets Relative energies within the KLL Auger series while changing the atomic number. Different coupling of the final state multiplet terms while spanning over Z Z L-S-coupling Intermediate coupling J-J-coupling

8 Gas vs. Solid Auger Emission from Mg: Very similar lines for CCC transitions, except for: an almost rigid energy shift (relaxation) some additional features related to many-body effects in solids (e.g. energy losses due to plasmons)

9 Primary Electron Beam Energy Dependence of the X-Level Ionization Cross-Section by Electron Impact The maximum is achieved at E p (3 4) E AX E AX appearance threshold

10 Comparison between electron-in electron-out and photon-in electron-out 1s Ionization Cross-Section for Selected Elements by Electron Impact and by X-Ray Excitation

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12 Auger Electron Spectroscopy Semi-Quantitative Analysis One can determine the atomic concentration (C i ) of the atomic species present in the near-surface region of a solid sample C i i I i s iii s i C i Atomic Concentration of the i-th species S i Orbital Sensitivity Factor of the i-th species I i Spectral Intensity Related to the i-th species E vac E kin Fragments of Theory The Auger process fulfills 1) Energy Conservation 2) Angular Momentum Conservation 3) Parity Conservation The process is governed by the Coulomb interaction between the electrons and related selection rules M if f e 2 r i j ij i E F VB M 2,3 3p M 1 3s L 2,3 2p L 1 2s K 1s

13 Auger Process Fragments of Theory Often i > and f > are expressed in terms of single-particle hole vector states, provided the total wave function is antisymmetric (Slater determinant) M if 3 p 3 p e 2 r ij 2s i j Thereby the initial and final S, L, M, J and S, L, M, J values are defined in terms of the s, l, m, j of the two holes in the each initial and in the final states. The transition probability of the Auger process can then be written as if 2 D E 2 E f (E f ) Density of final states conserving the energy D Direct matrix element E Exchange matrix element

14 Core-Valence-Valence (CVV) Auger Transitions 1) To a zero-th order approximation (i.e. no final state effects and assuming the matrix elements constant all over the transitions), the Auger emission is expected to mimic the self-folded DOS N(E), i.e. the Transition Density of States D(E) 2) Thereby, its width should be twice the value of G (width of the oneparticle state distribution) I E D 0 E N E N E E d

15 Core-Valence-Valence (CVV) Auger Transition To a more refined level, one has to take into account possible final state effects. Schematically speaking two limits are expected: 1) Pure Band-like Limit No final state effects are present 2) Atomic-like Limit The two final states holes are not effectively screened The band picture (i.e. the delocalized character of the valence wave functions along with the Bloch theorem) does not hold any more. The two holes left yield atomic-like multiplet effects. Since the overall energy of the process is conserved, the Auger electron does not have the kinetic energy expected in the pure bandlike scheme. Within a Hartree-Fock picture, Correlation and Exchange Potentials must be appropriately taken into account.

16 C KVV Auger Lines from a Variety of C-based Systems sensitivity to the chemical binding Experiment Self-folded DOS

17 Si L 2,3 VV Auger Line from Solid Silicon: Experiment vs. Theory Experiment Theory Comparison between the Si L 23 VV line shape with the calculated line shapes calculated from the optical transitions (Opt ---) and from a more refined theory

18 CMA Wide angular acceptance (conic element ±3 around 42 from axis 1 sterad). variable pass energy. Resolving power E 0 /DE ~200 for a diam 150 mm. E 0 /DE depends on E 0 High counting efficiency but limited resolution and operating distance

19 Auger Scanning Electron Microscopy V-shaped Filament Extractor Applications Deflecting Plates Primary e - Beam Image Display Backscattered Electrons Sample

20 Au N 6,7 VV Si L 2,3 VV Auger Spectra as measured at selected points of the self-organized agglomerated Au/Si(111) interface Island Flat region