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Introduction to X-ray Photoemission Spectroscopy. 제 5 회 표면분석 심포지움 Sep. 12, 2007. 경희대학교 물리학과 박 용 섭 parky@khu.ac.kr. Contents. Introduction What is XPS ? What can it do ? A Brief History of XPS Name of the Game Instrumentation Vacuum System Photon Source Electron Energy Analyzer.

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제 5 회 표면분석 심포지움 Sep. 12, 2007


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    1. Introduction to X-ray Photoemission Spectroscopy 제 5회 표면분석 심포지움 Sep. 12, 2007 경희대학교 물리학과 박 용 섭 parky@khu.ac.kr

    2. Contents • Introduction • What is XPS ? • What can it do ? • A Brief History of XPS • Name of the Game • Instrumentation • Vacuum System • Photon Source • Electron Energy Analyzer

    3. Contents • Data Processing • Smoothing & Integration • Background and X-ray Satellite removal • Peak Fitting and Deconvolution • Applications • Elemental Identification • Quantitative Analysis • Chemical Shifts • Angle-resolved Techniques • Advanced Topics • Conclusions

    4. PhotoElectron Spectroscopy (PES) photons in electrons out (hn, k) e- (E, S, p) XPS: X-ray Photoelectron Spectroscopy UPS: Ultraviolet Photoelectron Spectroscopy

    5. PES : Schematic Diagram UPS XPS

    6. What can it do ? • Non-destructive Elemental Identification • Quantification (concentration): ~ % • Chemical State Identification (e.g. Si, SiO2) • Surface/Adsorbate Structure • Electronic Structure • valence band level positions • band structure mapping • many body effects in e.g., high Tc materials, etc.

    7. A Brief History of PES • 1887: H. Hertz - Photoelectric effect • 1897: J. J. Thompson - Electron ! • 1900: M. Planck - Quantum theory • 1905: A. Einstein - Used Q theory for P effect • 1958: W. E. Spicer - UPS spectra DOS related ! • 1967: K. Siegbahn - XPS (ESCA) established • 1969: HP - Commercial XPS • 1970s: Synchrotron Radiation

    8. Early Photoemission Experiment

    9. Name of the Game • XPS: X-ray Photoelectron Spectroscopy • ESCA(XPS):ElectronSpectroscopyforChemicalAnalysis • UPS: Ultraviolet PS • PES: PhotoElectron (PhotoEmission) Spectroscopy • ARXPS, UPS, PES: Angle-Resolved XPS, UPS, PES • AES: Auger Electron Spectroscopy • IPES: Inverse PES • SPPES: Spin-Polarized PES • TRPES: Time-Resolved PES • XPD: X-ray Photoelectron Diffraction • PED: PhotoElectron Diffraction

    10. Instrumentation: Vacuum System IMFP

    11. Universal curve

    12. Surface: a Bird’s Eye View • 1 L (Langmuir) = 10-6 Torr sec • For 1 hour run, 10-6 /3600 = 3x10-10 Torr ! • Ultrahigh vacuum (UHV) is a MUST !

    13. Instrumentation: Photon Source • Intensity, Focus, Monochromatic, energy selection  dual anode, monochromatic, discharge lamp, SR

    14. Energy (eV) Width (eV) Line Y Mζ 132.3 0.47 Zr Mζ 151.4 0.77 Cr Lα 572.8 3.0 Cu Lα 929.7 3.8 Mg Kα 1253.6 0.7 Al Kα 1486.6 0.85 Si Kα 1739.5 1.0 Cu Kα 8048.0 2.6 α3 α4 α6 Line α12 α5 β Separation (eV) 0.0 17.5 48.5 8.4 10.2 20 8.0 4.1 0.45 Relative Intensity 100 0.55 0.5 Characteristic X-ray Lines Satellites of Mg Kα

    15. Energy (eV) Intensity (%) Resonance Line He I 21.2175 100 23.0865 2 He II 40.8136 Ne I 16.6704 15 16.8474 100 Ar I 11.6233 100 11.8278 50 Kr I 10.0321 10.6434 Xe I 8.4363 9.5695 Resonance Lines of Rare Gas Discharge

    16. Monochromatic X-ray

    17. Effect of Monochromatization • 10 ~ 40 times intensity reduction • Focusing at the sample

    18. Synchrotron Radiation • High Intensity, Tunability, Focused Beam • Big Facility means Big Money  Shared Facility

    19. Instrumentation: Energy Analyzer Cylindrical Mirror Analyzer : CMA

    20. Cylindrical Mirror Analyzer • Advantage • Compact Design • High Transmission • Integrated e-gun : ideal for AES • Disadvantage • Geometry Restriction  sample must be at focus • No angular resolution • Geometry-dependent Peak position & Resolution • Poor resolution • Double-pass Design  No panacea

    21. Hemispherical Energy Analyzer Concentric Hemispherical Analyzer : CHA

    22. Magnetic Lens System • Much improved performance !

    23. Instrumentation: Electron Detection Channeltron (CEM) Microchannel Plate (MCP)

    24. Position-sensitive Energy Analyzer Channeltron array MCP + CCD (or DLD)

    25. Data Processing: smoothing etc. • Smoothing • NOT recommended, but … • Savitsky-Golay method • Diff./Int. • Simple numerical procedure

    26. Background Removal • Background • Inelastic Energy Loss Mechanism • Complicated Process • Sample Dependent • Geometry Dependent • Instrument Dependent • Must be Removed for Quantifiation • Removal Methods • Linear, Shirley, Tougaard

    27. Linear Background Removal

    28. Shirley Method

    29. Shirley BG Removal

    30. Peak Fitting & Deconvolution Lorentzian Gaussian Voigt

    31. Least Square Peak Fitting

    32. Sample Charging • Sample Charging • Insulating Samples •  Peak Shift and Broadening • Dual Anode X-ray Source •  Static Equilibrium • Mono or SR Source •  Low Energy Electron Flood Gun, Metallic Grid

    33. Magnetic Lens System • Much better charging compensation

    34. Application: Elemental Identification

    35. Ti 2p O 1s Element Sr 3d C 1s RASF 1.843 0.296 2.001 0.771 6009.1 5339.7 Measured Intensity 6443.2 1080.6 17.5 40.8 Concentration 20.4 21.3 Quantitative Analysis: Example • Relative Atomic Sensitivity Factor (RASF) • Ionization cross-section, etc.

    36. IMFP - Need for Matrix Factor Quantitative Analysis: Matrix Factor • Probing different volumes  Correction must be made !!

    37. Matrix Factor Following Briggs and Seah ed. Practical Surface Analysis, the concentration of element Ain a multielement sample where,

    38. Free electron plasma frequency Number of valence electrons per atom or molecule Density in g/cc Atomic or molecular weight IMFP: Calculation This equation was fitted to the calculated IMFP from optical data

    39. inelastic scattering elastic scattering AL Elastic Scattering Correction • AL is really needed rather than  Define

    40. Calculation of Calculated, averaged and interpolated values of G0 to G3 for nearly all Z are tabulated in JVST A15, 2095(1997) & Surf. Sci. 364, 380 (1996). This formula was used for interpolation when calculation was not possible

    41. Qunatification: multielement sample The in is the weighted average value for the sample

    42. Qunatification: Remaining Problems • Complicated and iterative process • Established algorithm  Not difficult • Input data set is not widely used • Background removal • Shirley method: widely used, weak physical basis, end-points dependent • Touggard method: limited applicability, firm physical basis • Uncertainty in calculated IMFP

    43. Application: Chemical Shifts • Not always possible • Tabulated values vary widely

    44. Application: Angle-resolved XPS • Better surface sensitivity • Non-destructive depth profile

    45. Application: Work Function

    46. Conclusions • XPS/PES • Powerful Non-destructive Tool in Elemental, Chemical and Structural Analysis of Surface, Interface, and Thin Films • Related Other Techniques • Versatile and Extensible • Much Room for Further Development