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Surface analysis techniques part I

Surface analysis techniques part I. Yaniv Rosen. Surface Analysis Techniques. Chemical Analysis SIMS (Secondary ion mass spectroscopy) AES (Auger electron spectroscopy) Structural Analysis LEED (Low-energy electron diffraction) RHEED (Reflection high energy electron diffraction).

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Surface analysis techniques part I

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  1. Surface analysis techniques part I Yaniv Rosen

  2. Surface Analysis Techniques • Chemical Analysis • SIMS (Secondary ion mass spectroscopy) • AES (Auger electron spectroscopy) • Structural Analysis • LEED (Low-energy electron diffraction) • RHEED (Reflection high energy electron diffraction)

  3. Secondary ion mass spectroscopy • Sputter sample with high energy ions for example 4KeV Ar . • Surface material is released. • Use conventional ion mass spectrometers to determine composition. +

  4. Advantages of SIMS • Very sensitive – can reach parts per billion range. • Ability to do depth profiling.

  5. Disadvantages of SIMS • Only ionized particles are measured.

  6. Disadvantages of SIMS • Only ionized particles are measured. • Sputtering not necessarily even. • Different levels of ionization.

  7. Disadvantages of SIMS • Only ionized particles are measured. • Sputtering not necessarily even. • Different levels of ionization. • Intrinsically destructive: • Dynamic SIMS • 1nA/cm² • 1µm/hr • Static SIMS • 1mA/cm² • 1Å/hr

  8. Auger Electron Spectroscopy • Fire ~100eV-5keV electrons at sample • Electron knocked out of atomic core • Higher level electron falls into hole. • Outer shell electron emitted with excess energy. • Measure energy of emitted electron: KE=Ea-Eb-Ec* Ea Eb Ec

  9. Why use AES? • Easy to detect 1% impurity in monolayer. • Beam of electrons can be focused and moved easily – provides high resolution. • Image can be compared simultaneously with SEM (Scanning electron microscope) image. • Good transition rates for smaller elements – can get signal for Li.

  10. AES disadvantages • High resolution and fast rates can cause sample damage. • Theoretical predictions are complicated.

  11. AES disadvantages • High resolution and fast rates can cause sample damage. • Theoretical predictions are complicated. • Absolute quantification not attempted

  12. Low-energy electron diffraction • Fire 20-300eV electrons at sample in Ultra-High Vacuum (UHV~10^-9 torr) • Diffraction and elastic scattering occurs • Accelerate electrons towards florescent mesh. • Pattern should match reciprocal lattice.

  13. LEED Advantages • Small mean-free path through the material • Same instrumentation as AES – can be placed in same apparatus. • Averages over small defects in the periodicity.

  14. LEED Disadvantages • Adsorbates change the configuration. • Possible to have multiple configurations from one spectra. • Difficult theory when more then one atom in base cell.

  15. Reflection high energy electron diffraction • 30-100KeV. • Fire high energy electrons at a shallow angle. • Use phosphorus screen to detect diffraction pattern.

  16. RHEED Advantages • Electrons have high energy so they do not need help reacting with phosphorus. • Sensitive to local defects – used in MBEs (Molecular-beam epitaxy) systems to grow semi-conductors.

  17. RHEED Disadvantages • Mostly concerned with qualitative descriptions of surface as opposed to quantitative diffracted beam intensity. • Needs high vacuum so electrons are not deflected.

  18. Conclusion • Chemical Analysis • SIMS – Destructive but very sensitive to impurities. • AES – high resolution for chemical analysis. Uses same instrumentation as LEED. • Structural Analysis • LEED – Bulk structure analysis. • RHEED – Detects local defects.

  19. Photoemission Spectroscopy for Surface Studies

  20. Principle • X-ray (or UV) photons excite electrons to continuum states. • Electron kinetic energy (eKE) related to binding energy of the initial state by: eKE = hv – BE – δE eKE E=0 hv BE Core Level

  21. Method Energy Analyzer Electrostatic Lens Detector X-rays/ UV Photoelectrons Sample Huefner et. al. 1996

  22. Typical Properties • Resolution: • XPS ~.25eV (100 - >1000eV) • UPS ~.10eV (10 - 40eV) • Detection limits of 1 part in 10k to 100k for long measurements • Can sample first ~10nm

  23. Limitations • Surface properties interfere with attempts to measure bulk properties • Sample degradation • Charge loss • Radiation damage • Lower and upper bounds on analysis spot size (micron-mm) • UHV requirement, no magnetic field, low electric field

  24. Chemical Shift • PES lines affected by surroundings Neudachina et. al. 2005 Uhrberg et. al. 1998

  25. Ad/Desorption Properties Characterize potential energy surface of final ionized state adsorbed onto substrate AB+ + S (A+B+) + S AB + S hv (A+B) + S Fohlisch et. al. 1998

  26. ARPES Conservation laws eKE = hv – BE pef = phv + pei 0 θ Damascelli 2004

  27. Valence Band Characterization Damascelli 2004

  28. Inverse XPS Study unoccupied bands • Provides complimentary information to photoemission • Directly measure density of states above EF • Can use analogous techniques to PES Smith 1988

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