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.

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

9. 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

10. 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*
Small penetration depth. Depth profiling. Ea Eb Ec

11. 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.
Transition rates for electrons better then those for photons when at smaller atomic number ~<15

12. AES disadvantages
High resolution and fast rates can cause sample damage.
Theoretical predictions are complicated.
Backscattering can add random factors of two. Inner shells can add peaks.

14. AES disadvantages
High resolution and fast rates can cause sample damage.
Theoretical predictions are complicated.
Absolute quantification not attempted
Backscattering can add random factors of two. Inner shells can add peaks.

15. Low-energy electron diffraction
Fire eV 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.

16. 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.
Only retrieve surface ions

17. LEED Disadvantages Adsorbates change the configuration.
Possible to have multiple configurations from one spectra.
Difficult theory when more then one atom in base cell.
Possible to use other techniques to determine multiple configurations.
Most difficulties arise when have more then one atom per unit cell. More then two big scattering atoms becomes very difficult.

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

19. 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.

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

21. Conclusion Chemical Analysis Structural 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.

22. Photoemission Spectroscopy for Surface Studies

23. 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

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

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

26. 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

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

28. 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

29. ARPES Conservation laws eKE = hv – BE pef = phv + pei θθ
Damascelli 2004

30. Valence Band Characterization
Damascelli 2004

31. 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