Surface analysis techniques part I

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Surface analysis techniques part I

• Yaniv Rosen

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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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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.

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Advantages of SIMS

• Very sensitive can reach parts per billion
  range.
• Ability to do depth profiling.

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Disadvantages of SIMS

• Only ionized particles are measured.

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Disadvantages of SIMS

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

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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

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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 KEEa-Eb-Ec

Ea
Eb
Ec
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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.

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AES disadvantages

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

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AES disadvantages

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

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Low-energy electron diffraction

• Fire 20-300eV electrons at sample in Ultra-High
  Vacuum (UHV10-9 torr)
• Diffraction and elastic scattering occurs
• Accelerate electrons towards florescent mesh.
• Pattern should match reciprocal lattice.

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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.

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LEED Disadvantages

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

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Reflection high energy electron diffraction

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

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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.

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RHEED Disadvantages

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

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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.

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Photoemission Spectroscopy for Surface Studies
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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 dE

eKE
E0
hv
BE
Core Level
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Method
Energy Analyzer
Electrostatic Lens
Detector
X-rays/ UV
Photoelectrons
Sample
Huefner et. al. 1996
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Typical Properties

• Resolution
• XPS .25eV (100 - gt1000eV)
• UPS .10eV (10 - 40eV)
• Detection limits of 1 part in 10k to 100k for
  long measurements
• Can sample first 10nm

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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

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Chemical Shift

• PES lines affected by surroundings

Uhrberg et. al. 1998
Neudachina et. al. 2005
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Ad/Desorption Properties
Characterize potential energy surface of final
ionized state adsorbed onto substrate
AB S
(AB) S
AB S
hv
(AB) S
Fohlisch et. al. 1998
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ARPES
Conservation laws
eKE hv BE pef phv pei
0
?
Damascelli 2004
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Valence Band Characterization
Damascelli 2004
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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