Methods and Tehniques in Surface Science

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Methods and Tehniques in Surface Science
Prof. Dumitru LUCA Alexandru Ion Cuza
University, Iasi, Romania

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Ioni Spectroscopies

• SIMS/SNMS (Secondary Ion/Neutral Mass
  Spectroscopy)
• the most sensitive elemental.
• Difficulties in interpretation of spectra
• Sputtering incident ion ? emergent ion.
• Requires HV/UHV conditions.
• Detectioin mass spectrometry
• RBS (Rutherford Backscattering Spectroscopy)
  scattering of high-energy (MeV) incident ions on
  sample NUCLEI.
• Probing depth a copuple of ?m!
• Scattering incident ion emergent ion
• Sputtering effect is minor (sputtering
  cross-section sectiunea is almost nil at that
  energy values).
• Detection solid-state scintillators.
• Equipment particle accelerator.
• LEISS (ISS) (Low-Energy Ion scattering
  Spectroscopy) scattering of incident ions
  imprastierea on the atoms in the topmost layer of
  the surface.
• Requires UHV.
• Equipment Dedicated LEIS spectrometer
• Detection electrostatic analizer.

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Low Energy Ion Scattering Spectroscopy (LEIS)
?ltlt dij ? Classical Mechanics Interaction
potential ??
Coulmb Potential Screening function a0 Bohr
radius of the scattering atom. Incident ions
experience to a lesser extent the presence of
nucleus due to electrostatic screening. (in RBS
the screening function 1)
E1 kinetic energy of scattered ions E0
kinetic energy of the incident ion (100 10 000
eV) M1 mass of the incident iont M2 mass
of the scattering atom ?L scattering angle.
Allows for qualitative and semi-quantitative
analysis of surface composition.
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Low Energy Ion Scattering Spectroscopy (LEIS)

• For a scattering angle values of ?L 900
  (forward scattering) and ?L 1800 (backward
  scattering), the previous equation becomes even
  simpler

• The optimum performance in terms of mass
  discrimination involves
• Due to very high neutralization probability of
  the incident ions by impact with the atoms in the
  surface, the LEIS technique provides information
  on the nature of the ions in the topmost layer of
  the sample surface, exclusively.

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Low Energy Ion Scattering Spectroscopy

• The intensity of the detected current, I, is a
  function of the number of the atoms of a species
  k, Nk, via the equuation
• 
  I K Ip Nk ?
  Pi W
• where
• ? - scattering cross-section ( probability
  that an incident ion be scattered towards the
  detector, after a collision with an atom of
  species k),
• Ip incident beam current,
• Pi the probability that an ion remains
  un-neutralized after a collision,
• W entrance solid angle of the detector.
• The above equation is seldom used for
  quantitative analysis, since the Pi parameter is
  hardly known.
• Data processing involves to know the scattering
  cross-section and the probability for impact
  neutralization/re-ionization. Usually we rather
  want to calibrate the LEISS machine by using
  standard samples.
• Most frequently, LEISS is associated
  with complementary techniques.

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LEIS - Instrumentation
Schematics of the LEIS setup, using the TOF
spectroscopy to detect forward- and backward
scattered particles.
Nuclear Instruments and Methods, Vol. 162, 1979,
p 587.
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TOF basics
In reality, correction factors should be taking
into accountm ostly in the case of reflectron
configuration
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LEISS applications
Incident beam 3 keV 3He Detection angle 1350
A LEIS spectrum showing the evolution of a
topmost layer of the Ti during titanium
nitridation
The evolution of the LEIS Ti peak area with
pressure. A Ti surface is exposed to a nitrogen
atmosphere in UHV.
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N/O substitution at Ti surface