Methods and T ehni ques in Surface Science - PowerPoint PPT Presentation

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Methods andTehniquesin Surface Science

Prof. Dumitru LUCA

“Alexandru Ion Cuza” University, Iasi, Romania

Hamamatsu, November 2007

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.

Low Energy Ion Scattering Spectroscopy (LEIS)

<< 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.

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 LEIStechnique provides informationon the nature of the ions in the topmost layer of the sample surface, exclusively.

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 = KIpNk PiW

where:

 - scattering cross-section (= probability that an incident ionbe scatteredtowards 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.