Methods and T ehni ques in Surface Science

1. Methods andTehniquesin Surface Science Prof. Dumitru LUCA “Alexandru Ion Cuza” University, Iasi, Romania Hamamatsu, November 2007

2. 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. Hamamatsu, November 2007

3. 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. Hamamatsu, November 2007

4. 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. Hamamatsu, November 2007

5. 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. Hamamatsu, November 2007

6. 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. Hamamatsu, November 2007

7. TOF basics In reality, correction factors should be taking into accountm ostly in the case of reflectron configuration : Hamamatsu, November 2007

8. LEISS applications Incident beam: 3 keV 3He Detection angle = 1350 A LEIS spectrum showingthe 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. Hamamatsu, November 2007

9. 300 pO2 = 5 x 10-9 mbar O peak 280 N peak Time (s) 260 2015 240 1860 1705 220 1550 1395 200 Intensity (cps/nC) 1240 1085 180 930 775 160 620 465 140 310 120 155 0 100 1000 1100 1200 1300 1400 1500 1600 Final energy (eV) N/O substitution at Ti surface Hamamatsu, November 2007