X-Ray Photoelectron Spectroscopy (XPS)

  • quantitative element analysis at surfaces of solid materials by spectroscopy of emitted photo electrons, information of the chemical states by analysis of peak-shift or peak-shape changes
  • excitation of electrons with medium energy X-rays (Al-Ka or Mg-Ka), or He-UV-radiation
  • analysis of only elastically scattered electrons (no energy loss) with a hemispherical energy analyzer
  • depth resolution 2 ... 10 nm depending on electron energy
  • analysis of non-conducting samples,  additionally low energy electron flood gun
  • sputtering by noble gas ions (e.g. Ar+) for surface cleaning and depth profiling
  • analysis at UHV-conditions, base pressure 10-8 Pa (@ 10-10 mbar)
  • Analytical features:
    • spectra registration (survey or multiplex spectra) of selected sample areas, peak identification, atomic concentration(a. c.)-calculation, spectra manipulation (peakfit, background subtraction, factor analysis ...)
    • depth profiling in connection with ion sputtering
    • line-scan and map registration (including spectral shape = “chemical imaging”)
    • (automatic non-destructive) depth profiling at the nm-scale by angle dependent measurements (ARXPS) with model calculations
    • off-line spectra classification by mathematical methods like factor analysis
detection limit: depends on element, typ: 0.1 at %.
sample requirement: UHV-compatibility (low vapour pressure, no hydrocarbons),<br>sample diameter up to 50 mm, max. height 20 mm.<br>flat surface for depth profiling
application fields: surface reactions (oxidation, segregation),<br>interface reaction at thin films, film growth, <br>determination of chemical bonding, chemical valences.<br>analysis of insulating material, <br>direct monitoring of electron structure
materials: thin films (semiconductors, metals, insulators), <br>ceramics, superconductors (density of states),<br>oxide powders (segregation, surface states),<br>corroded, surface-modified material (chem. bonding), polymers

Examples:

Identification of different chemical structures of Pb (during depth profiling of PbF/PbS films).
Depth profile of an insulating STO/BTO-titanate-superlattice with single layer thickness of about 10 nm (1.5 keV, Kr+, sample rotation).