X-ray photoelectron spectroscopy
X ray photoelectron spectroscopy (XPS) is a surface sensitive quantitative spectroscopic technique that measures the elemental composition of the materials. XPS spectra are obtained by irradiating a material with a beam of X-rays while simultaneously measuring the kinetic energy and number of electrons that escape from the top 0-10 nm of the material being analysed. It requires high vacuum conditions. XPS can be used to measure the elemental composition, empirical formula, chemical state and electronic state of the elements in a material.
Ultraviolet photoelectron spectroscopy (UPS) operates on the same principles as XPS, the only difference being that ionisation radiation at energies of 21.2 eV (He I) and 40.8 eV (He II) are used to induce the photoelectric effect. Ultraviolet photons are produced using a gas discharge lamp, filled with helium gas. UPS can be used to study valence band spectra, position of the Fermi level and for work function measurements.
Ion scattering spectroscopy (ISS) is a technique in which a beam of ions is scattered by a surface and the kinetic energy of scattered ion is measured. Peaks are observed corresponding to elastic scattering of ions from atoms at the surface of the sample. Each element at the sample surface produces a peak at a different measured kinetic energy, caused by the momentum transfer between the incident ion and atom. The scattered ion and the scattering atom are normally of different masses. But the total momentum of the atom and ion is conserved. Therefore as the initially stationary atoms recoils, some kinetic energy is lost from the scattered ions, and quantity of lost energy depends on the relative masses of the atoms and ions. The technique is uniquely sensitive to the outermost layer of the surface and is complementary to SIMS (secondary ion mass spectrometry). Typical uses include contamination identification, surface segregation and atomic layer deposition (ALD) studies.
Reflected electron energy loss spectroscopy (REELS) is a specialized technique that is usually performed as an adjunct to Auger electron spectroscopy (AES) using the same instrumentation but much lower primary electron beam energies (100 ev to 3 kev). It involves analyzing the energy distribution of those primary beam electrons which have been reflected back from a solid sample with well defined energy losses from the initial energy, E0. This technique can be used to study the composition and atomic coordination in surface layers of a material.
Depth profile measurement
Monatomic depth profiling uses an ion beam to etch layers of the surface or surface contamination, revealing sub-surface information. Combining a sequence of ion gun etch cycles with XPS analyses provides quantified information as well as layer thicknesses. Before removing material from the sample, a spectra or a set of spectra, is recorded from the surface of the sample. The surface is etched by rastering ion beam over a square of rectangular area of the sample. After the cycle, the ion beam is blanked and another set of spectra is recorded. This sequence of etching and spectrum acquisition is repeated until profiling has proceeded to the required depth. Hence, this technique is best suited for investigating the different layers on a multi-layered material, including thin films.