The Electron Spectroscopy Lab is equipped with two powerful analytical instruments:
PHI 4200 Thin Film Analyzer for AES depth profiles
PHI 545-590 Multiple Analytical System (XPS, UPS, AES)
Auger Electron Spectroscopy (AES)
is one of the most widely used analytical techniques for chemical analysis of the outermost layers of a material (surface or interface). Auger electrons result from an atomic rearrangement involving 3 electrons and following the creation of a core hole. Though the latter can be generated in a number of different ways, i.e. by energetic electrons or ions as well as by X-Rays, in a dedicated instrument electron beams are always employed. This is because the ability to finely focus and handle electrons beam directly gives the technique a high lateral resolution. Excited Auger electrons are analyzed according to their kinetic energy, which is in turn a fingerprint of the emitting element.
Elemental identification is possible as well as quantification of elements relative abundance (0.5-1% is the typical detection limit for most elements). Information on the chemical state of surface is additionally derived from line-shape analysis of Auger transitions involving valence electrons. All elements in the periodic table (except hydrogen and helium) are detected over a wide energy range (generally 0 to 2000 eV) with moderate matrix effects. Characteristic of the technique is its high surface sensitive (few monolayers) due to the limited mean free path of AES secondary electrons.
By combining Auger analysis with ion etching of the surface, a composition profile of elements as a function of depth may be achieved. The depth profile raw data consist of peak-to-peak elemental signals as a function of the sputtering time but a more commonly used in-depth composition image is given by the elemental concentration as a function of the erosion depth. Therefore, a conversion of the measured sputtering time into sputtering depth and that of the acquired signal intensities to elemental concentration are required. In order to determine the erosion rate, a reference material of certificate thickness is used.
Photoelectron spectroscopy utilizes photo-ionization and analysis of the kinetic energy distribution of the emitted photoelectrons to study the composition and electronic state of the surface region of a sample. Traditionally, when the technique has been used for surface studies it has been subdivided according to the source of exciting radiation into :
- X-ray Photoelectron Spectroscopy (XPS) using soft x-rays to examine core-levels
- Ultraviolet Photoelectron Spectroscopy (UPS) using UV radiation to examine valence leves.
Ultraviolet Photoelectron Spectroscopy (UPS)
is a powerful method to investigate valence energy levels and chemical bonding of the near surface region of materials. In this technique, the sample under investigation is irradiated using vacuum UV photons of energy hν, created in a continuous discharge source by applying high voltage to a gas to cause breakdown. Helium lamp emitting at 21.2 eV (He I radiation) or 40.8 eV (He II radiation) is typically used.
Such photons exciting energy is of the same order of magnitude as the binding energies of outermost levels of atoms and that's why only the photoelectrons emitted from the valence band or the shallow core levels can be detected while the deep core electron levels cannot be excited.