Electron Spectroscopic Methods
The CAICISS system is equipped with both XPS and AES for analysis of chemical composition and bonding environment of atoms in the near surface region.
An overview of XPS process is shown schematically in the figure below. An X-ray photon of energy hν penetrates the surface and is absorbed by an electron with binding energy Eb, leading to the emission of the electron from the atom. In the most simple model, the emitted electron would have a kinetic energy, Ek, given by:
Ek = hν − Eb − φ
where φ is the work function of the irradiated material. However, when a core hole is created, the surrounding electrons react to the reduced screening potential by relaxing to lower energy states to partially screen the core hole. This gives the outgoing photoelectron an extra energy, Ea, known as the intra-atomic relaxation shift. The generation of a core hole may also lead to an electron from a higher energy level dropping down to fill the hole, causing the emission of a second (Auger) electron with a kinetic energy characteristic of the energy levels involved in the emission. This forms the basis of Auger electron spectroscopy (AES) and is described below.
The above description would be adequate for a free atom if photoionisation and photoemission were slow processes and allowed the system to reach a stable equilibrium. In reality, these processes are extremely rapid (∼ 10−14 s) and can lead to final state electrons residing in excited bound states, or to the emission of another electron to an unbound state above the vacuum level. Both of these possibilities lead to a reduction in the kinetic energy of the photoelectron. Inelastic scattering of photoelectrons during their escape from the solid results in a loss of kinetic energy and contributes to the background of the XPS spectrum. For a solid surface, the picture is again modified. In metals, weakly bound (valence) electrons are particularly mobile and help to screen the core hole. Therefore an extra interatomic relaxation shift, Er must be incorporated to calculate the photoelectron’s kinetic energy, so:
Ek = hν − Eb + Ea + Er − φ
Schematic of the physical principles behind XPS. (a) An incident photon with energy hν which is absorbed by a core level electron. (b) A photoelectron is emitted with a given kinetic energy if hν is greater than the binding energy of the electron, leaving behind a core hole.
The surface specificity of XPS comes from the relatively short inelastic scattering mean-free-path of the photoelectrons, Λ. This can be described as the thickness of matter which attenuates an incident electron flux, I0, by a factor of e−1 (see figure below). The detected electron flux at a take-off angle (TOA), θ, after passing through a material of thickness d, in which their inelastic mean-free-path is Λ, without undergoing an inelastic scattering event is given by:
I(E) = I0(E)e−d/Λsinθ
Approximately 90% of the emitted flux comes from the outermost 2Λ of the material at a TOA of 90◦, although the incident X-rays penetrate several μm in to the sample. With typical Λ values being around 3 nm, this limits the XPS probing depth to around 10 nm. In addition, as the TOA is decreased, the emitted flux becomes more surface specific, with 90% of the detected signal originating from the outermost Λ/2 at a 15 degree take-off angle.
With each atomic species having a unique electronic structure, XPS allows both highly surface specific studies (at glancing θ values), and overlayer thicknesses can be determined by comparing the intensity at a given θ from the substrate material prior to and following deposition. For details on how compositions and overlayer thicknesses can be determined with XPS, please consult the group theses.
A demonstration of the effects of varying the TOA in XPS. (a) shows detection of photoelectrons normal to the surface. By rotating the sample to glancing angles, as in (b), the escape depth, d, is reduced, and hence the XPS spectra are more surface specific.
In AES the target is irradiated with an electron beam in the energy range 1-10 keV. This results in the emission of backscattered, secondary and Auger electrons. The latter are emitted when a core electron falls in to a lower level, releasing energy which an electron in a higher level can absorb before being emitted from the atom. Auger electrons have well defined energies based on the atomic species from which they are emitted and the chemical environment of that atom.