Surface sensitivity of XPS

Owing to the limited escape depth of photoelectrons, the surface sensitivity of XPS can be enlianced by placing the analyser at an angle to the surface nonnal (the so-called take-off angle of the photoelectrons). This can be used to detemiine the thickness of homogeneous overlayers on a substrate. 

Figure Bl.25.5. (a) XPS spectra at take-off angles of 0° and 60° as measured from the surface nonnal from a silicon crystal with a thin layer of Si02 on top. The relative intensity of the oxide signal increases significantly at higher take-off angles, illustrating that the surface sensitivity of XPS increases, (b) Plot of...

Xps is a surface sensitive technique as opposed to a bulk technique because electrons caimot travel very far in soHds without undergoing energy loss. Thus, even though the incident x-rays penetrate the sample up to relatively large depths, the depth from which the electron information is obtained is limited by the "escape depth" of the photoemitted electrons. This surface sensitivity of xps is quantitatively defined by the inelastic mean free path parameter which is given the symbol X. This parameter is defined to be the distance an electron travels before engaging in an interaction in which it experiences an energy loss. 

The surface sensitivity of XPS generally requires an unrealistic UHV environment for analysis. This problem may sometimes be circumvented by employing IR or Raman spectroscopies. Peri... 

As the kinetic energy is always lower than /rv, the electrons ejected have low energies and can easily be absorbed by matter. Only photoclcctrons generated near the surface, typically at depths of less than 10 nm, will be emitted from the solid and analysed, giving rise to the surface sensitivity of XPS. 

The extreme surface sensitivity of XPS may be a handicap under certain circumstances. It may also be interesting to examine the sub-surface sample composition. This is possible using an ion gun to erode the surface of the sample. As erosion rates arc of the order of a nm/min... 

Non-Silicon Semiconductor Materials. III-V compounds are finding wider uses in the semiconductor Industry. One area of primary Interest is the concern over the cleanliness of substrates used in Molecular Beam Epitaxy (MBE) studies. Substrate preparation is Important because the substrate may play a role in failure that occur after additional processing steps. The surface sensitivity of XPS makes it Ideally suited for such investigations. 

The photoelectrons in XPS with typical energies in the 100-1500 eV range can only travel a short distance in the solid before they lose energy. This is the reason for the high surface sensitivity of XPS. If Iq is the flux of electrons originating at depth the flux detected at the peak energy, i.e., without energy loss is... 

This hardware is all enclosed in a UHV chamber with pressure below 10 mbar, constructed of or shielded by, a low magnetic permeability material such as mu-metal. This is necessary to avoid the field from the Earth and nearby magnetic objects to affect the electron trajectory. The UHV requirement is, in part, a result of the very surface sensitivity of XPS itself. At a vacuum level of 10 mbar a monolayer of gas is adsorbed on a solid surface in a few seconds if the surface atoms are reactive to the gas. At a vacuum of 10 mbar (UHV) the time to form an adsorbed monolayer is several hours. 

The surface sensitivity of XPS is the result of the limited depth below the surface from which electrons can escape elastically (without energy loss). This depth depends on the... 

The surface sensitivity of XPS is illustrated by comparison of the spectra of a clean surface and one contaminated with hexatriacontane (Figure 9.18). The presence of the contamination at the polymer surface is easily identified." ... 

XPS may be used to non-destructively depth profile a surface. This is an important ability since the most common depth profiling method used in surface techniques is acheived by sputtering surface atoms from the material with the use of a high flux ion gun, such an approach w ould be inappropriate for analysing polymers due to the extreme levels of damage w hich would occmr The surface sensitivity of XPS derives from the low inelastic mean free path of electrons in solids. This means that photoelectrons emitted from deep in the sample have a very small probability of... 

The surface sensitivity of XPS arises not from the penetration depth of the X-rays but from the limited distance an electron of a given kinetic energy can... 

---