Auger Electron Spectroscopy

Auger electron spectroscopy is another tool which provides useful information about the elemental composition of a surface. This spectroscopic technique measures the electrons emitted by the auger effect. The auger effect is the phenomenon by which an electron is emitted from an atom after a series of internal relaxation events that occur following the removal of a core level electron from that atom. When a core level electron is removed from an atom, either by bombarding the sample with high energy photons or

discussed in the previous section, for this higher energy electron to fill the core level vacancy, the energy difference between the two orbitals must be released by the electron. This often occurs via the emission of a photon. However in some cases, this energy is instead transmitted to another electron of lower binding energy, typically in the valence shell, which results in its emission from the atom. The emitted electron is referred to as an Auger electron, named after the French physicist Pierre Victor Auger, who was one of the first scientists to observe this phenomenon in 1923. Ultimately, this process results in the creation of two vacancies in the affected atom.

Given the energy transfer process described above for emission of an Auger electron, the kinetic energy of the Auger electron is given by the equation

𝐸 = 𝐸 − 𝐸 − 𝐸 (2.9) where 𝐸 , 𝐸 , and 𝐸 are the binding energies of the core electron, the higher level electron, and the emitted valence electron. According to this equation, the difference in binding energy between the core level and the higher level electrons represents the energy that is transmitted to the valence electron. Thus, the kinetic energy of the Auger electron is only determined by the difference between this energy transfer and the binding energy of the valence electron and not by the energy of the source used to create the core level vacancy. As with the photoelectrons emitted during x-ray irradiation, the kinetic energy of these Auger electrons are unique to a given element and may therefore may be used to unambiguously identify the elemental composition of a surface.

Similar to XPS, for these Auger electrons to be accurately measured by the detector it must first escape the material without colliding with other atoms. Consequently, both

AES and XPS yield elemental analysis results for only the top ~10 nm of a material. In contrast however, given that the kinetic energy of Auger electrons arises from only the difference in binding energies of the involved electrons, the kinetic energy of the Auger electron emission process is not impacted by the oxidation state of the atom. As a result, this technique was used primarily to confirm the cleanliness of a sample before use in TPD studies.

For taking AES measurements, a separate UHV chamber similar to the one illustrated in Figure 2.1 was used that was equipped with a high energy electron gun and a cylindrical mirror analyzer. The electron gun consisted of a tungsten filament, electrostatic accelerating anode, and focus lens, and is illustrated below in Figure 2.8. The filament produces electrons by thermionic emission as described in the previous section. The accelerating anode was held at a potential of 2 kV relative to the filament, producing a high electrostatic field that accelerates the electrons toward the anode. The anode is cylindrical, allowing a large portion of the electrons to pass through the center, forming a beam of high velocity electrons. The electron beam then passes through a focus lens, consisting of another cylindrical anode held at a potential about 80 % of the accelerating anode. By tuning the potential of the focus lens, the diameter and focal point of the electron beam may be adjusted. This electron gun was positioned on the vacuum chamber such that the focal point of the electron beam is on the sample. By bombarding the sample with these high velocity electrons, core level vacancies are created, initiating the Auger electron emission process.

The emitted Auger electrons were detected using a cylindrical mirror analyzer (CMA). The CMA consisted of two concentric cylinders held at different potentials and operates on the same principles as the hemispherical electron energy analyzer discussed previously. An aperture is located at one end of the assembly and the CMA is positioned on the chamber below the electron gun such that the aperture is in front of a sample when positioned in the beam from the electron gun. During bombardment of the sample with high energy electrons, Auger electrons emitted from the sample surface enter this aperture and their trajectory is influenced by the electrostatic field between the charged cylinders. A detector is positioned at the opposite end of the CMA assembly to detect electrons that possess the requisite energy to navigate the cylindrical passage. A schematic of the CMA is given below in Figure 2.9, which also illustrates its location on the vacuum chamber relative to the electron gun.

As with the hemispherical electron energy spectrometer used for XPS, the potential difference between the cylinders is held at a fixed value and a negatively charged grid is placed at the entrance of the aperture. By varying the charge on the grid, incoming electrons over a range of energies may be measured.

The spectrum generated by sweeping through a voltage range will provide a spectrum similar to that obtained during XPS. In fact, several Auger electron peaks are evident in the XPS spectrum given in figure 2.6. However, as shown in that figure, these Auger peaks are often associated with a large, sloping background signal due mainly to the inelastic scattering of these secondary emission electrons, making analysis of these peaks difficult. As a result, AES spectra are often represented as the first derivative of the electron intensity signal, dN(E)/dE, as illustrated below by the Auger spectrum for Ti. In this spectrum, multiple Auger electron peaks are evident. Given the differential form of this spectrum, the electron energy at which these peaks occur may be determined by the energy at which the signal sharply crosses over from positive to negative, relative to the baseline, signifying the apex of the peak.

The CMA used in this research project was operated using an external PC, which controlled the bias voltage and sweep rate of the retarding grid and simultaneously recorded

Sample Inner cylinder Outer cylinder UHV chamber Auger electrons Electron beam Electron gun Electron detector Retarding grid

Figure 2.9. CMA schematic and configuration on the UHV chamber relative to the electron gun.

with the CMA through a lock-in amplifier, which provided additional signal processing capabilities such as gain modification and noise filtering.