Introduction 

Auger electron spectroscopy (AES) is used to analyze the surface of solids. Pierre Auger discovered the Auger effect in 1923 and after 30 years it became a valuable characterization technique for solid surfaces^[1]. AES has many applications in industry and research, it can be used for example to determine the properties of a material, to examine the possible contaminations of a surface or to study layered thin films^{[2, p.126-135]}. It can also be used in depth profiling of a material^[3]. In AES a high energy electron beam excites the surface of the sample, and photons and electrons are then emitted from the surface, but only a small percentage of them are Auger electrons ^{[2, p.1-11]}.

Principle

Auger electron emission results from the relaxation following the excitation. After the excitation, a secondary electron is emitted which leaves a hole in lower energy level of an atom. This hole is then filled with an electron from a higher level and another electron is emitted, the Auger electron. ^{[2, p.24-28]} Figure 1 shows how the electron from the core level is emitted, then an electron from outer shell fills the hole in the core level and an Auger electron is emitted due to the energy changes from this transition. 

Figure 1. The relaxation following the excitation of electrons. Figure: Tytti Virtanen.

Kinetic energy of the emitted electron E_kin can be calculated with the help of Planck's constant h, frequency of the photon v, first outer shell E_B and second outer shell E_C electron binding energies. 

But this equation and model is too simple to be used. More accurate approximation is called "Z+1" and it assumes that the atom number is one greater than the actual number, because binding energies of electrons increase. E_A is the binding energy of the electron in a core level. ^{[2, p.24-28]}

Instrumentation

The instruments include electron gun, electron energy analyzer, electron detector, sample stage and data recording. Additional instruments can also be used, for example ion gun, X-ray source/detector or secondary electron detector SED. The AES is operated under vacuum, mostly ultra-high vacuum UHV. Vacuum is used, because it prevents the electron scattering and keeps the sample surface clean. Electron gun is the source of the electron beam. There are different kinds of electron guns available. Because images of the sample surfaces are being produced with AES, the electron beam must be scanned. The image is produced by recording the Auger signal as a function of the position of the spot in the sample. Electron detector measures the number of electrons emitted from the sample per second. ^{[2, p.89-126]} The analyzer must be able to measure electron kinetic energy and it disperses the secondary emitted electrons from the sample according to their energies. The electrons that are transmitted through the analyzer are then counted by the detector ^[1]. Figure 2 shows the simple instrumentation of AES. 

Figure 2. Instrumentation of AES, including electron gun, sample holder, electron detector and data acquisition. Figure: Tytti Virtanen.

Figure 3 shows a SEM image of magnesium-doped lithium niobate (MgLN) crystal sample with a triangular poled and etched -Z domain on the left, and Auger spectra for the survey areas showing the Auger oxygen KLL transition peak^[4].

Figure 3. (a) SEM image of MgLN sample and (b) Auger spectra for the survey areas ^[4]. (License: OSA Open Access License)

Example

Romero et al.^[5] grew Cu(In,Ga)Se₂ (CIGS) thin films and analyzed them through X-ray diffraction (XRD) and Auger Electron Spectroscopy (AES) depth profile measurements. The AES results can be seen in the Figure 4.

Figure 4. AES depth profile analysis of Cu(In,Ga)Se₂ thin films deposited (a) in two stages and (b) in three stages ^[5]. (License: CC BY-NC 4.0)

References