Auger electron spectroscopy (AES) is a powerful tool for analyzing surface composition. It works by detecting electrons emitted during atomic relaxation after core electron removal. AES is incredibly surface-sensitive, probing just the top few atomic layers of a material.

AES complements other surface spectroscopy techniques like XPS and UPS. While XPS and UPS rely on photon excitation, AES uses electron bombardment. This makes AES great for high spatial resolution analysis and depth profiling when combined with ion sputtering.

The Auger Process in Surface Analysis

Fundamentals of the Auger Process

• The Auger process involves the emission of electrons from atoms due to the relaxation of excited states created by the removal of a core electron
• In the Auger process, an electron from a higher energy level fills the core hole, and the excess energy is transferred to another electron, which is ejected as an Auger electron
• Auger electrons have kinetic energies characteristic of the atomic energy levels involved in the transition, making them element-specific
• The Auger process is named after Pierre Auger, who discovered this phenomenon in the early 20th century while studying the interaction of X-rays with matter

Surface Sensitivity of Auger Electron Spectroscopy (AES)

• AES is a surface-sensitive technique because Auger electrons have relatively low kinetic energies (typically 20-2000 eV), limiting their escape depth to a few nanometers
• The surface sensitivity of AES arises from the short inelastic mean free path of electrons in solids, which is the average distance an electron can travel before losing energy through inelastic scattering
• The escape depth of Auger electrons depends on their kinetic energy and the material properties, but it is typically in the range of 0.5-3 nm, corresponding to 2-10 atomic layers
• The surface sensitivity of AES makes it an ideal technique for studying the composition and properties of the topmost atomic layers of materials (surfaces, interfaces, thin films)

Applications of AES in Surface Analysis

• AES is used for elemental analysis and mapping of surfaces, providing information about the composition and spatial distribution of elements in the top few atomic layers
• AES can detect all elements except hydrogen and helium, with a typical sensitivity of 0.1-1 atomic percent, depending on the element and the experimental conditions
• AES is widely used in materials science, surface chemistry, catalysis, and semiconductor technology for characterizing the elemental composition and homogeneity of surfaces and interfaces
• Examples of applications include studying the surface segregation of alloys, monitoring the growth of thin films, investigating the surface composition of catalysts, and analyzing the elemental distribution in microelectronic devices

Auger vs Photoemission: Electron Generation

Electron Emission Mechanisms

• In the Auger process, electrons are emitted due to the relaxation of excited states created by the removal of a core electron, while in photoemission, electrons are emitted due to the absorption of photons
• Auger electrons are generated through a three-electron process involving the filling of a core hole and the ejection of another electron, while photoelectrons are generated through a single-electron process involving the direct excitation of an electron by a photon
• The Auger process is an example of a secondary electron emission process, where the initial excitation (core hole creation) leads to the emission of another electron, while photoemission is a primary electron emission process, where the incident photon directly causes the ejection of an electron

Energy Dependence of Emitted Electrons

• The kinetic energy of Auger electrons depends on the atomic energy levels involved in the transition, while the kinetic energy of photoelectrons depends on the energy of the incident photons and the binding energy of the electron
• Auger electron energies are characteristic of the elements involved and are not affected by the primary excitation source, while photoelectron energies are determined by the photon energy and the binding energy of the electron according to the photoelectric effect equation: $KE = hν - BE - ϕ$, where $KE$ is the kinetic energy, $hν$ is the photon energy, $BE$ is the binding energy, and $ϕ$ is the work function
• The energy range of Auger electrons is typically 20-2000 eV, depending on the atomic number of the elements involved, while the energy range of photoelectrons can be much broader, extending from a few eV to several keV, depending on the photon source (UV, soft X-rays, hard X-rays)

Excitation Sources and Intrinsic Processes

• Auger electron emission is an intrinsic process that does not require an external excitation source, while photoemission requires an external source of photons, such as an X-ray or UV light source
• In AES, the initial core hole can be created by various processes, such as electron impact ionization (using an electron beam), X-ray absorption, or ion bombardment, while in photoemission, the excitation is provided by a photon source, such as an X-ray tube, a synchrotron, or a gas discharge lamp
• The intrinsic nature of the Auger process makes it suitable for studying the electronic structure and composition of materials without the need for an external excitation source, while photoemission requires a tunable and monochromatic photon source to probe the electronic structure and band structure of materials

Instrumentation for Auger Electron Spectroscopy

Electron Excitation and Detection

• AES typically employs an electron gun to generate a focused beam of primary electrons with energies ranging from a few hundred eV to several keV
• The primary electron beam is directed onto the sample surface, causing the emission of Auger electrons and other secondary electrons
• An electron energy analyzer, such as a cylindrical mirror analyzer (CMA) or a concentric hemispherical analyzer (CHA), is used to measure the kinetic energy distribution of the emitted electrons
• The electron energy analyzer is positioned close to the sample surface to collect electrons emitted at a specific angle, typically 30-60 degrees relative to the sample normal
• The energy resolution of the analyzer determines the ability to resolve closely spaced Auger peaks and is typically in the range of 0.1-1 eV, depending on the analyzer type and the operating parameters

Sample Manipulation and Vacuum Requirements

• The sample is usually mounted on a manipulator that allows for precise positioning and rotation to optimize the Auger signal and enable depth profiling or spatial mapping
• The sample manipulator can also include heating or cooling capabilities to study temperature-dependent phenomena or to perform sample cleaning or annealing
• Ultra-high vacuum (UHV) conditions (typically < 10^-9 torr) are necessary to minimize surface contamination and ensure a long mean free path for the Auger electrons
• UHV conditions are achieved using a combination of pumps (turbomolecular, ion, titanium sublimation) and baking the system to remove adsorbed gases from the chamber walls
• The UHV environment also enables the use of other surface analysis techniques, such as low-energy electron diffraction (LEED) or X-ray photoelectron spectroscopy (XPS), in conjunction with AES for a comprehensive characterization of the surface

Spatial Resolution and Depth Profiling

• The spatial resolution of AES is determined by the size of the primary electron beam and can range from a few nanometers to several micrometers, depending on the electron gun and the focusing optics
• High spatial resolution (< 10 nm) can be achieved using field emission electron guns or by combining AES with scanning electron microscopy (SEM) to obtain Auger electron maps of the surface
• Depth profiling in AES is performed by combining the technique with ion sputtering, where an ion beam (typically Ar+) is used to remove material from the surface layer by layer, allowing the analysis of the elemental composition as a function of depth
• The depth resolution of AES depth profiling depends on the sputtering rate and the escape depth of the Auger electrons and is typically in the range of 1-10 nm, depending on the material and the experimental conditions

Interpreting Auger Spectra: Key Features

Peak Positions and Elemental Identification

• Auger spectra plot the derivative of the electron energy distribution (dN/dE) as a function of the electron kinetic energy, with Auger peaks appearing as characteristic features
• The kinetic energy of an Auger peak is element-specific and corresponds to the difference in energy between the involved atomic levels, allowing for the identification of elements present on the surface
• Auger peaks are labeled using the X-ray notation for the atomic levels involved in the transition, such as KLL, LMM, or MNN, where K, L, M, and N refer to the principal quantum numbers 1, 2, 3, and 4, respectively
• The presence of multiple Auger peaks for a given element, such as the KLL, LMM, or MNN series, can help confirm the elemental identification and provide additional information about the electronic structure

Peak Intensities and Quantitative Analysis

• The intensity of an Auger peak is related to the concentration of the corresponding element on the surface, enabling quantitative analysis using sensitivity factors
• Sensitivity factors are empirically determined coefficients that relate the Auger peak intensity to the elemental concentration, taking into account factors such as the ionization cross-section, the escape depth, and the analyzer transmission function
• Quantitative analysis in AES requires the use of reference materials with known compositions to calibrate the sensitivity factors and account for matrix effects
• The accuracy of quantitative AES analysis is typically in the range of 5-20%, depending on the elements involved, the sample matrix, and the experimental conditions
• The background in Auger spectra arises from inelastically scattered electrons and can be used to normalize peak intensities for quantitative analysis

Chemical State Information and Peak Shapes

• Peak shapes and widths can provide information about the chemical state of the elements, such as oxidation state or bonding environment, due to small energy shifts or changes in the peak shape
• Chemical shifts in Auger spectra arise from changes in the atomic energy levels due to variations in the local electronic environment, such as the presence of bonding partners or the redistribution of valence electrons
• The magnitude of chemical shifts in AES is typically smaller than in XPS (0.1-1 eV vs. 1-10 eV) due to the involvement of multiple atomic levels in the Auger process, which can partially cancel out the energy shifts
• Peak shape analysis in AES can provide information about the presence of different chemical states or bonding configurations, such as the distinction between metallic and oxidized forms of an element
• Examples of chemical state information in AES include the differentiation between carbidic, graphitic, and amorphous carbon based on the shape of the C KLL Auger peak, or the identification of different oxidation states of transition metals based on the fine structure of the LMM or MNN Auger peaks