Auger electron escape depth

Local surface structure and coordination numbers of neighbouring atoms can be extracted from the analysis of extended X-ray absorption fine structures (EXAFS). The essential feature of the method22 is the excitation of a core-hole by monoenergetic photons modulation of the absorption cross-section with energy above the excitation threshold provides information on the distances between neighbouring atoms. A more surface-sensitive version (SEXAFS) monitors the photoemitted or Auger electrons, where the electron escape depth is small ( 1 nm) and discriminates in favour of surface atoms over those within the bulk solid. Model compounds, where bond distances and atomic environments are known, are required as standards. 

As such, it competes directly with X-ray fluorescence (XRF) but it is not limited by the dipole operator selection rules. All energetically allowed transitions are observed in an Auger electron spectrum. In addition, the electron escape depth is also a few tens of angstroms, unlike XRF where typical escape depths are on the order of tens of thousands of angstroms. 

Alternatively, the total electron yield from the sample due to cascades initiated by the Auger processes, can be detected (Citrin, 1978). The signal is measured with a charmel-tron detector, or simply by measuring the photoionization current from the sample. This method has the peculiarity of probing a few thousand A under the sample surface (due to the limited electron escape depth) and can be useful in studying macroscopically layered structures, e.g. ion-implanted materials. 

The availability of high-intensity, tunable X-rays produced by synchrotron radiation has resulted in the development of new techniques to study both bulk and surface materials properties. XAS methods have been applied both in situ and ex situ to determine electronic and structural characteristics of electrodes and electrode materials [58, 59], XAS combined with electron-yield techniques can be used to distinguish between surface and bulk properties, In the latter procedure X-rays are used to produce high energy Auger electrons [60] which, because of their limited escape depth ( 150-200 A), can provide information regarding near surface composition. 

When the electron beam enters the sample, it penetrates a small volume, typically about one cubic micron (10-18m3 ). X-rays are emitted from most of this volume, but Auger signals arise from much smaller volumes, down to about 3 x 10 25m3. The Auger analytical volume depends on the beam diameter and on the escape depth of the Auger electrons. The mean free paths of the electrons depend on their energies and on the sample material, with values up to 25 nm under practical analytical conditions. 

The least energetic emissions will not reach the surface from lower depths of the sample. For instance, Auger electrons that are emitted from deeper regions of the sample lose their energy through collisions with sample atoms before they reach the surface. As a result, AES is a very sensitive technique to probe the chemical composition of only the top 50-100 A (i.e., 15-30 monolayers). In comparison, the maximum escape depth of secondary electrons has been estimated as 5 nm in metals, and 50 nm in insulators. 

Both electron microprobe analysis and SAM use an electron beam for excitation of the specimen. The difierence between these techniques is in the detection of emitted x-rays in the microprobe technique while SAM measures emitted electrons. For both techniques, the energy of the detected particles is characteristic of the parent atom and thus identifies the atomic species present. The lateral spatial resolution in SAM is superior due to the much shorter mean free path of the emitted energy (electrons). The escape depth of auger electrons is approximately 10 A versus 1000 A in microprobe analysis. This phenomenon makes SAM a highly specific surface analysis technique. 

The above considerations show that only the electrons which have escaped from the host material without energy loss contribute to the Auger peak. This contribution is made in an exponentially decreasing manner ) for successive deeper layers ) with a characteristic distance d ). Thus the extent to which such measurements are specific to the surface region depends on the ejection depth d of the Auger electrons and not on the penetration depth of the exciting particle into the specimen. 

The surface absorption of clean surfaces can be detected by recording Auger electrons at kinetic energies in the range 50-100 eV which have a very short escape depth. 

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