AUGER ELECTRON SPECTROSCOPY (2024)

AUGER ELECTRON SPECTROSCOPY

INTRODUCTION

·Thesecondary electron emitted due to the incident electron beam produces a vacancyin the lower energy level (usually K shell)

·Thisvacancy can be filled by an electron in an outer shell with higher energy

·Ifthe electron with the higher energy fills this vacancy, there are twopossibilities

oItcan emit X rays (Radioactive process)

oItcan emit an electron (Non-radioactive process)

·Thelatter is known as an Auger electron which is produced by the knocking out ofa higher energy level electron by the energy which comes out when the vacantspace is filled by an electron with higher energy

·3electrons participate in emission of Auger electron

oEmittedsecondary electron

oElectronwhich jumps from higher level to occupy the vacancy

oElectron(Auger) which is emitted due to the energy from jumping of the electron

oHydrogenand Helium don’t produce Auger electron as they have less than 3 electrons

·Theenergy of the Auger electron is a fixed amount and hence is lost if theelectron is coming from an atom which is away from the surface of the specimen

·Onlyatoms very close to the surface of the specimen can emit the Auger electron

·Theenergy of the Auger electron is a characteristic of the element. Hence bydetermination of the energy of the Auger electron an idea about the compositionof the specimen can be obtained

·Considerthat the K Shell electron was emitted as a secondary electron and it wasoccupied by an L shell electron which in turn knocked out another L shellelectron as Auger electron. This case would be represented by the notation KLL

·Theenergy of the Auger electron as detected by the detector can be obtained by theexpression

Where is the Kinetic energy of the electron asdetected by the detector and is the energy of the electron in the Kshell and is the energy of an electron in the L Shelland is the work function of the detector

HOW IT WORKS?

The schematic of the experimentalarrangement for basic AES is shown in Fig. below. The sample is irradiated withelectrons from an electron gun. The emitted secondary electrons are analyzedfor energy by an electron spectrometer. The experiment is carried out in a UHV(Ultra high vacuum) environment because the AES technique is surface sensitivedue to the limited mean free path of electrons in the kinetic energy range of20 to 2500 eV. The essential components of an AES spectrometer are

·UHVenvironment

·Electrongun

·Electronenergy analyzer

·Electrondetector

·Datarecording, processing, and output system

Electron Energy Analyzer & ElectronDetector

The function of an electron energy analyzeris to disperse the secondary emitted electrons from the sample according totheir energies. An analyzer may be either magnetic or electrostatic. Becauseelectrons are influenced by stray magnetic fields (including the earth’smagnetic field), it is essential to cancel these fields within the enclosedvolume of the analyzer. The stray magnetic field cancellation is accomplishedby using Mu metal shielding. Electrostatic analyzers are used in all commercialspectrometers today because of the relative ease of stray magnetic fieldcancellation.

The dispersed secondary electrons arereceived in the electron detector. Detector communicates the energy withrespect to time data to the computer attached with it. The data is analyzed tofind out the Auger peak. An auger analysis is normally based on measurements ofthe strengths of Auger peaks in a plot of, the back scattered electron energy perunit energy interval, versus E, the energy of electrons. This energy flux issimply equal to the electron energy times the number of electron per unitenergy interval, that is,. Such a spectrum is depicted for a puresilver specimen bombarded by 1000 V electrons in the lower curve of figureshown below. Note that it is very difficult to resolve the Auger peaks on thiscurve because they are small and super imposed on a strong background signaldue to backscattered electrons. If is multiplied by 10 and the resultsreplotted, the intermediate curve in the figure is obtained. The Auger peaksare now more pronounced, but still difficult to resolve. This problem can besolved by plotting the derivative of with respect of E as in the uppermostcurve of figure. The resulting curve clearly shows evidence of severalpeaks that fall within the range of energies between approximately 240 and 360eV.

WHAT ITDOES?

The high surface sensitivity of AES is due to the limited meanfree path of electrons in the kinetic energy range 20 to 3000 eV. Augerelectrons, which lose energy through plasma losses, core excitations, orinterband transitions, are removed from the observed Auger peaks and contributeto the nearly uniform background on which the Auger peaks are superimposed.Because phonon losses are small compared with the natural width of Auger peaks,they do not affect the Auger escape depth. Hence the Auger yield is notdependent on the sample temperature.

Because the Auger transition probability and Auger electron escapedepth are independent of the incident electron beam energy, Ep, the dependenceof the Auger peak amplitude on Ep is governed completely by the ionizationcross-section of the initial core level. Ionization occurs primarily by theincident electrons during their initial passage through the escape depth region(5 to 25Å thick). The backscattered primary electrons can also contribute tothe Auger yield when the incident beam energy is substantially greater than thebinding energy of the core level involved.

Variables Involved in the Production of Auger Electrons

An inner shell vacancy can be produced through a variety ofmethods, such as irradiation with electrons and X rays or bombardment withargon ions. Electron impact is usually used for producing Auger lines foranalytical purposes. It provides an intense beam that can be brought to a finefocus. X-ray irradiation has its value in providing less radiation damage andbetter peak-to-background ratios.

High-Energy Satellite Lines

High-energy satellite structures have been observed in the Augerspectra of solids. The presence of such a structure has been interpreted asbeing due to plasmon gains. It is also believed that the high-energy linesarise from an initial multiple ionization or perhaps resonance absorption. Thequestion of Auger satellites in solids is still under active consideration.

Characteristic Energy Losses

Electrons ejected from a solid can suffer characteristic energylosses, usually due to plasmaon losses. Because Auger spectra are generallyrather complex and often not well resolved and are spread over a considerablerange of energies, peaks from characteristic energy losses are much moredifficult to disentangle from the normal Auger spectrum than is usual in thecase of photoelectron spectroscopy.

Also, the surface contamination will alter the nature of thecharacteristic loss peaks considerably.

Charging in Nonconducting Samples

Charging as a result of an impinging beam of electrons on anonconductor is a particularly serious problem in Auger spectroscopy. Often thecharging and the resulting non uniform surface potential prevent a meaningfulAuger spectrum. However, this problem often can be overcome by choosing theproper angle of incidence and the energy of the primary electron beam. Theimportant factor is the ratio δ (the number of secondary electrons leavingthe target to the number impinging on the target). If δ = 1, the charge isstabilized. If δ < 1, the charge is negative, and if δ > 1, itis positive. The choice of impact energy is also important. The factor δbecomes less than 1 if the energy of the impinging beam of electrons is eithertoo large or too small. Generally, the primary beam energy lies between 1.5 and3.0 keV depending on the application and the resolution required.

Scanning Auger Microscopy

With a finely focused electron beam for Auger excitation, AES canbe used to perform two-dimensional surface elemental analysis. In this setup,the electron gun operation is similar to that used in conventional scanningelectron microscopy (SEM). A set of deflection plates raster the electron beamon the sample. The scanning Auger system can be used to perform point Augeranalysis with a spatial resolution on the order of 3 µm by using a minimum beamsize of about 3 µm or to obtain a two-dimensional mapping of the concentrationof a selected surface element. The low-energy secondary electron or absorbedcurrent displays are used to monitor the surface topography and locate theareas of interest on the sample. To obtain an elemental map, the intensity ofthe display is controlled by the magnitude of the selected Auger peak. The mostnegative excursion in the differentiated Auger spectrum is taken as a measureof the Auger current. A two-dimensional elemental map of the surface isobtained by set-ting the pass energy of the electron spectrometer at thenegative excursion of the Auger peak of interest. While the output of thelock-in amplifier is used to modulate the intensity of the record display asthe electron beam is rastered across the sample. Three-dimensional analysis ofthe surface of a sample can be obtained by using a combination of scanningAuger microscopy and sputter etching.

Applications:

Auger electron spectroscopy is a very powerful surface analyticaltechnique that has found applications

in many fields of solid-state physics and chemistry.

·AES is used to monitor the elemental composition

·Several phenomena such as adsorption desorption, surfacesegregation from the bulk, measurement of diffusion coefficients, and catalyticactivity of surfaces have been investigated using AES.

·It has also been used to study the surface compositional changesin alloys during ion sputtering. Chemical properties such as corrosion, stresscorrosion, oxidation and catalytic activity and mechanical properties such asfatigue, wear, adhesion, resistance to deformation processes, and surfacecracking depend on surface properties.

·Similarly, grain boundary chemistry Influences mechanicalproperties such as low- and high-temperature ductility and fatigue, chemical propertiessuch as inter-granular corrosion and stress corrosion, and electricalproperties.

·AES has been used to relate surface and grain boundary chemistryto properties of materials. AES has proved to be extremely valuable compared tomost other techniques, which are limited by either large sampling depth or poorsensitivity.

ADVANTAGES AND DISADVANTAGES

The main advantages of AES can be summarized as follows:

• Spatial resolution is high.

• Analysis is relatively rapid.

• Surface or subsurface analysis can be performed.

• It is sensitive to light elements (except H and He).

• It provides reliable semi quantitative analysis.

• Chemical information is available in some cases.

The disadvantages of this technique are as follows:

• Insulators are difficult to study due to surface charging.

• Surface may be damaged by the incident electron beam.

• Precise quantitative analysis may require extensive work.

• Sensitivity is modest (0.1 to 1 atom %).

• Depth profiling by ion sputtering or sectioning is destructive.

CONCLUSION:

Auger electron spectroscopy is useful fordetermining the compositions of surface layers to a depth of about 2 nm forelements above He. It also has a spatial resolution greater than or equal to100 nm, which is about a tenth of that of the electron probe X-ray microanalyser. This makes this technique well suited to the studies of grainboundaries in metals and alloys, especially with specimens susceptible tobrittle grain boundary fractures. It is also useful for surface segregationstudies as in the solving of stress-corrosion problems.