AES is an abbreviation for Auger Electron Spectroscopy
AES is an analytical technique which is based upon the measurement of the kinetic energy of electrons ejected from an atom, usually following the bombardment of that atom by high energy electrons. The Auger process is named after its discoverer, Pierre Auger.

The Auger Process
When an atom is struck by a high energy electron (typically in the range 1 to 25 keV) there is some probability of a core level electron being ejected. The atom is then in an energetic ionic state with an electron missing from a core level. One mechanism by which the atom can relax into a lower energy state is for another electron, from the same atom, to fill the core level hole with the release of energy. This energy can then cause the emission of another electron (the Auger electron).

The process of Auger electron emission

The Auger electron has a characteristic energy which depends upon a number of factors: 

• The chemical element involved   
• The energy level within which the initial hole was formed   
• The energy level of the electron which eventually fills the hole   
• The initial energy level of the electron which eventually becomes the the Auger electron

Auger Electron Spectroscopy
If we measure the energy of the ejected Auger electrons we can learn some important facts about the sample under investigation:

• The elements from which it is made   
• The relative quantity of each element   
• The chemical state of the elements present (provided the energy is measured with sufficient resolution)

SEM and SAM
SEM is scanning electron microscopy
SAM is scanning Auger microscopy

The Auger process relies upon the sample being struck by a beam of high energy electrons. If this beam is scanned over the surface of the sample and the secondary electrons are detected then we have an SEM. Similarly, if we detect the Auger electrons as the beam is being scanned then we can map the distribution of elements at the surface of the sample.
Modern Auger instruments can produce images or maps showing the distribution of the elements or their chemical states over the surface. A good instrument would have an ultimate spatial resolution of about 10 to 20 nm. A multitechnique XPS instrument should be capable of an ultimate spatial resolution of <100nm.
Electrons can only escape from the solid if they originate close to the surface. If they originate from too deep within the bulk of the solid then they will be reabsorbed. The information available from AES is, therefore, highly specific to the surface of the material.

Components of an AES Instrument
An AES instrument has three main components:

• An electron source, this should have variable energy and be capable of producing a very small spot of electrons.   
• An electron energy analyser, for example, a spherical sector analyser   
• A secondary electron detector for the production of SEM's

The measurements must be made in ultra-high vacuum (uhv), for two reasons: 

• To allow the Auger electrons to travel from the surface of the sample to the detector without striking a gas atom   
• If a clean surface is prepared for analysis, it would become contaminated if it were not under uhv.

Other, optional, items may also be present on an Auger electron spectrometer: 

• A backscattered electron detector, this produces an image similar to an SEM but the contrast mechanism is different and it is less surface sensitive. This type of detector is often fitted to a scanning electron microscope   
• An ion source which is used both to clean a surface prior to analysis and to erode the surface of the sample so that concentration depth profiles can be measured.

Many Auger electron spectrometers have other analytical techniques associated with them for example: 

• An X-ray source to allow XPS measurements to be made (XPS is described elsewhere)