Low-energy electron microscopy

The low-energy electron microscope is a device for the investigation of surface structures by means of electron, which was invented by Ernst G. Bauer in 1962, but has been only in 1985 fully developed. With his help, atomically smooth surfaces, atom -surface interactions and thin ( crystalline ) films can be microscopically examined.

Principle of operation

The principle of low-energy electron is comparable to the light microscopy. The sample is a large surface area (diameter of up to about 50 microns ) is illuminated with an electron beam. In electron optics, these electrons have an energy of a few thousand eV. Before the sample, the electrons are decelerated to a few eV and therefore penetrate only a few angstroms into the sample. Low-energy electron microscopy is thus a surface sensitive to a few atomic layers method. The reflected (more precisely, as in LEED backscattered ) electrons will be guided through an imaging optical system which, as in a light microscope generates an image which is made visible from an electron detector and recorded by a camera.

In this way, dynamic surface processes can be monitored.

With LEEM conductive, crystalline samples can be investigated, which can be aligned to the incident electron beam in a defined manner. Enhancements include Aberationskorrektur on the image side (AC- LEEM ) and spin -polarized LEEM ( SPLEEM )

Imaging system

Electrons are emitted from an electron gun with 15-20 keV, focused by a condenser lens and sent through a magnetic beam splitter ( 60 ° or 90 °). The fast electrons fly through a objective lens, and are decelerated in the direction of the sample surface facing the electron gun is similar to a potential. The electrons are then surface- sensitive ( 1-100 eV) and the depth of penetration by varying the electron energy ( potential difference samples / electron gun potential minus the work function of the sample ) can be changed. The backscattered electrons fly back through the condenser lens ( which is at ground potential ), accelerate the electron gun and re-run the beam splitter. Now the electrons move away from the condenser optics and fly into the projector lens. The projection of the focal plane of the objective lens by means of a relay lens in the object plane of the projection lens produces a diffraction pattern (LEED - Low Energy Electron Diffraction ) in the image plane that can be taken in various ways. The intensity distribution of the diffraction pattern is dependent on the sample surface and direct impact of the wave nature of the electron. The individual intensities of the diffraction spots can be measured by switching off the lens and insert a contrast aperture.

Differences to the usual electron

The sample must be illuminated from the side of the imaging optics, as materials for low-energy electrons are not transparent. For dividing the incident and scattered electron beam, a magnetic prism is used, the focused electrons from both the electron as well as in plane to avoid interference in the image.

Further, the potential of the sample is not held at the ground, but brought about by an electronic immersion objective lens close to the electron gun, to slow electrons. In addition, the device under ultra-high vacuum ( UHV) have to work.

  • Electron optics
  • Surface Physics
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