RHEED

Scheme of a RHEED structure. Electrons fly from the electron source to the sample and hit there at an angle theta of less than 5 °. The diffracted electrons then fly to the screen (shown here as CCD). The mirror reflex is shown here, for which the angle of incidence is equal to the angle of reflection.

RHEED ( English reflection high-energy electron diffraction , German 'diffraction of high-energy electrons during reflection') is a physical method for analyzing smooth material surfaces with the help of electron diffraction . The electrons have an energy in the kiloelectronvolt range (mostly approx. 10–50 keV). The angle between the surface and the electron beam is very small (typically around 2 °) so that the electrons are reflected on the surface. The reflected electrons hit a fluorescent screen, where the diffraction pattern becomes visible. The type of crystal lattice and the lattice parameters can be determined from the diffraction pattern .

Since electrons are slowed down very quickly by air, this process is operated in an ultra-high vacuum .

The advantage of the RHEED method over the LEED method is that there are no equipment in the way along the surface normals, so that the growth of thin layers on surfaces can be observed during molecular beam epitaxy . This is the main use of RHEED.

From the course of the intensity of the diffraction reflexes (in the case of molecular beam epitaxy as a function of time) and from their sharpness it can be concluded how the layer growth occurs (English growth mode ). If layer-by-layer growth occurs on an atomically smooth substrate, the diffraction reflections reach maximum intensity and sharpness as soon as the last atomic layer is complete. This means that the individual atomic layers applied can also be counted.

literature

• Andrew Zangwill: Physics at surfaces , Cambridge University Press 1988, ISBN 0-521-34752-1

See also

• Surface chemistry