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Refection high energy electron diffraction (RHEED) is widely used as a sensitive technique for the in situ investigation of surface structures and growth processes in MBE. The geometry of RHEED is quite simple. A high energy beam (3-100 keV) is directed at the sample surface at a grazing angle (< � 3). The electrons are diffracted by the crystal structure of the sample and then impinge on a phosphor screen mounted opposite to the electron gun. Upon refection, electrons diffract, forming a diffraction pattern that depends on the structure and the morphology of the probed surface. The grazing incidence angle ensures that only a few atomic layers are probed despite the high energy of the incident electrons. Conceptually, perfectly flat surfaces should result in a diffraction pattern that consists of spots arranged on Laue rings. However, because of the non-idealities in the electron beam and the sample's surface, streaks appear instead of spots. [1,2] The distance between the streaks is an indication of the surface lattice unit cell size. If a surface is atomically flat, then sharp RHEED patterns are seen. If the surface is rough, then the RHEED pattern is more diffuse. In many cases, because of the surface roughness, the diffraction pattern is produced by transmission through the three-dimensional crystalline islands. In kinematic scattering theory, [3] the diffraction results when the Laue condition is satisfied:

where ks and k0 are the wave vectors for the diffracted and the incident beams, respectively and G is the reciprocal-lattice vector. In the special case of elastic scattering, |k0|= |ks| and the diffraction condition can be obtained from the geometrical construction of Ewald sphere. An Ewald sphere is a sphere that has its origin as the origin of the k0 and a radius |k0|. Hence, the Laue condition may be re-formalized as "diffraction occurs for all connecting the origin of the sphere and a reciprocal-lattice point".

For detailed discussions on RHEED theories please see [2,3]

Find the New BOOK on RHEED (highly recommended)

Reflection High-Energy Electron Diffraction
by Ayahiko Ichimiya, Philip I. Cohen, Cambridge University Press (2004)

also see useful links

Literature on RHEED

[1] J. E. Mahan, K. M. Geib, G. Y. Robinson, and R. G. Long, J. Vac. Sci. Technol. A 8, 3692 (1990).

[2] W. Braun, Applied rheed, in Springer Tracts in Modern Physics, edited by G. H�hler et al., volume 154, Springer-Verlag, Berlin and Heidelberg, 1999.

[3] E. Bauer, Reflection electron diffraction (red), in Techniques of metals research, edited by R. F. Bunshah, volume II, Part-2, page 501, Interscience, New York, 1969.

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