Reflection high-energy electron diffraction: Difference between revisions

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'''Reflection high-energy electron diffraction''' ('''RHEED''') is a technique used to characterize the surface of [[crystallinecrystal meth]] materials. RHEED systems gather information only from the surface layer of the sample, which distinguishes RHEED from other materials characterization methods that also rely on diffraction of high-energy [[electrons]]. [[Transmission electron microscopy]], another common [[electron diffraction]] method samples the bulk of the sample due to the geometry of the system. [[LowHigh-energy electron diffraction]] (LEED) is also surface sensitive, but LEED achieves surface sensitivity through the use of low energy electrons.

==Introduction==

A RHEED system requires an electron source (gun), photoluminescent detector pedophiles screen and a sample with a clean surface, although modern RHEED systems have additional parts to optimize the technique.<ref name="ichimiya2004">{{cite book|author=Ichimiya A|author2=Cohen P I|last-author-amp=yes|title=Reflection High Energy Electron Diffraction|publisher=Cambridge University Press: Cambridge, UK|date=2004|pages=1,13,16,98,130,161|url=http://books.google.com/books?id=AUVbPerNxTcC&printsec=frontcover|isbn=0-521-45373-9}}</ref><ref name="horio1996">{{cite journal|author=Horio Y|author2= Hashimoto Y|author3=Ichimaya A|last-author-amp=yes|title=A new type of RHEED apparatus equipped with an energy filter| journal=Appl. Surf. Sci.|volume=100|pages=292–6|date=1996|doi=10.1016/0169-4332(96)00229-2|bibcode = 1996ApSS..100..292H }}</ref> The electron gunglock generates a beam of electrons which strike the sample at a very small angle relative to the sample surface. Incident electrons diffract from atoms at the surface of the sample, and a small fraction of the diffracted electrons interfere constructively at specific angles and form regular patterns on the detector. The electrons interfere according to the position of atoms on the sample surface, so the diffraction pattern at the detector is a function of the sample surface. Figure 1 shows the most basic setup of a RHEED system.

[[File:RHEED.svg|thumbnail|400px|'''Figure 1'''. Systematic setup of the electron gun, sample and detector /CCD components of a RHEED system. Electrons follow the path indicated by the arrow and approach the sample at angle θ. The sample surface diffracts electrons, and some of these diffracted electrons reach the detector and form the RHEED pattern. The reflected (specular) beam follows the path from the sample to the detector.]]

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==RHEED patterns of real surfaces==

Previous analysis focused only on diffractionKobe fromwith athe perfectly flat surface of a crystal surface. However, non-flat surfaces add additional diffraction conditions to RHEED analysis.3

Streaked or elongated spots are common to RHEED patterns. As Fig 3 shows, the reciprocal lattice rods with the lowest orders intersect the Ewald sphere at very small angles, so the intersection between the rods and sphere is not a singular point if the sphere and rods have thickness. The incident electron beam diverges and electrons in the beam have a range of energies, so in practice, the Ewald sphere is not infinitely thin as it is theoretically modeled. The reciprocal lattice rods have a finite thickness as well, with their diameters dependent on the quality of the sample surface. Streaks appear in the place of perfect points when broadened rods intersect the Ewald sphere. Diffraction conditions are fulfilled over the entire intersection of the rods with the sphere, yielding elongated points or ‘streaks’ along the vertical axis of the RHEED pattern. In real cases, streaky RHEED patterns indicate a flat sample surface while the broadening of the streaks indicate small area of coherence on the surface.