Reflection high-energy electron diffraction: Difference between revisions

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{{short description|TechniqueElectron indiffraction crystallographyby reflection from surfaces}}

'''Reflection high-energy electron diffraction''' ('''RHEED''') is a [[analytical technique|technique]] used to characterize the surface of [[crystalline]] materials. RHEED systems gather information only from the surface layer of the sample, which distinguishes RHEED from other [[material characterization|materials characterization]] methods that also rely on diffraction of high-energy [[electrons]]. [[Transmission electron microscopy]], another common [[electron diffraction]] method samples mainly the bulk of the sample due to the geometry of the system, although in special cases it can provide surface information. [[Low-energy electron diffraction]] (LEED) is also surface sensitive, but LEED achieves surface sensitivity through the use of low energy electrons.

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===Vacuum requirements===

Because gas molecules diffract electrons and affect the quality of the electron gun, RHEED experiments are performed under vacuum. The RHEED system must operate at a pressure low enough to prevent significant scattering of the electron beams by gas molecules in the chamber. At electron energies of 10keV10 keV, a chamber pressure of 10⁻⁵ mbar or lower is necessary to prevent significant scattering of electrons by the background gas.<ref name="dobson1988"/> In practice, RHEED systems are operated under ultra high vacuums. The chamber pressure is minimized as much as possible in order to optimize the process. The vacuum conditions limit the types of materials and processes that can be monitored in situ with RHEED.

==RHEED patterns of real surfaces==

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===MCP-RHEED===

MCP-RHEED is a system in which an [[electron beam]] is amplified by a [[micro-channel plate]] (MCP). This system consists of an [[electron gun]] and an MCP equipped with a [[fluorescence|fluorescent]] screen opposite to the electron gun. Because of the amplification, the intensity of the electron beam can be decreased by several orders of magnitude and the damage to the samples is diminished. This method is used to observe the growth of [[Electrical insulation|insulator]] crystals such as [[Organic compound|organic]] films and [[alkali halide]] films, which are easily damaged by electron beams.<ref name="saiki">{{cite journal|author=Saiki K|author2=Kono T|author3=Ueno K|author4=Koma A|name-list-style=amp|title=Highly sensitive reflection high-energy electron diffraction measurement by use of micro-channel imaging plate|journal=Rev. Sci. Instrum.|volume=71|pages=3478|date=2000|doi=10.1063/1.1287625|bibcode = 2000RScI...71.3478S|issue=9 |s2cid=43346059|url=https://semanticscholar.org/paper/5d36f788118b97e81778e722e235469ca542cc63}}</ref>

==References==

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*Introduction to RHEED, A.S. Arrot, Ultrathin Magnetic Structures I, ''Springer-Verlag'', 1994, pp.&nbsp;177–220

*A Review of the Geometrical Fundamentals of RHEED with Application to Silicon Surfaces, John E. Mahan, Kent M. Geib, G.Y. Robinson, and Robert G. Long, ''J.V.S.T.'', A 8, 1990, pp.&nbsp;3692–3700

{{Crystallography}}

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[[Category:Crystallography]]