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.

==Introduction==

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A RHEED system requires an electron source (gun), photoluminescent detector 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|name-list-style=amp|title=Reflection High Energy Electron Diffraction|publisher=Cambridge University Press: Cambridge, UK|date=2004|pages=1,13,16,98,130,161|url=https://books.google.com/books?id=AUVbPerNxTcC|isbn=0-521-45373-9}}</ref><ref name="horio1996">{{cite journal|author=Horio Y|author2= Hashimoto Y|author3=Ichimaya A|name-list-style=amp|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 gun 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.]]

==Surface diffraction==

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The Ewald's sphere is centered on the sample surface with a radius equal to the magnitude of the wavevector of the incident electrons,

<{{center>|<math>k_i=\frac{2\pi}{\lambda}</math>,</center>}}

where λ is the electrons' [[Matter wave|de Broglie wavelength]].

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===Dynamic scattering analysis===

The dynamically, or inelastically, scattered electrons provide several types of information about the sample as well. The brightness or intensity at a point on the detector depends on dynamic scattering, so all analysis involving the intensity must account for dynamic scattering.<ref name="ichimiya2004"/><ref name="braun1999"/> Some inelastically scattered electrons penetrate the bulk crystal and fulfill Bragg diffraction conditions. These inelastically scattered electrons can reach the detector to yield kikuchiKikuchi diffraction patterns, which are useful for calculating diffraction conditions.<ref name="braun1999"/> Kikuchi patterns are characterized by lines connecting the intense diffraction points on a RHEED pattern. Figure 6 shows a RHEED pattern with visible [[Kikuchi line (solid state physics)|Kikuchi lines]].

[[File:TiO2 Surface with Kikuchi Lines.gif|thumbnail|400px|'''Figure 6'''. A RHEED pattern from a TiO₂ (110) surface with visible Kikuchi lines. The Kikuchi lines pass through the Laue circles and appear to radiate from the center of the pattern.]]

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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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===RHEED-TRAXS===

Reflection high energy electron diffraction - total reflection angle X-ray spectroscopy is a technique for monitoring the chemical composition of crystals.<ref>{{cite journal|last1=Hasegawa|first1=Shuji|last2=Ino|first2=Shozo|last3=Yamamoto|first3=Youiti|last4=Daimon|first4=Hiroshi|title=Chemical Analysis of Surfaces by Total-Reflection-Angle X-Ray Spectroscopy in RHEED Experiments (RHEED-TRAXS)|journal=Japanese Journal of Applied Physics|volume=24|issue=6|pages=L387–L390|date=1985|doi=10.1143/JJAP.24.L387|bibcode = 1985JaJAP..24L.387H |s2cid=94132569 }}</ref> RHEED-TRAXS analyzes X-ray spectral lines emitted from a crystal as a result of electrons from a RHEED gun colliding with the surface.

RHEED-TRAXS is preferential to X-ray microanalysis (XMA)(such as [[Energy-dispersive X-ray spectroscopy|EDS]] and [[Wavelength dispersive X-ray spectroscopy|WDS]]) because the incidence angle of the electrons on the surface is very small, typically less than 5°. As a result, the electrons do not penetrate deeply into the crystal, meaning the X-ray emission is restricted to the top of the crystal, allowing for real-time, in-situ monitoring of surface stoichiometry.

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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]]