Reflection high-energy electron diffraction: Difference between revisions

Revision as of 21:32, 24 August 2020

Revision as of 12:50, 15 October 2020

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==Introduction==

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|lastname-authorlist-ampstyle=yesamp|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&printsec=frontcover|isbn=0-521-45373-9}}</ref><ref name="horio1996">{{cite journal|author=Horio Y|author2= Hashimoto Y|author3=Ichimaya A|lastname-authorlist-ampstyle=yesamp|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.]]

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RHEED users construct [[Ewald's sphere]]s to find the crystallographic properties of the sample surface. Ewald's spheres show the allowed diffraction conditions for kinematically scattered electrons in a given RHEED setup. The diffraction pattern at the screen relates to the Ewald's sphere geometry, so RHEED users can directly calculate the reciprocal lattice of the sample with a RHEED pattern, the energy of the incident electrons and the distance from the detector to the sample. The user must relate the geometry and spacing of the spots of a perfect pattern to the Ewald's sphere in order to determine the reciprocal lattice of the sample surface.

The Ewald's sphere analysis is similar to that for bulk crystals, however the reciprocal lattice for the sample differs from that for a 3D material due to the surface sensitivity of the RHEED process. The reciprocal lattices of bulk crystals consist of a set of points in 3D space. However, only the first few layers of the material contribute to the diffraction in RHEED, so there are no diffraction conditions in the dimension perpendicular to the sample surface. Due to the lack of a third diffracting condition, the reciprocal lattice of a crystal surface is a series of infinite rods extending perpendicular to the sample's surface.<ref name="oura2001">{{cite book|author=Oura K|author2=Lifshits V G|author3=Saranin A A|author4=Zotov A V|author5=Katayama M|lastname-authorlist-ampstyle=yesamp|title=Surface Science: An Introduction|publisher=Springer-Verlag: Berlin|date=2001|pages=59–65|isbn=3-540-00545-5}}</ref> These rods originate at the conventional 2D reciprocal lattice points of the sample's surface.

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

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[[File:Azimuth angles comparison.svg|thumb|400px|'''Figure 5'''. The incident electron beam is incident on an identical surface structure at a different azimuth angles in a) and b). The sample is viewed from the top in the figure, and the points correspond to the reciprocal lattice rods, which extend out of the screen. The RHEED pattern would be different for each azimuth angle.]]

Users sometimes rotate the sample around an axis perpendicular to the sampling surface during RHEED experiments to create a RHEED pattern called the azimuthal plot.<ref name="oura2001"/> Rotating the sample changes the intensity of the diffracted beams due to their dependence on the azimuth angle.<ref name="mitura1993">{{cite journal|author=Mitura Z|author2=Maksym P A|lastname-authorlist-ampstyle=yesamp|title=Analysis of reflection high energy electron diffraction azimuthal plots|journal=Phys. Rev. Lett.|volume=70|pages=2904–2907|date=1993|doi=10.1103/PhysRevLett.70.2904|pmid=10053683|issue=19|bibcode=1993PhRvL..70.2904M}}</ref> RHEED specialists characterize film morphologies by measuring the changes in beam intensity and comparing these changes to theoretical calculations, which can effectively model the dependence of the intensity of diffracted beams on the azimuth angle.<ref name="mitura1993"/>

===Dynamic scattering analysis===

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[[File:TiO2 Terraced and Streaked RHEED Pattern.gif|thumbnail|400px|'''Figure 7'''. Streaked RHEED pattern from the TiO₂ (110) surface. The sample had a terraced surface, which caused noticeable streaking compared to the RHEED pattern from the flat TiO₂ (110) surface shown above.]]

Surface features and polycrystalline surfaces add complexity or change RHEED patterns from those from perfectly flat surfaces. Growing films, nucleating particles, crystal twinning, grains of varying size and adsorbed species add complicated diffraction conditions to those of a perfect surface.<ref name="bozovic2001">{{cite book|author=Bozovic I|author2=Eckstein J N|author3=Bozovic N|lastname-authorlist-ampstyle=yesamp|title=In Situ Real-Time Characterization of Thin Films|publisher=John Wiley and Sons: New York|date=2001|pages=29–56|editor=Auceillo O|editor2=Krauss A R|url=https://books.google.com/books?id=kmyVA1OYB_UC&pg=PA29|isbn=0-471-24141-5}}</ref><ref name="brewer2001">{{cite journal|author=Brewer R T|author2=Hartman J W|author3=Groves J R|author4=Arendt P N|author5=Yashar P C|author6=Atwater H A|lastname-authorlist-ampstyle=yesamp|title=Rheed in-plane rocking curve analysis of biaxially-textured polycrystalline MgO films on amorphous substrates grown by ion beam-assisted deposition|journal=Appl. Surf. Sci.|volume=175|issue=1–2|pages=691–696|date=2001|doi=10.1016/S0169-4332(01)00106-4|bibcode = 2001ApSS..175..691B }}</ref> Superimposed patterns of the substrate and heterogeneous materials, complex interference patterns and degradation of the resolution are characteristic of complex surfaces or those partially covered with heterogeneous materials.

==Specialized RHEED techniques==

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===Film growth===

RHEED is an extremely popular technique for monitoring the growth of thin films. In particular, RHEED is well suited for use with [[molecular beam epitaxy]] (MBE), a process used to form high quality, ultrapure thin films under ultrahigh vacuum growth conditions.<ref name="atwater1997">{{cite journal|author=Atwater H A|author2=Ahn C C|author3=Wong S S|author4=He G|author5=Yoshino H|author6=Nikzad S|lastname-authorlist-ampstyle=yesamp|title=Energy-Filtered Rheed And Reels For In Situ Real Time Analysis During Film Growth|journal=Surf. Rev. Lett.|volume=4|pages=525|date=1997|doi=10.1142/S0218625X9700050X|bibcode = 1997SRL.....4..525A|issue=3 }}</ref> The intensities of individual spots on the RHEED pattern fluctuate in a periodic manner as a result of the relative surface coverage of the growing thin film. Figure 8 shows an example of the intensity fluctuating at a single RHEED point during MBE growth.

[[File:Oscilatting function.gif|thumbnail|400px|'''Figure 8'''. The curve is a rough model of the fluctuation of the intensity of a single RHEED point during MBE deposition. Each peak represents the forming of a new monolayer. Since the degree of order is at a maximum once a new monolayer has been formed, the spots in the diffraction pattern have maximum intensity since the maximum number of diffraction centers of the new layer contribute to the diffracted beam. The overall intensity of the oscillations is dropping the more layers are grown. This is because the electron beam was focused on the original surface and gets out of focus the more layers are grown. Note that the figure is only a model similar in shape to those used by film growth experts.]]

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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|lastname-authorlist-ampstyle=yesamp|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 |url=https://semanticscholar.org/paper/5d36f788118b97e81778e722e235469ca542cc63}}</ref>

==References==