RHEED

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RHEED stands for Reflection High Energy Electron diffraction. RHEED is quite similar to Low energy electron diffraction (LEED).

RHEED uses an electron gun and a display or recording device (e.g. a fluorescent screen) for obtaining information on the structure and/or morphology of a crystal surface.

Schematic of a RHEED setup. The distance from sample to screen, L, and the energy of the electron beam must be known to derive the lattice spacings of the crystal

RHEED uses much higher electron energy (typically 5-100keV) and lower impact angles (<5°) than LEED. The higher energy sharpens the picture, while the lower angle makes the electrons just pass a few atomic layers into the crystal. This makes RHEED pictures represent the structure of the surface, not the whole crystal.

Due to this surface sensitivity, single crystal materials that have a flat surface on the nanometre scale will produce a RHEED pattern with lines instead of dots (as would be seen in most other electron diffraction techniques). Relatively rough surfaces will produce a RHEED pattern with dots present.

This is because the glancing angles used in RHEED mean that a very flat surface is effectively a 2-dimensional structure. In reciprocal space, this will produce a 1-dimensional rod, rather than a 0-dimensional dot as is (approximately) produced with most other techniques. Consequently, the reciprocal lattice lines touch the Ewald sphere over a wider range than normal, and the Laue diffraction condition is fulfilled over a range in the forward direction of the electron beam, producing streaks as opposed to dots.

RHEED is often used for real-time analysis and monitoring of crystal growth, for example in Molecular-beam epitaxy growth systems. The technique is particularly suited to this application as it does not block the direction vertical to the surface of the crystal which is observed, and is particularly sensitive to surface roughness, down to monolayer sensitivity. This allows growth rate of layers of atoms on the surface to be monitored, by analysing the 'RHEED oscillations' - the change from a RHEED pattern consisting of streaks to one of dots, and back. ^[1]

By knowing both the distance from the sample to the screen or recording medium and the energy of the electron source, it is possible to calculate the lattice spacings of the sample analysed.

Especially for the crystals that can be easily damaged, MCP-RHEED is used.

[edit] Further reading

Introduction to RHEED, A.S. Arrot, Ultrathin Magnetic Structures I, Springer-Verlag, 1994, pp. 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. 3692-3700