Reflection High Energy Electron Diffraction (RHEED)

A high energy electron beam (3-100 keV) is directed at the sample surface at a grazing angle. The electrons are diffracted by the crystal structure of the sample and then impinge on a phosphor screen mounted opposite to the electron gun (compared with LEED). The resulting pattern is a series of streaks. The distance between the streaks being an indication of the surface lattice unit cell size. The grazing incidence angle ensures surface sensitivity despite the high energy of the incident electrons. If a surface is atomically flat, then sharp RHEED patterns are seen. If the surface is rough, the RHEED pattern is more diffuse. This behavior can lead to "RHEED oscillations" as a material is evaporated onto a surface following the layer-by-layer growth mode. RHEED is therefore of particular use with MBE. Jason Drotar in our lab developed a energy filter RHEED that allows new applications such as the quantitative determination of carbon nanotube correlations. See his paper in Phys. Rev. B 64, 12517 (2001) [Full Text PDF].

RHEED Procedure Manual

High Resolution Low Energy Electron Diffraction (HRLEED)

There are about 50 Henzler types of HRLEED system (U. Scheithauer, G. Meyer, and M. Henzler, Surface Science 178, 441, (1986)) in the world. We believe our group is one of the most productive users of HRLEED besides Henzler's group in Hannover, Germany. The HRLEED system not only can resolve domain size from a few angstroms to 2000 angstroms but also has millisecond temporal resolution. Numerous novel surface, overlayer, and thin film phenomena have been observed using this system. HRLEED is particularly ideal for real-time monitoring of 2D island growth (or shrinking) of overlayer and multilayer structure on surfaces quantitatively. HRLEED is complementary to the grazing incidence X-ray diffraction technique for surface ordering studies. The latter uses an intense synchrotron radiation source and can achieve very high spatial resolution. For the study of the dynamics of growth, HRLEED has the advantage of high temporal resolution. Because of the strong interactions between the electrons and surface atoms, the number of counts per seconds is very high for each data point for electron diffraction. One can obtain an angular profile within a very short period of time (e.g. milliseconds). We have built an atomic resloution STM in house and have used it for the study of rough surfaces. The STM is powerful for real space imaging of surfaces and submonolayer or monolayer films. However, when it is used for multilayer thicker film study the maximum depth that the tip can probe is limited to less than ten layers high unless the tip is very sharp. That is the valley of rough surfaces or rough films can not be reached by the tip. We believe for dynamic study of rough surfaces and multilayer thick films the diffraction technique has certain advantages and is statistically quantitative.

Auger Electron Spectroscopy (AES)

Auger Electron Spectroscopy studies the releasement of electrons with characteristic energies from elements in the surface of the material. This technique is derived from the Auger effect. AES identifies the chemical elements in the surface of the material.