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Brief Summary of RHEED, LEED, XPS and AES

RHEED - reflection high energy electron diffraction.

Commonly used as an in situ monitoring technique for MBE. The grazing incidence electron beam does not interfere with the effusion sources. Scattering from a crystalline surface produces a diffraction pattern on a phosphor screen whose features are characteristic of the periodicity of the crystal, and the periodicity of the surface reconstruction in particular. Both MBE chambers in the group are equipped with RHEED systems.

LEED - low energy electron diffraction.

Instead of high energy grazing incidence electrons, in LEED one uses low energy (100 eV or so) electrons at normal incidence. These are diffracted through large angles by the target crystal and in a modern LEED optics, the diffracted electrons are accelerated back past the miniature electron gun on to a rear-view phosphor screen. All of the non-MBE chambers in the group are equipped with LEED systems.

XPS - X-ray photoelectron spectroscopy.

Commonly used chemical analysis technique: incident X-ray photons generate photoelectrons which are energy-analysed. Peaks in the energy spectrum are characteristic of the core levels of individual elements near the surface of the sample. The basic equation is: electron energy = photon energy - core level binding energy - work function, but actually other small shifts occur as well. Perhaps the most useful is the 'chemical shift': the core level binding energy is modified slightly (~ 1 eV) by the local chemical bonding environment of the atom. For example, at a partly oxidised GaAs surface, Ga bonded to As shows and Ga bonded to oxygen show slightly different binding energies in XPS.

AES - Auger electron spectroscopy.

Similar in concept to XPS, AES also allows chemical analysis of surfaces. An incident electron beam (energy ~ 5 keV) is used to excite Auger transitions in near-surface atoms. The energies of emitted Auger electrons are again characteristic of different elements.