LOW-ENERGY-ELECTRON MICROSCOPY

Low energy electron microscopy (LEEM) is a new technique for surface imaging, based on the wave nature of the electron. Although it makes use of electron lenses as in conventional electron microscopes it differs from them in that the electrons have energies of the order 1-100 eV when they interact with the surface. As a consequence, the method is very surface-sensitive and the probing depth may be tuned by varying the energy. Contrast is mainly produced by diffraction. Resolution is determined mainly by the chromatic and spherical aberration of the decelerating/accelerating field in front of the specimen. LEEM is easily combined with low energy electron diffraction (LEED), photoemission electron microscopy (PEEM) and other emission microscopies. The specimen is easily accessible which allows a wide variety of in situ studies over a wide temperature range. The review gives an introduction to the basic electron-specimen interaction processes, to the instrumentation and to the factors which govern contrast and resolution. The remaining, larger part of the review deals with applications to clean surfaces, atom-surface interactions and thin films. Topics discussed include topography, phase transitions, adsorption, reaction, segregation, growth, sublimation and magnetic microstructure. Related techniques are also discussed briefly.