LEEM and SPLEEM

Low-energy electron microscopy (LEEM: http://www.leem-user.com) is an imaging method that makes use of elastically backscattered electrons with energies below about 100 eV, frequently with less than 10 eV. In contrast to transmission electron microscopy (TEM), which generally works with electrons in the 100 keV range where backscattering is negligible, the backscattering cross sections for low-energy electrons are large enough to make them useful for surface imaging. This was already evident in the classical diffraction experiments of Davission and Germer, but it took 35 years before the use of slow diffracted electrons for surface imaging was suggested and another 23 years before convincing images could be published. Thus, although diffraction of slow electrons and imaging with slow emitted electrons with resolution in the micrometer range were demonstrated before TEM reached submicron resolution, LEEM became a viable imaging method only much later. This chapter reviews the basics of LEEM, its present state of art, and its applications. An associated imaging method, spin-polarized LEEM (SPLEEM), which gives magnetic information, will also be discussed. Other methods, such as mirror electron microscopy (MEM), which gives information mainly on the local surface potential, Auger electron emission microscopy (AEEM), which gives chemical information, electron energy loss microscopy (EELM), which gives some electronic information, and secondary electron emission microscopy (SEEM), will be mentioned only briefly because they have been used much less frequently, although they are also useful. Photoelectron emission microscopy (PEEM), in particular XPEEM, is included only in connection with the discussion of SPELEEM because it is the subject of another chapter.