Quantitative Electron Diffraction from Thin Films
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periodic arrangement of atoms, i.e., the correlation function of positions, and is sensitive to disorder in this arrangement. The power of electron diffraction lies in its relatively easily achievable surface sensitivity, because of the high probability for both elastic and inelastic scattering of electrons by atoms. Another distinct asset of electron diffraction is its ease of implementation. It is straightforward to obtain a monoenergetic electron beam; signal levels are high because of the high reflectivity of surfaces for electrons; and detection is easy because electrons are charged particles that can be manipulated with electric and magnetic fields. For these reasons, some form of electron diffraction is found in nearly all molecular beam epitaxy systems and other vacuum stations devoted to the growth of epitaxial films or to the initial formation of films on crystalline surfaces. There are, of course, also limitations to electron diffraction techniques. One is the need for a reasonable vacuum (~10~'4 Torr), because electrons scatter easily off gas atoms in the vapor (for the same reasons that one has surface sensitivity). Thus one does not, for example, find electron diffraction as a tool in most chemical vapor deposition systems. Another limitation is the requirement for structures that are coherent in the surface plane, i.e., epitaxial films or at least films with a preferred growth orientation. We will discuss these and other limitations in greater detail later. The names and acronyms most commonly associated with surface-sensitive electron diffraction are low-energy electron diffraction (LEED) and reflection high-energy electron diffraction (RHEED). In addition, there is medium-energy electron diffraction (MEED). The distinctions implied by such different names are more
historical than actual. The primary difference is the energy of the incident electrons. The information content in all methods is similar. RHEED is typically associated with an electron energy between 10 and 100 keV For such energies, surface sensitivity is achieved with grazing incidence: even though the electron mean free path at these energies is hundreds of Angstroms, a small enough incident angle (0.1 to 5°, depending on the energy) forces the beam to remain near the surface. The same objective can be achieved as well with x-rays (see the article by Fullerton et al. in the December MRS Bulletin), but the much lower scattering cross section of atoms for x-rays requires more intense sources, e.g., a synchrotron. Nevertheless, surfacesensitive x-ray diffraction is becoming an increasingly important tool. Low-energy electron diffraction is typically associated with an electron energy between 10 and 500 eV and with normal incidence. At these energies, the electron mean free path is of the order of only a few Angstroms. The electron energy for MEED lies between these two regimes. A distinct advantage of a grazing geometry is the accessibility of the sample— for example to sources of atoms for the growth of films and to optical or scan
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