The use of low-energy electrons for surface analysis is due to two main reasons:

1. The de Broglie wavelength of electrons with energies of 30-200 eV is ca. 0.1-0.2 nm, which satisfies the condition of diffraction on atomic structures, i.e. the wavelength is equal or smaller than interatomic distances.

2. The average path length of low-energy electrons equals to several atomic layers. As a result, most of the elastic scattering occurs in the uppermost layers of the sample; therefore, they make the maximum contribution in the diffraction pattern.

The figure shows the experimental setup for direct observation of the LEED patterns. In an electron gun the electrons emitted by the cathode (at a negative potential – V) are accelerated to an energy eV, and then move and scatter on the sample in the fieldless space, since the first grid of the diffractometre and the sample are grounded. The second and third grids, at a potential slightly less than the cathode potential (V – ΔV), are used to cut off the inelastically scattered electrons. The fourth grid is grounded and shields the other grids from the fluorescent screen with a potential of about +5 kV. Thus, the electrons are elastically scattered on the sample surface, pass through the retarding grids and then get accelerated to high energies to cause fluorescence of the screen, which represents the diffraction pattern. As an example, the figure shows a LEED pattern of atomically clean Si(111)7×7.

The LEED method allows:

1) qualitative assessment of the structural perfection of the surface (a well-ordered surface gives a LEED pattern with clear bright reflections and low background);

2) determination of the reciprocal lattice of the surface from the diffraction pattern geometry;

3) evaluation of the surface morphology on the basis of the diffraction reflex profile;

4) determination of the atomic structure of the surface by comparing the dependencies of the diffraction reflection intensity on the energy of the electrons (I-V curves), calculated for the structural models with dependencies obtained in the experiment.

The LEED and RHEED methods differ in the energy of the electrons and, correspondingly, in geometries (in LEED the electron beam is practically perpendicular to the surface, while in RHEED it falls at a grazing angle of ca. 1-5º). Both methods provide similar information about the surface structure. The advantage of the LEED is its simpler arrangement, as well as more visual and easy to interpret results. The advantage of the RHEED is the possibility of carrying out research during the process of film growing on the sample surface.