I. Introduction
URGENT DIRECTION of modern solid physics is investigation of atomic diffusion processes on crystal surface during epitaxial growth, adsorption and sublimation. Surface diffusion determines the processes of formation of atomically-smooth surfaces and interfaces, that makes these investigations actual for technological applications. The control and analyzes of these processes is difficult at high temperatures. High mobility of adsorbed atoms (adatoms) and big amplitude of thermal fluctuations does not allow to resolve atomic structure by scanning tunneling and high resolution electron microcopy techniques. However the information about atomic mechanisms of surface diffusion can be obtained from investigations of atomic steps motion kinetics. One of the methods allowing to visualize structure and morphology of crystal surface with high spatial and temporal resolution is in situ ultra-highvacuum reflection electron microscopy (UHV REM) [1]. This method allows to visualize atomic step which height on Si(111) face is 0.314 nm at wide temperature range (from room temperature to the melting point).