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The endotaxia is the process of growth of one crystal structure inside the volume of another. In this case we are talking about the formation of a film of silicon carbide in a silicon substrate. A silicon substrate is placed in the gas chamber. The sample is exposed to the stream of methane gas CH4 at temperature of 1360 - 1380 °C and at normal pressure. Moreover, gas contains both the stable carbon isotope C12 and the radioactive carbon isotope C14, and hydrogen H2 in the gas acts as a carrier of carbon. The process lasts up for 1 hour, during which methane molecules chemically react with sili-con atoms, forming silicon carbide and hydrogen: СН4 + Si → SiC + 2H2. As a result, a silicon carbide SiC film grows in the surface of the sample, and the hydrogen formed dur-ing the reaction leaves the sample and is of no further interest. As a result of the chemical reaction, the crystal lattice of the sample changes and, due to the different parameters of the Si and SiC lattices, defects are formed. The defects in the crystal lattice are various stable violations of the translational symmetry of a crystal. Such defects represent an empty lattice site, and thus are a so-called vacancy for other atoms. In fact, at temperatures above absolute zero, in a solid some of the lattice sites will always be empty, which means that vacancies in a solid will always exist. Carbon atoms, which are already part of the structure, can undergo self-diffusion and diffuse into the interior. What is more, an interstitial diffusion mechanism is distinguished, when an impurity atom moves from one internode to an empty neighboring one, and a vacancy mechanism, when an atom moves from one lattice site to the site where the vacancy is located. We consider a physical model that allows one to describe diffusion in a solid, which takes into account the vacancy mechanism of motion of impurity atoms, i.e. substitution mechanism.