Two semiconductor compounds exist in Ca-Si system: Ca2Si [1] and Ca3Si4 [2]. It is known that CaSi2, as a compound in the Ca-Si system with a maximum silicon concentration [1], is a metal according to the theoretical work [3]. At atmospheric pressure a rhombohedral CaSi2 structure with the space group R-3m in two modifications hR3 and hR6 were observed [4]. The main problems for the CaSi2 film’s growth on Si substrate are the parameter mismatch of CaSi2 and Si(111) crystal lattices (0.4-1.0%) and near threefold increase in CaSi2 thermal expansion coefficient as compared with one for silicon at the growth temperature. Therefore, after CaSi2 film cooling to room temperature the lattice extension in the direction of [001] and compression in the direction of [100] were observed when the film thickness increases.    The grown at 500 oC Ca silicide film on Si(111) substrate [5] was assigned to semiconducting Ca3Si4, since a transparency region in the photon energy range from 0.2 eV to 0.7 eV with band gap width of 0.63 eV and a plasma reflection region at energies less than 0.2 eV [5] were observed, which is typical of a degenerated semiconductor. Indicated contradiction, that Ca3Si4 film or stressed CaSi2 film with changes in the energy band structure being formed at the reactive deposition at T = 500 oC on Si(111) substrate, can be resolved by investigating the cross-section structure and properties of Si/Ca silicide/Si (111) double heterostructures fabricated at the same substrate temperature (500 oC) in addition to possible strong stresses in the CaSi2 crystal lattice planes. Such stresses are not related to epitaxial CaSi2/Si (111) interface.  In this work we studied the peculiarities in the CaSi2 crystalline structure in the cross sections of CaSi2/Si(111) and Si(111)/CaSi2/Si(111) heterostructures and electronic band structure of rhombohedral CaSi2 structure under the constant stress.  It was established that stressed CaSi2 films are formed at the reactive deposition epitaxy of Ca on the Si(111)7×7 surface at a temperature of 500 oC, but at temperature of 500 oC the twinning epitaxial growth of silicon layer was observed in the Si(111)/CaSi2/Si (111) double heterostructures. Since the compession and stretching with units of percents are conserved both in double heterostructures HBS and CaSi2 films on Si (111) substrate, one can suggest strong distortions in the energy band structure of CaSi2, sufficient for appearance of the band gap. However our first principle’s calculations showed that small density of states conserves on the Fermi level in different points of Brillouin zone that correlates with semimetal properties of h-CaSi2, but not with optical measurement’s data [6]. For a precise answer to the question the stressed CaSi2 film on Si(111) substrate is a generated semiconductor or it is a semimetal as a bulk CaSi2 the additional temperature Hall effect measurements in the temperature range of 20-500 K are required.  This work was carried out with financial support from the Russian Found of Basic Researches grant (No. 1652-00074) and Belarusian Science Foundation for Fundamental Research (No. F16R-048).