Growth of Mg film on amorphous Si (a-Si) at room temperature in UHV conditions was studied in situ with optical differential reflection spectroscopy and electron energy loss spectroscopy. The phase composition of the film was also studied by high-resolution transmission electron microscopy. The mechanism of suicide film growth on a-Si is considered. The origin of internal stress within the growing film and its role in the silicide film growth process are discussed. Due to high pressure occurring within the growing film, the first phase to form is the hexagonal suicide phase h-Mg2Si. According to the DRS data, the phase h-Mg2Si is semiconducting. The new peak in the differential reflectance spectrum is assigned to the h-Mg2Si. At later stages of Mg deposition the cubic silicide phase c-Mg2Si grows. (C) 2014 Elsevier B.V. All rights reserved.