Crystal structure of thin CaSi2 films grown by radiation-induced epitaxy

•The electron radiation-induced phase transformation of CaF2 into the CaSi2 film.•CaSi2 is formed during CaF2 epitaxy under electron beam exposure at temperature more than 300 °C.•The transition of CaSi2 polymorph 3R to 6R was found with increasing thickness of CaF2. The electron-beam irradiation (electron energy 20 keV, current density 50 μA/m2) effects on the CaF2 films epitaxially grown on a Si(111) substrate have been studied with Raman spectroscopy (RS). It was found that the radiation-induced phase transition of CaF2 into CaSi2 films takes place at electron-beam irradiation both as during the epitaxial CaF2 film growth and as after completed growth of the film. The temperature dependence studies of the observed radiation-induced phase transition show needs to use substrate heating above 300 °C. The crystal structure of CaSi2 films was found to depend on the thickness of the grown CaF2 film. For thin films (less than 20 nm) crystal structure of CaSi2 films belongs to the trigonal rhombohedral modification 3R (space group R3̅m with a three-layer translational period of silicon substructures in the unit cell 3R). The transition of CaSi2 crystal structure to the trigonal rhombohedral modification a six-layer period material 6R was found for thicker films (>20 nm).