Strain measurement of epitaxial CaF2 on Si (111) by MeV ion channeling

Planar strain in CaF2 films, grown by molecular beam epitaxy at 700 °C on (111) Si substrates, has been measured by MeV 4He+ ion channeling. For CaF2 films thinner than 200 nm, the planar strain was found to be tensile. No strain was observed for films thicker than 200 nm. The observed tensile strain cannot be explained by a simple pseudomorphic growth model since the planar strain would be compressive due to the larger lattice constant of CaF2 relative to Si. A plausible explanation of the results is that defects are nucleated at the growth temperature to relieve stress. These defects then result in a tensile planar strain as the sample is cooled down after growth due to the large difference in thermal expansion coefficients between CaF2 and Si.