Surface and interface phonons of CaF₂ epitaxial layers on Si(111) measured by high resolution electron energy loss spectroscopy

Dipole scattering high resolution electron energy loss spectroscopy (HREELS) experiments were carried out in specular geometry on CaF₂ epilayers on Si(111). Surface and interface optical phonons (Fuchs–Kliewer modes) were measured with intensities and frequencies depending on layer thickness. At low (<50 Å) and intermediate (200 Å) thicknesses, clear deviations were observed between the experimental data and the predictions of the dielectric theory. These deviations can be due to residual stress in the layers. A characterization of the stress was attempted by using impact scattering HREELS to investigate stress‐induced modifications of the surface acoustical phonons. The dispersion curve of the Rayleigh mode was measured for CaF₂(111) and for a 15 Å layer CaF₂/Si(111) in the two principal directions of the surface Brillouin zone. Striking differences are observed at the M̄ point where the layer frequency is at variance with the single crystal one. Theoretical calculations are in progress in order to extract from these data, a quantitative evaluation of the stress.