CONTRIBUTION OF SURFACE MICROSCOPIC PHONONS TO THE ELECTRON ENERGY-LOSS SPECTRUM OF IONIC FILMS ON AN INTRINSIC SEMICONDUCTOR

Starting from shell-model lattice dynamical equations, a microscopic formalism is developed for computing the high-resolution electron energy-loss spectroscopy (HREELS) spectrum of a thin ionic crystalline film in the dipolar-scattering regime. Test calculations are performed for KF(001) and RbF(001) films on a Si or Ge substrate and compared with the predictions of the standard dielectric approximation. It is shown that the dielectric theory fails in producing all the loss features predicted by the microscopic calculations for very thin ionic overlayers, a few nm thick. We find in particular a contribution of the S2 surface microscopic phonon of the RbF and KF overlayers which is responsible for a non-negligible loss intensity in the region of the low-frequency Fuchs-Kliewer (interface) mode predicted by electrostatics. Theoretical results for these latter systems are extrapolated to available experimental data for CaF2(111)/Si. We claim that the contribution of the S2 phonon should be large enough to be observed experimentally in HREELS spectra of the systems like RbF/Ge.