Vibrational Properties of ultrathin Perovskite Films

Schumann1, K. Meinel1 and W. Widdra1,2

1.Institute of Physics, Martin-Luther-Universität Halle-Wittenberg, Von-Danckelmann-Platz 3, 06120 Halle, Germany

2.Max Planck Institute of Microstructure Physics, Weinberg 2, 06120 Halle, Germany

Phonons and their softening are key elements for the phase transitions in ferroelectrics and multiferroics. In thin films, the phonons depend sensitively as well as the phase transition temperatures on the strain within the film. Here we report on high-resolution electron energy loss spectroscopy (HREELS) on the (001) surfaces of BaTiO3, SrTiO3, and the Ruddlesden-Popper series of Srn+1RunO3n+1 with n of 1 and 2. HREELS reveals three dipole-active phonon-polariton (Fuchs-Kliewer) modes, which are derived from the known transversal optical bulk phonons. In addition, it will be demonstrated that the complex dielectric function in the energy range from 4 to 1000 meV (30 to 8000 cm-1) can be quantitatively extracted from the experimental loss function.

The full dielectric characterization for (001)-oriented single crystals of BaTiO3 and SrTiO3 with different intentional doping levels will be presented. The extracted surface dielectric function will be quantitatively compared with available bulk infrared data and allows the experimental determination of the surface-near doping level in oxides. For ultrathin films of BaTiO3(001) as grown by MBE or magnetron sputtering on various substrates, HREELS data allow for the first time the determination of the complex dielectric functions of ultrathin oxide films down to single unit cell thicknesses. These data will be discussed with respect of a possible two-dimensional electron gas (2DEG) at the surface, electron-phonon coupling, as well as strain-driven phonon softening.

Support by the Sonderforschungsbereich SFB-762 “Functional oxide interfaces” is gratefully acknowledged.