Lattice dynamics and dielectric functions of multiferroic BiFeO3/c-sapphire films determined by infrared reflectance spectra and temperature-dependent Raman scattering

Abstract

Multiferroic BiFeO3 (BFO) films have been grown on c-sapphire substrates by pulsed laser deposition under different laser energies (EL). The X-ray diffraction and Raman spectra indicate that the films are polycrystalline and exhibit the single rhombohedral (R) phase. The crystal distortion becomes weaker with decreasing the EL, which is described by the ratio of c/a. It was found that different EL values also lead to the variation of the Bi/Fe ratio. Temperature-dependent Raman spectra were carried out to study the phonon mode evolution behaviors. The three A1 transverse optical (TO) phonon modes located at 219, 172, and 142cm−1 shift towards a lower energy side with the temperature due to thermal expansion, thermal disorder and the anharmonic effects of lattice. The E(TO) and three A1(TO) phonon frequencies slightly increase with increasing the EL of the growth condition, which results from the Bi vacancies, the changes of the length and intensity of BiO bonds and the local structure distortion in the FeO6 octahedra. The dielectric functions of the BFO films in the frequency range of 50–8000cm−1 have been extracted by fitting infrared reflectance spectra with the Lorentz multi-oscillator dispersion model. The variation trend of the dielectric functions with different EL can be observed and related to the packing density, surface roughness, and defect states. It was concluded that the EL corresponding to changing the c/a ratio has an obvious influence on the lattice vibrations and intraband transitions of the BFO films.