Structural and optical properties of BiFeO3 thin films

Thin films of BiFeO3 (BFO) were prepared at 550 & DEG;C annealing temperature on very thin glass and aluminum substrates by spin-coating technique. Bismuth acetate and iron nitrate were utilized as precursors materials, while ethylene glycol was used as a solvent. Structural measurement by XRD indicates a BFO amorphous phase on the Al substrate. Raman and PL spectroscopies were used to determine the structural optical phonons and defects, respectively. While XPS was necessary to describe the chemical properties of the BFO surface. Transmittance and reflectance spectra, from 190 to 15500 nm (UV-Visible-infrared), were collected and they were fitted using Tauc-Lorentz and Lorentz classical dispersion models of the complex dielectric function to obtain additional optical properties, such as refraction index and extinction coefficient. The results indicate that the BFO that grows on a thin glass substrate presents an R3c crystalline structure, while the one that grows on aluminum shows an amorphous phase. Both films are dense and without cracks, although the presence of porosity in BFO/glass was observed. The optical response and the valence analysis of the chemical elements suggest the existence of oxygen vacancies in both samples. Furthermore, a blue shift in some vibrational modes and optical transitions can be explained through a compressive stress at the film-substrate interface due to differences in coefficients of thermal expansion. In addition, a first principles calculation using Density Functional Theory (DFT) of crystallized BFO is performed to observe the structural and optical properties. These theoretical results were compared with those obtained experimentally.