Fabrication and optical properties of SrBi4Ti4O15/BaBi4Ti4O15 graded ferroelectric thin films

SrBi4Ti4O15 (SBTi) and BaBi4Ti4O15 (BBTi) thin films as well as SBTi/BBTi graded thin films with gradients in composition were prepared by sol-gel technique. A special heating treatment procedure was employed in this study to form the uniform composition gradients at the temperature below 850degreesC. The smooth graded thin films were obtained by rapid thermal annealing (RTA) at 750degreesC for 10 min and then annealing at 800degreesC for 60 min. The microstructural evolution of SBTi, BBTi and SBTi/BBTi graded films and their optical properties were studied. The band edge absorption of SBTi/BBTi graded thin films is between that of BBTi and SBTi in the wavelength 250 nm-400 nm from the results of spectrophotometric measurements of the transmittance. The band gap energy of SBTi/BBTi graded film is found to be 4.10 eV, which is between that of BBTi (3.59 eV) and SBTi (4.44 eV). It indicates that the structure of the graded thin films will affect the band gap energy and optical absorbancy of this material. Annealing conditions also have a great influence on the optical properties of graded thin films. It is found that the impurity absorption beyond the intrinsic absorption appears in the graded films, and the impurity absorption peak blue-shifts with the increase in the annealing time.