Effect of annealing temperatures on the structural and optical properties of sol–gel processed nanocrystalline BiFeO3 thin films

Abstract Nanocrystalline bismuth ferrite (BiFeO 3 ) thin films were deposited onto a glass substrate by sol–gel process. The effects of annealing temperature on the structural and optical properties of BiFeO 3 thin films have been studied. The as-fired film, annealed at 300 °C was found to be amorphous that crystallized to hexagonal structure after annealing at 500 °C for 2 h in air. The X-ray diffraction pattern shows that the sample is polycrystalline in structure with the lattice constants of a = 5.575 A and c = 6.912 A, respectively. The average grain size of the film is found to increase from 16.1 to 35.4 nm, with the increase in annealing temperature from 400 to 500 °C, respectively. Atomic force microscopy studies revealed the increase in the root mean square roughness with the increase in annealing temperature. The optical transmittance spectra showed a decrease in the transmittance with the increase in annealing temperature. The absorption edge of the films is also found to be sharper and shifted towards the lower energy as the annealing temperature increases. The refractive indexes of the films are found to increase while the optical band gaps are found to decrease with annealing temperature. Nanocrystalline BiFeO 3 thin film exhibits a characteristic luminescence spectrum with two maxima around 2.53 and 2.36 eV.