Influence of Gd doping and thickness variation on structural, morphological and optical properties of nanocrystalline bismuth ferrite thin films via sol-gel technology

Pure and gadolinium(Gd)-doped bismuth ferrite(BiFeO3, BFO) thin films were successfully fabricated via sol-gel processing technique onto the corning glass substrate using a programmable spin coating unit. The effect of thickness of the multilayered films coated and the influence of Gd doping on the structure, surface morphology, chemical composition and optical parameters of the thin films were systematically investigated. X-ray diffraction analysis of the annealed thin films confirmed the existence of rhombohedral distortion perovskite with preferred orientation along (101) plane. Atomic force microscopy (AFM) and Field Emission Scanning electron microscope (FESEM) images demonstrated that the average grain size decreased with Gd incorporation in the BFO lattice. Bi-site substitution by Gd in BFO lattice was confirmed by the Raman spectra analysis measured at room temperature. Energy Dispersive X-ray Analysis (EDAX) further confirmed the elemental composition with the presence of Gd element in Gd-doped BFO thin films. Various optical parameters including extinction coefficient, optical bandgap energy, near-edge absorption ratio, and Urbach energy were determined from Ultraviolet-Visible spectra of the multilayered bismuth ferrite (BFO) thin films of different thickness and Gd-doped bismuth ferrite (Bi0.95Gd0.05FeO3, BGFO) thin films. The increase in film thickness (or the number of layers coated) leads to a reduction in the optical bandgap of the material, from 2.65 eV to 2.44 eV, which may be useful for applications in thin film solar cells and optoelectronic devices. The transmittance and the optical bandgap decreased with Gd doping and a reduced bandgap of 2.51 eV was observed in BGFO thin film.