Fabrication of high quality multiferroic BiFeO3 based thin films using cost-effective processing route

Abstract

Spin coated thin films of bismuth ferrite (BiFeO3) were produced using cost effective sol-gel processing route. Precursor solutions were made using Bi(NO3)3.5H2O and Fe(NO3)3.9H2O as the metal sources in a 2-methoxyethanol solvent. Acetic acid was used as the chelating agent. The solution was dispensed on a preheated soda-lime glass or FTO-coated glass substrate and spun in a spin coater to obtain the thin films. The selection of annealing temperature was revealed to be an important aspect since low annealing temperatures such as 450° C was found to be insufficient for producing phase pure bismuth ferrite. Increasing the annealing temperature had a beneficial effect on phase formation as well as microstructure development. Annealing at 550° C for 30 mins was enough to produce phase pure bismuth ferrite and well crystallized films with low variation in grain size. A pronounced effect of substrate preheating was observed on the film topography and continuity. Increasing the substrate preheat temperature from 30° C to 70° C produced increasingly continuous and smooth thin films. Some degree of preferred orientation or texture was induced by rapid heating of the dried films. Annealing a thin seed layer first, before subsequent coatings were deposited on top of the seed layer, improved the texture and phase purity of the rapidly heated films. On the other hand, Bi non-stoichiometry of 5% seemed to change the microstructure along with the unit cell size. The presence of different types of defects in these Bi-deficient or Bi-excess films were argued to be the main reason behind the observed changes. Interestingly, Bi-deficiency was found to be beneficial for producing a less porous microstructure with a slightly larger grain size. Electrical measurements could not be made reliably due to the presence of excessive dust in the films. Dust was reduced appreciably by filtration of the solution but measurements were still unreliable, indicating the unsuitability of the working environment for producing high quality thin films. Magnetic properties of the films were evaluated and weak ferromagnetism of bismuth ferrite was confirmed. The magnetization seemed to be the highest in the Bi-excess film, owing to its many vacancies in the Fe sub-lattice. Optical properties of the films were measured and the band gap (~2.73 eV) was revealed to be unaffected by Bi non-stoichiometry