Optical, conductive, and ferroelectric properties of the first layer of dip-coated BiFeO3 films from methoxyethanol and acetic acid-based chemical dissolvents

Abstract

The overall performance of the multilayer resulting in a sol-gel BiFeO3 film will be primarily determined by the properties of the first layer, but this has yet to receive much attention, even though chemical and morphological defects of this layer can accumulate as the number of layers increases. Here, we perform an optical, conductive, and ferroelectric study of first layer (L1) dip-coating sol-gel BiFeO3 films using two routes that vary only in the dissolvent; the first one is based on 2-methoxyethanol (MOE), and the second one on acetic acid (AA) with some MOE (AA-MOE). Tauc plots reveal a band gap of 2.43 eV and 2.75 eV for MOE (30±5 nm thick) and AA-MOE (35±5 nm thick) films, respectively. MOE films showed a dielectric function with features at ~2.5 eV, ~3.1 eV, and ~3.9 eV, which were associated with charge-transfer transitions, but such features are absent in AA-MOE films. Advanced atomic force microscopy (AFM) techniques were used to identify the fine features or defects of the BiFeO3 films: The conductive maps show that the charge transport pathways in both film routes are controlled by nanometer defects rather than grain or grain boundary defects. Current-voltage curves reveal high conductive pathway at a lower voltage for the MOE films than for AA-MOE films. The piezoelectric coefficient for MOE films was ~20% higher than AA-MOE films. Both deposition methods yield ferroelectric films with an electromechanical strain controlled by the piezoelectric effect and minimal contribution from electrostriction. An optimization for the AA-MOE-based route in the withdrawal speed results in a significant reduction of morphological defects and a more than twofold increase in the piezoelectric coefficient. Our results broaden the understanding of optical and ferroelectric BiFeO3 films based on a chemical solution by dip-coating.