Thickness Dependence of the Physical Properties of Mn and Cu Co-Doped BiFeO₃

Thin films of BiFeO₃ (BFO) co-doped with manganese (Mn) and copper (Cu) were deposited on FTO/glass substrates using the spin-coating technique. The stoichiometry used was BiFe_0.94Mn_0.04Cu_0.02O₃, ensuring an appropriate proportion of elements to study the effects of co-doping on the physicochemical properties of the films. The films were fabricated with different thicknesses, varying the number of layers from 1, 3, 5, 10, and 15. Additionally, a pure BFO sample with 15 layers was fabricated to quantify the effects of co-doping on the material. X-ray diffraction (XRD) analyses revealed that co-doping and increased thickness significantly improved crystallinity, showing a transition from a mixed R3c-P1 structure in the thinner films to a predominantly R3c structure in the thicker ones. Scanning Electron Microscopy (SEM) images showed an increase in grain size and better adhesion with an increased number of layers, suggesting improved nucleation and growth of the grains. Raman spectra indicated a reduction in structural defects and the appearance of a Jahn-Teller band in the co-doped samples, suggesting additional distortions in the FeO₆ structure due to co-doping. XPS analyses confirmed stable oxidation states for Bi, Fe, Mn, and Cu in the co-doped films, with a significant reduction in oxygen defects. Hysteresis (P-E) measurements showed an improvement in remnant polarization (Pr), reaching 1.1208 μC/cm² for BFMCO₁₅films compared to 0.0215 μC/cm²in for BFO₁₅, and an increase in coercive field (Ec) from 75.73 kV/cm in BFO₁₅ to 508.32 kV/cm in BFMCO₁₅, indicating better alignment of ferroelectric domains and greater structural stability. Regarding optical properties, the co-doped films showed slightly lower transmittance in the visible range, attributed to increased light absorption due to the introduction of dopants. The band gap energy decreased with the increase in the number of layers and co-doping, from 2.34 eV in BFO₁₅to 2.32 eV in BFMCO₁₅, suggesting greater interaction between the dopants and the BFO matrix. Additionally, the absorption coefficient and refractive index varied significantly in the range of 200 to 800 nm. The results demonstrate that both Mn and Cu co-doping, as well as thickness control, are effective strategies for improving the properties of BFO films for various technological applications.