Phase transition, interband electronic transitions and enhanced ferroelectric properties in Mn and Sm co-doped bismuth ferrite films

Mn and Sm co-doped BiFeO₃ (BFO) polycrystalline films have been fabricated on F-doped SnO₂ (FTO) conductive glass substrate by a chemical solution deposition method. X-ray diffraction patterns and Raman spectra both imply a coexistence of rhombohedral and tetragonal phases in Mn-Sm co-doped BFO films. Scanning Electron Microscopy (SEM) images indicate more impact morphology with greater particle size in doped films. X-ray photoelectron spectroscopy (XPS) analysis suggests that high-valence Mn ions induced by Sm-doping lead to efficient decrease in oxygen vacancies, and thus improve leakage and polarization properties. Optical spectra analysis by using a Kubelka-Munk function and corresponding XPS valence band spectra analysis reveal that the optical band gap (E_g, varied from 2.38 eV to 2.26 eV) and Fermi level (E_F, varied from 1.18 eV to 1.01 eV) decrease continually with the introduction of Sm dopants, further confirming the decreased defects and vacancies in layer interfaces or grain boundaries. Consequently, larger saturation polarization and lower leakage current densities were obtained in Mn-Sm co-doped BFO films. Furthermore, possible energy band structure schematic proposed on the basis of above analysis shows that the shift in Urbach impurities energy level induced by ions dopants outsiders may be quite crucial in future photovoltaic application.