Connecting electrical current conduction and Urbach energy in doped BiFeO3 thin-films

The present study explores the influence of Co2+ and Mn4+ ions on the optical and electrical properties of bis-muth ferrite thin films prepared on indium tin oxide-coated glass substrates. In particular, a correlation between the Urbach energy tails and the current conduction is established. Doping-dependent variations in Urbach energy tail widths, optical bandgap energies and J-V curves of the fabricated films are investigated. The Urbach energy tail width of Co-doped BiFeO3 is found to increase by 36%, while that of Mn-doped one is found to reduce by 15%. The optical bandgap of Co-doped and Mn-doped BiFeO3 was found to reduce by 44% and 40% respectively compared to undoped BiFeO3. A correlation between the induced defect states and the optical parameters has been observed. Analysis of J -V characteristics reveals the dependence of the current conduction upon the valence state of the dopant. In particular, the dark current density decreased by almost one order of magnitude with Mn-doping and increased by more than one order with Co-doping. A defect-dependent space-charge-limited conduction (SCLC) mechanism is proposed to understand the J -V curve in the Co-doped and Mn-doped BFO samples. A direct variation of the leakage current density with the Urbach energy suggests that Urbach energy studies could be considered as an alternative indicator of the leakage current density in thin-film BiFeO3-based devices.