Polarity-Dependent Charge Transport and Resistance Degradation Enabled by Oxygen-Vacancy Gradients in BiFeO₃ Films

The long-term reliability of Mn-doped BiFeO₃ thin films remains a key challenge for their implementation in piezoelectric microelectromechanical systems. In this work, the electronic transport mechanisms and DC resistance degradation of (100) Mn-doped BiFeO₃ (BFMO) epitaxial films grown on (100) Si substrates by RF magnetron sputtering were systematically investigated. The leakage current characteristics exhibit a strong polarity dependence, originating from a nonuniform distribution of oxygen vacancies across the film thickness. Under field-up bias, where a high concentration of oxygen vacancies is present near the top interface, leakage is dominated by Poole-Frenkel emission from $V_O^{̈}$ related traps in the low-field regime and evolves into trap-controlled space-charge-limited conduction at higher fields, with transport controlled by ${\mathrm{Fe}}_{\mathrm{Fe}}^{\text{'}}$ sites. In contrast, under field-down bias, where the $V_O^{̈}$ concentration near the bottom interface is comparatively low, transport is Ohmic at low electric fields and transitions to Poole-Frenkel emission at higher fields, with conduction governed primarily by ${\mathrm{Fe}}_{\mathrm{Fe}}^{\text{'}}$ trap levels. The asymmetric $V_O^{··}$ distribution also gives rise to polarity-dependent DC resistance degradation, governed by field-driven $V_O^{̈}$ drift and subsequent accumulation at the cathode interface. Since the $\left[V_O^{̈}\right]$ is initially higher near the top interface, $V_O^{̈}$ redistribution proceeds more rapidly, resulting in accelerated degradation kinetics and a reduced lifetime. The extracted $V_O^{̈}$ diffusion coefficient for BFMO (∼2.7 × 10^-14 cm² s^-1) is approximately an order of magnitude higher than values reported for PZT films, consistent with the comparatively shorter lifetime observed under similar stress conditions. These findings highlight the pivotal role of defect engineering particularly in controlling oxygen vacancy concentration and migration for optimizing the reliability and performance of BiFeO₃-based MEMS devices.