Migration behaviors of oxygen vacancies in polarized multilayer ceramic capacitors studied by Kelvin-probe force microscopy and thermally stimulated depolarization current spectroscopy

The migration behaviors of the oxygen vacancies in multilayer ceramic capacitors (MLCCs) due to polarization under high temperature and heat treatment after the polarization were studied by frequency modulation Kelvin-probe force microscopy (FM-KFM) and thermally stimulated depolarization current (TSDC) spectroscopy. The TSDC spectra of the MLCC showed two TSDC peaks at temperatures of about 240 and 300 °C, which correspond to the migration of the oxygen vacancies within the grains and across the grain boundaries, respectively. The potential distributions of the cross-sectioned MLCCs of unpolarized, polarized at 220 °C, and heat-treated at 240 °C after the polarization were measured using FM-KFM. Before polarization, a step-like potential distribution with a steep potential gradient was observed at the electrode/dielectric interfaces and grain boundaries. In the polarized MLCC, the electric field was relatively higher at the anode/dielectric interface and within the grains, while it was low at the grain boundaries and cathode/dielectric interface. After the heat treatment, a potential decrease was observed at the grain boundaries, and the asymmetry between the anode and the cathode interface was alleviated. These potential changes are attributed to the migration of the oxygen vacancies. It can be interpreted that the oxygen vacancies migrate toward the cathode side within the grains and to the grain boundaries during polarization and are depolarized within the grains but not in the grain boundaries by the heat treatment at 240 °C, which is consistent with the TSDC spectra.