Analysis of the Fermi energy position and its correlation to defect levels in acceptor doped BaTiO3 ceramics
A major restriction of multi-layer ceramic capacitors is the increasing leakage current as a result of a decreasing insulation resistance, which is supposed to be based on the electromigration of oxygen vacancies (VO˙˙) accompanied by an O2-exchange at the electrode interfaces.
The O2-exchange is affected by the space charge region, which is determined by the difference in Fermi energy at the interface and in the bulk. This can be manipulated by doping and varying the electrode material, which should enable systematic tuning of the O2-exchange and accordingly resistance degradation.
Therefore, we analyzed the barrier formation of Mn/Fe doped BTO ceramics to different high and low work function electrodes as a function of doping and VO˙˙ concentration using X-ray photoelectron spectroscopy. Different VO˙˙ concentrations were established by reducing and oxidizing treatments. The chosen doping concentrations of 3.2x10^19 to 4.0x10^20 cm-3 were supposed to be sufficiently high to pin the Fermi level at the specific defect level states.
However, the experiments reveal no lower Fermi energy than 1.7 eV above the valence band maximum, which strongly contrasts with defect chemistry calculations, which predict a Fermi energy of approx. 1 eV for oxidized acceptor doped BTO. Furthermore, no pinning was observed at the known defect levels (in the accessible range). These results reveal a significant discrepancy in Fermi level position between defect chemistry calculations and experiment.