Controlling Resistance Degradation of High-Permittivity Dielectrics by Bulk and Interface Fermi Level Engineering

Polycrystalline Mn- and Fe-doped BaTiO3 ceramics were synthesized using the solid-state synthesis route with different doping concentration. Resistance degradation is considerably suppressed with increasing doping concentration. In order to quantify defect energy levels, enthalpies of segregation of oxygen vacancies, electrical current, and oxygen migration paths, samples have been strongly reduced in Ar/H2 atmosphere and slowly re-oxidized by temperature cycling in dry air or similar mixtures of 1-20% oxygen with inert gases such as Aror N2. The re-oxidation behavior is monitored by continuously recording the dc electrical conductivity. Substantial differences are observed in dependence on dopant species and concentration, which can partially be related to the different defect energy levels of Mn and Fe. The current paths are obtained from simulations of the electrostatic potential distribution, taking account of the segregation of oxygen vacancies to the grain boundary core following the widely accepted model and procedure introduced by Souza. By comparing the simulations with measured activation energies as a function of conductivity (oxygen vacancy concentration), it should be possible to extract the energy levels of the defects and the enthalpy of segregation.