DEFECT CHEMISTRY MODEL OF THE FERROELECTRIC-ELECTRODE INTERFACE

The interactions between the electrodes and titanate ceramic materials are examined using numerical defect chemistry models. A time independent model solves for the steady-state concentrations of electrons and holes, ionized accepters, and oxygen vacancy concentrations throughout the ferroelectric film as a function of electrical potential, temperature, and oxygen partial pressure. These calculations reveal a basic mechanism by which electrically active space charge regions may form in the ferroelectric material. The combination of atmospheric oxygen partial pressure with ionization energies of naturally occurring acceptor impurities of around one electron-volt prevents full ionization of the accepters. However, the accepters respond strongly to an electric field, and the percentage of ionized accepters in the interface region may range from 0% to 100% with the application of a few tenths of a volt. The response of the partially ionized impurities to the electric field is the proposed mechanism for the space charge activity in thin-film devices. These models also show that the electrochemical potential induced by the contacts can produce a substantial increase of the oxygen vacancy concentration at the metal/ceramic interface. A negative contact potential reduces the ceramic, lowering the oxygen concentration in the interface region. The reduction is increased by an externally applied negative bias, which lowers the oxygen concentration at the interface even more. The calculations confirm previous models that treat the space charge layer as a depleted n-type semiconducting region. These results give further support to the Schottky barrier models that have been proposed to explain the capacitance vs. voltage and current vs. voltage characteristics of ferroelectric capacitors.